Cactus e Dintorni

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Home Cultivation guide The diseases
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The diseases

AUTHOR’S DISCLAIMER

The author of this website declines all responsibility for any damage resulting from use or misuse of the following information or from improper manipulation and application of the agricultural chemicals mentioned in this article, all of which are registered trademarks belonging to the legitimate owners. Insecticides, fungicides, acaricides and other chemicals referred to in this webpage are recommended solely for informational purposes. If an active principle has never been used before it is advisable to test it for phytotoxicity on a single plant first. The retail sale of certain chemicals may be legal in some countries and illegal in others; you may also require a private license to purchase some restricted products that are potentially toxic or harmful to humans. When handling or applying a chemical product you must be aware that you are doing it at your own risk and that the instructions printed on the package label or insert have priority over the information provided in this section.

Before using any chemical product it is essential that you read the manufacturer’s instructions carefully, especially with regard to its active principle, dosage, mode of action against parasites, composition, directions for use, toxicity class, compatibility with other products, time of application, the possible dangers to wildlife, the interval after which the chemical is no more harmful (PHI), the nature of risk (exposure by inhalation, ingestion and skin or eye contact). All chemicals must be locked in a cabinet and stored in a room or building away from living areas and out of the reach of children; the preparation and application of a solution should always be done outdoors, never inside a building or a greenhouse. Take care not to spray chemicals against the wind; do not eat, drink or smoke when handling dangerous substances; always wear protective clothing including gloves and goggles and avoid the inhalation of the fumes or the contact with eyes or mucous membranes. Wash yourself thoroughly after every treatment; do not pollute the environment and dispose of empty containers properly. In case of contact wash affected skin areas with soap and water immediately; if symptoms arise consult a doctor carrying the product label with you.

NOTICE: beginners usually tend to become too anxious about an unhealthy plant and start looking for some expert advice in order to save it, not really knowing the gravity of the problem. I would like to point out that, exceptions aside, even a phytopathologist needs to examine a specimen closely and possibly run some laboratory tests to establish an accurate diagnosis.
You should also be aware of the fact that most cryptogamic diseases cannot be cured as symptoms usually become visible only when the pathogen has already spread within the vascular system of the host plant and harmed the organism irreparably.
Since prevention is the best remedy for this kind of infections, you will find further information which should give you some clues to identifying a plant disease correctly, especially when you have taken a first-hand look at the specimen; once determined the stage and seriousness of the infection you can start the right treatment straightaway.

FOREWORD. A plant is in good health if it can carry out its physiological functions efficiently. According to Owens "A disease is a disturbance to or a deviation from the normal structure or physiology of a plant, either localized or generalized, which can be identified through specific signs or symptoms and causes damage to the plant".
At least three factors play a fundamental part in the parasitisation of a plant: the receptivity of the specimen, the virulence of the pathogen and a suitable environment; when these occur simultaneously they create the conditions for the pathogen to enter the host. This concept, developed by Bateman, is known as ‘Disease Triangle’: Host, Pathogen, Environment; sometimes a fourth factor, Time, is considered, understood as the duration of favourable conditions for the infection which can influence the extent and seriousness of the damage.

PHYTOPATHIES AND PESTICIDES

VEGETAL PARASITES
Vegetal parasites are usually divided into four groups:
a) Fungi: these microscopic organisms are incapable of movement and reproduce both sexually and asexually; they invade vegetal tissues by means of hyphae feeding on the plant sap. Fungal spores can survive cold weather and drought and when conditions are suitable for germination they might be easily spread through water used for irrigation. Many of the problems concerning fungal diseases are caused by our carelessness as we often introduce pathogens unintentionally into the greenhouse and if they become established eventually they lead to serious consequences. Fungi can enter a host through wounds in its stem or roots, natural openings such as stomata and some of them can even bore through the plant cuticle using a specialized hypha. Most fungi thrive in high humidity associated with low temperatures and poor or absent ventilation; altering these parameters can stop the spread of the disease which can, however, start again if conditions worsen. Vegetal debris and certain kinds of plants may harbour fungi such as Alternaria, Cercospora, Colletotricum; others like Phytophtora and Fusarium are hidden in the soil; Rhizoctonia, Pythium, Sclerotium are among the saprophytes found in dead organic matter. This is why using garden soil as a potting mix is not recommendable as it might contain resistant spores or other survival structures of pathogens and bacterial infections unless it is sterilized either with steam at 90°C (194°F) or fumigants (methyl bromide, carbon tetrachloride); both procedures, however, cannot be easily carried out at home. The prevention of diseases is achieved through the accurate sanitization of potting mixes, propagators and cold frames, greenhouses and any other room where plants are kept, ensuring at the same time plenty of ventilation, good drainage, a balanced fertilization as well as the disinfection of seeds..

b) Bacteria: (Schizomycetes): these simple monocellular prokaryotic organisms with a cell membrane but no distinct nucleus usually reach a diameter of a few microns and fall halfway between the animal and the vegetal kingdoms. They can be either aerobic or anaerobic, double their number by cell division every 20 minutes and survive extreme conditions which do not allow any other life form to develop. They are found in the air, in the water, inside and outside every living thing and abundantly in the soil (between 1 and 50 millions per gram of soil). Most of them are saprophytic while very few are autotrophs. Many bacteria have a major role in soil improvement with their oxidation process and decomposition of organic matter; some are able to fix nitrogen from the air combining it with hydrogen making it available to plants. They thrive in a temperature range of 10°C to 35°C (50-95°F), the optimal growth temperature being around 25°C (77°F), high soil moisture content and in the presence of infected or non-decomposed organic matter and can be transmitted by insect vectors. Penetration into the host occurs indirectly through wounds or natural openings (stomata, scars due to leaf or flower abscission); once inside the plant they anchor themselves to the cell walls, multiply and invade adjacent cells or find their way through the parenchymal tissues or the vascular bundles. That’s why bacterial infections may be systemic, vascular, parenchymal or hyperplastic. Systemic infections cause leaf wilt and blackening of vascular tissues resulting in canker formation. Vascular infections produce wedge-shaped necrotic lesions with black rot. Treat seeds with copper compounds or water at 50°C (122°F) for 20 minutes as a preventive measure.
The bacteria Pseudomonas and Erwinia cause rot and tissue blackening, root canker, vascular occlusion and dieback.

c) Viruses: varying from 0,05 to 0,2 micron in size, these biological intracellular entities are composed of nucleic acids like RNA (ribonucleic acid) or DNA (deoxyribonucleic acid) enclosed in a protein coat (capsid). They do not have a cell structure, cannot move or grow but they are able to evolve and mutate in order to adapt to different environments. They cannot, therefore, enter plant tissues directly so possible ways of transmission are grafting, vegetative propagation of contaminated specimens, plant-to-plant contact (through epidermal lesions in the healthy specimen) and finally infected pollen which may give rise to diseased seedlings. Viruses can also be transmitted by insects, such as aphids, leafhoppers, mealy bugs, whiteflies, thrips, beetles as well as by mites, nematodes, fungi and of course humans through careless cultivation. Once inside they can spread within the plant travelling through the phloem. Viruses can only reproduce within a ling host cell which is penetrated and forced to replicate the virus genetic material. They can induce chromatic alterations, stem or flower malformation, chlorosis and dieback. Yet sometimes infections may account for interesting variations in stem and flowers, producing spots, deformities, mosaic patterns or chlorotic ring spots. We should keep an eye on any alterations in the colour of flowers as these may indicate the presence of a pathogen.
Correct diagnosis can be obtained using an electronic scan microscope, serological test kits and fluorescent probes.
Viroses can be prevented through sterilization of tools, suppression of disease vectors, protection of plants with horticultural fleece, weeding and keeping your greenhouse clean and tidy. Thermotherapy may be effective in removing the virus while meristem culture is used to produce virus-free plants. Moreover, genetic engineering helps to develop virus-resistant species. On the other hand biotech vaccines are of little or no use since plants lack a real immune system.

d) Mycoplasmas: these microscopic prokaryotic organisms do not possess a cell wall but are instead enclosed by a three-layered membrane, are gram-negative, have no flagella and do not produce spores. Mycoplasmas are neither bacteria nor viruses but something in between and live inside the phloem of the plants. These parasites can cause serious disorders in the host that can result in anomalous growth, not to be confused with the cristate form which is not caused by an infection. They interfere with the metabolic hormone stimulating the production of a substance which encourages the development of new shoots from axillary buds. Because the disease is relatively benign, it is common practice among most nurserymen to graft infected plants onto others in order to obtain monstrose forms to be sold as oddities. All cacti are believed to be prone to this infection, particularly the genera Opuntia, Echinopsis, Gymnocalycium, Cereus. So far researchers have never been able to isolate the pathogen responsible for such malformations. The symptoms are: deformities and aberrant growth; proliferation of axillary buds particularly near the apex; weak and stunted basal offsets; leaf yellowing and pronounced internodal elongation, nanism of apical meristem, reduced growth, virescence. Affected plants do not need to be eliminated since there’s no risk of the disease spreading naturally. Avoid using them for grafting unless you want to experiment; sterilize tools with denaturated alcohol before using them on other plants. Try to keep your greenhouse free of pests like leafhoppers, psyllids, mealy bugs, aphids, whiteflies which can transmit the pathogen. Mycoplasmas can be treated with hot water (immerse the plant in water at 45°C/113°F for five hours) or antibiotics such as Tetracyclines and Streptomycin (the latter is prohibited in some countries).

Diseases caused by groups b, c and d are still quite rare though becoming increasingly common and hardly curable. Mycoses, though, are usually treatable but once the disease has spread through a large collection it is almost impossible to get rid of; prevention is therefore essential through appropriate horticultural practices and hygienic-prophylactic measures to be observed in any place where plants are raised or grown.

Here are some general rules that are always valid: disinfect and quarantine plants that are newly introduced in your collection; always use good, wholesome potting soil with a low amount of organic matter; spray plants and soil once a year with systemic products; sanitize cutting and pruning tools using a solution of water and bleach in a 10:1 ratio; always ensure good ventilation; apply water early in the morning so that plants stay dry at night, preferably avoiding overhead watering; make sure that the soil is never waterlogged, that plants are fed with a balanced fertilizer, that the air moisture is not too high; germinate seeds that have been treated with fungicide or bought from specialized nurseries; throughout the year remove plant debris, spent flowers and fallen leaves from the greenhouse; disinfect the place with a powdered broad-spectrum fungicide in late autumn before overwintering plants. Destroy those specimens that have shown symptoms of infectious disease in order to prevent the proliferation of the pathogen and the contamination of other plants and get rid of the soil and pots in which they have been grown; do not forget to wash your hands before handling other containers or plants. Always bear in mind that some microorganisms cannot enter vegetal tissues but for our carelessness, finding their way through cuts and wounds, spreading during repotting operations or vectored by pests, which therefore must be dealt with immediately.

Among diseases attributable to vegetal parasites some are worth examining a little closer: Helminthosporiosis, Fusariosis, rot due to Rhizoctoniosis, Pythium, Phytophthora, necrosis produced by Coniothyrium, Monosporiosis, spots caused by Septoriosis, Anthracnosis, Moulds. Because many diseases manifest themselves in a similar way and yet require different approaches, it is best to have valuable specimens tested in a laboratory of phytopathology for a correct diagnosis.

HELMINTHOSPORIOSIS-ALTERNARIOSIS/Bipolaris/Drechslera. It is probably the most dreaded and widespread fungal disease to hit succulents and is caused by Helminthosporium cactivorum (Drechslera cactivora). In young plants it gives rise to brown rot and leads rapidly to death. On mature specimens it is known to follow a slower course, manifesting itself at first as yellowish spots which then turn black and rotten when the pathogen penetrates into deeper tissues. The most vulnerable parts are plant collar and concavities in the surface where humidity usually lingers. High levels of air moisture speed up the progression of the disease which sometimes shows itself as a black velvety efflorescence formed by tiny dome-shaped clusters.
Favourable conditions for the proliferation of the fungus are temperatures between 16 and 30°C (61-86°F), constant humidity and overhead watering, closely packed plants, the presence of wounds in the leaves or stems and poor ventilation.
The recommendation is to take preventive action against the problem, sterilizing the soil with suitable products, removing and destroying affected plants and avoiding waterlogging or excess wet. Under critical conditions or at the onset of the first symptoms start treatments with systemic products such as thiabendazole, zineb, mancozeb, ferbam, chinosol, captan (the use of certain substances may be subjected to limitations). Similar symptoms are associated with Alternariosi (Alternaria tenuis).

FUSARIOSIS. The symptoms of this disease, caused by Fusarium oxysporum, are wilt, dry rot, canker, yellowing of leaf margins, reduced growth with slight tissue alterations in the basal region which appears opaque and less turgid with soft round spots. A straight cut across the plant stem will show that sections of the vascular system are red or brown-coloured; this means that the fungus, which was lying dormant in the soil, has entered the host plant through its roots, invaded the vascular bundles obstructing them and released a toxin that reduces osmotic pressure. In its most acute form it leads to basal rot and eventually death of affected plants. There is also a milder form of the disease which tends to become chronic, causing dry rot, a sort of suberization, which spreads over the surface quite slowly and usually allows the host to survive for some time. Certain species of Fusarium may infect the aerial parts instead and then work their way downwards to the rest of the plant.
The control of Fusariosis and Verticilliosis (Verticillium) along with other vascular diseases is very difficult; you might try using benzimidazolic fungicides (carbendazim, benomyl, etc.) with a broad-spectrum systemic action or thiophanate, dodine (Syllit 35). Prevention is therefore the best cure: do not grow on soil that is suspect of contamination; use wholesome horticultural products only; always sanitize pots, tools etc; avoid excess water and unbalanced fertilizers.

A common problem affecting succulents, especially seedlings and plants propagated from cuttings is ROT which can suddenly attack the part of stem in contact with the soil. Unfortunately, if the disease spreads to young seedlings it is fatal so it is best to treat preventively with systemic fungicides like propamocarb (Previcur) or iprodione (Rovral).

ROOT ROT AND COLLAR ROT are attributable to different fungi yet producing analogous alterations and cause tissues to soften and become reddish-brown with the discolouration spreading from the collar to the roots and leading to plant collapse. Other symptoms are interruption of growth and yellowing of stem and/or leaves. You might be able to save a diseased plant by immediately removing the infected portion of stem with a sharp cutter until the exposed surface looks healthy and shows no dots or tiny spots near the vascular rings. Make sure to sterilize the blade with alcohol each time you make a cut so that you won’t spread the disease upwards to the healthy part. The upper portion may be grafted, if short, otherwise rooted as cutting after being dusted with sulphur-copper powder or zineb. Such problems may also stem from excess watering and poor drainage and can be avoided by repotting the plant in better soil, by following a correct watering schedule and in case of fungal attack by treating with Benomyl or Thiram. If the disease is too advanced for treatment, stems can be cut off to be rooted as cuttings.

ROT CAUSED BY RHIZOCTONIOSIS. The pathogen responsible for this disease is Rhizoctonia solani, a parasite attacking mainly Opuntia, Mammillaria, Cucurbitaceae and young seedlings. This saprophyte fungus is found in the soil surviving within dead tissues and germinates at temperatures between 8°C and 30°C (46/86°F) invading the host plant through the root system. The first symptom is basal rot which progresses rapidly through the whole stem. At the first appearance of the disease the epidermis looks tight and glossy then the inner tissues start to deteriorate and eventually collapse. The pathogen is easily detectable through a microscope. 
Effective measures against the disease are preventive soil sterilization with steam or fumigants and treatments with pencycuron, tolcofos metil, iprodione once infection has occurred. Benzimidazolics (carbendazim, benomyl, thiabendazole) also work quite well.

ROT CAUSED BY PYTHIUM SP. There are different species of this fungus that are known to attack roots and collar of seedlings with lethal consequences. In mature specimens the affected parts are covered in light-coloured mildew. Treat with Previcur.

ROT CAUSED BY PHYTOPHTHORA AND ARMILLARIA.
Although this disease is rather infrequent in succulent cultivation in some cases it may harm plants severely, especially young seedlings. The fungus responsible for the infection is Phytophthora cactorum  which is commonly found in garden soil. The infection first manifest itself as soft, spongy rot which spreads rapidly and later causes the formation of cracks along the stem. The affected tissues appear dark-coloured and sticky and in the more advanced stages of the disease the parenchymal tissues eventually become dry. Nematodes are likely to be found in association with this infection. Lesions caused by Phytophthora usually harbour other fungi that give rise to dry rot (Fusarium) and soft rot (Erwinia).
Shoestring root rot is caused by a fungus called Armillaria mellea and its symptoms differ slightly from those of Phytophthora root rot in that there is no gum exudation and the infected mass has a distinct smell of mushrooms.
The disease can be effectively controlled only through prophylaxis which means disinfecting the soil, eliminating waterlogging and excess humidity problems, isolating and destroying any infected specimens. We might also resort to specific chemical products if necessary, such as dithiocarbamates (ziram, mancozeb, propamocarb, iprodione) after having perfomed a phytotoxicity test on the plant. 

NECROSIS CAUSED BY CONIOTHYRIUM. The infection, which some Euphorbias seem to be particularly susceptible to, first attacks the main stem, sometimes secondary branches, and is recognisable by the presence of dark-coloured roundish spots. As the infection progresses the affected parts become either soft or withered and the whole plant gets weak and eventually dies. It is believed that the pathogen can enter the plant only through wounds. The disease produces fruiting bodies containing the parasite that look like black specks, often arranged in concentric circles. When mature these structures exude cirrus-like masses of conidia that dispersed by water rapidly spread the infection. Once again prevention consists in avoiding causing injuries to plants, ensuring sharp drainage, feeding with balanced fertilizers and finally eliminating infected material. Captan is the best product for treating the disease, but should it be difficult to get, benzimidazolics are also recommended.

MONOSPORIOSIS AND MINOR FUNGAL DISEASES.
Monosporium cactacearum and Sporotrichum cactorum are other pathogens that might cause serious damage to certain succulents. The genus Echinopsis and some Cerei are particularly prone to the aforesaid diseases. The infection starts attacking the basal cladodes manifesting itself as black spots, at first small then becoming bigger. It soon penetrates the inner tissues causing plant death. The recommendation is to remove infected parts, isolate diseased specimens and treat with products whose main ingredient is methyl, ziram or dodine (Syllit 35); otherwise use copper oxychloride.

SPOTS CAUSED BY SEPTORIOSIS and ANTHRACNOSIS. The fungi Septoria cacticola, Ascochyta opuntiae and Colletotrichum are known to attack mainly Euphorbia, Opuntia, Cereus, Echinocactus, Ferocactus, Agave although hypothetically all cacti could be infected, particularly those grown outdoors and therefore exposed to hail damage. These fungi spread in the autumn favoured by the high humidity and the shorter day-length. The sticky droplets found over or near the spots are the remainders of the fungus after it has penetrated into the plant epidermis. Following this, dry lesions begin to appear on the cladodes along with little yellowish spots. The disease spreads rapidly causing stems to dry out and eventually killing the plant so it is essential to take measures at the onset of first symptoms in order to stop the pathogen from infecting the rest of your collection.
On the other hand, symptoms of anthracnosis caused by Colletotrichum are round sunken spots with regular margins in which at first you might distinguish three or four different colours like black, light brown and yellow. After a while the spots turn brownish-grey as a result of the formation of black spores.
Prevention: take care not to cause any damage to plants; do not keep them too crammed together; do not overwater plants especially diseased ones in order to avoid spore dispersal; ensure good ventilation; destroy affected parts at the first appearance of an infection; discard old soil and apply a systemic fungicide (thiabendazole) to the new one. Cure: use anilides such as diclofluanide but also ziram, zineb, dodin, benomyl (Benlate), mancozeb, maneb, captan, bordeaux mix.

MOULDS. Botrytis cinerea is a pathogen that is commonly known as grey mould’. It is usually harboured by plant debris that accumulates on the floor and attacks mainly seedlings and succulents grown in a humid and poorly ventilated environment. The temperature range favourable to the development of grey mould is between 0 and 33°C (32/91°F), preferably 15-23°C (59/73°F), at a relative humidity of 80% or above. It thrives inside greenhouses therefore it is essential that infected parts be removed and destroyed at once in order to minimize the presence of spores in the air.

Symptoms: initially, dark green specks or little spots with yellow margins appear on the leaves and stems and later turn brown or black until they are finally covered in a fuzzy coating (mycelium) whose colour may vary from silvery to greenish grey; the affected plant soon rots and dies. 
Prevention: place potted plants at a reasonable distance from one another; sterilize your conservatory or greenhouse each Autumn; remove plant debris; install an adequate ventilation system in the glasshouse especially in cold and damp locations; keep humidity levels in check; do not use a sprinkler irrigation system; avoid using high-nitrogen fertilizers; apply fungicide to plant wounds; eliminate weeds from your greenhouse. Affected plants should be treated with systemic fungicides like benzimidazolics (Benlate) in combination with one of the following products: ronilan, iprodione, euparen, copper oxychloride. You might treat preventively with captan and benomyl.
If a plant shows signs of infection it has to be placed inside a plastic bag and isolated from the other specimens so as to avoid further contamination; after that remove all the infected parts of the diseased plant and treat the cut surface with a fungicide.
Biological control agents against Botrytis are Gliocladium roseum, Myrothecium verrucaria, Trichoderma spp., Cladosporium cladosporioides.

POWDERY MILDEW is a fungal disease that might affect Spurges and Cucurbits and some other plants, particularly during late Summer; it appears as a white powdery coating usually causing leaves to curl up. Favourable conditions to the development of the infection are temperatures of about 20-25°C (68/77°F) and a relative humidity between 60 and 80%. Its spores are spread by the wind; it can be controlled spraying wettable sulphur.

SOOTY MOULD is caused by saprophytic fungi (Antennaria, Capnodium, Cladosporum) which feed on honeydew, a sugary substance produced by some parasites such as aphids, mealy bugs, planthoppers, white flies etc. or on nectar drops exuded by the glands of cacti belonging to the genera Ferocactus and Opuntia. The disease manifest itself as a greyish black encrustation or powder resembling soot. These fungi are not pathogenic as they do not penetrate the epidermis yet they give the plant a sick, unsightly appearance. The recommendation is to remove the coating using a toothbrush dipped in a solution of sulphur-copper powder. This should be done without delay especially when the affected area is quite wide otherwise the plant loses vigour as a result of poor photosynthesis and gas exchange. The most effective way to deal with the problem is preventing it by eradicating insect pests that excrete honeydew and, with regard to Ferocactus, by washing away the nectar drops exuded by the plants.


NOXIOUS ANIMALS
When it comes to the management of pest animals it is extremely important to look into their life cycle, physiology, natural enemies, reaction to chemical substances and how this is influenced by environmental factors.

MEALY BUGS and SCALE INSECTS belong to the order Hemiptera; they are deleterious and prolific (up to eight generations per year); they suck the plant sap through a piercing-sucking mouthpart (stylet) simultaneously injecting saliva thus weakening the plant, causing epidermal discoloration and increasing its vulnerability to sunburn. Other symptoms are an increment in the respiration rate and damages to the apex and basal part of the stem as well as to the roots. They are covered in a waxy coating, some having the appearance of cotton wool, others of small bumps. They number a few hundred species but those attacking cacti are Diapsis echinocacti (cactus scale, family Diaspididae), Eriococcus coccineus (woolly cactus scale) and Hypogeococcus festerianus or pungens (cactus mealy bug) both in the family Pseudococcidae as well as Icerya purchasi and Gueriniella serratulae (family Margarodidae). Coccus hesperidum is known to attack Euphorbias and Bromeliads. Hypogeococcus festerianus has been inadvertedly introduced from the Canary Islands into continental Europe around the ‘80s. This species is a serious threat to cacti; in affected plants growth is distorted if not arrested and clumps of new shoots may arise from the tubercles. It can be distinguished from other mealy bugs by its pinkish colour and smaller size. All mealy bugs can be recognized fairly easily and when squeezed they ooze a yellowish liquid.

The sugary liquid waste(honeydew) excreted by these parasites attracts ants and is an excellent medium for pathogenic fungi which eventually cause cryptogamic diseases like sooty mould. If allowed to proliferate unchecked they are likely to kill the affected plant. Males move by means of legs and wings so they can easily reach nearby plants. The small nymphs, an immature stage of the insect, are just as mobile and take advantage of the wind to be dispersed and expand the colony. Female mealy bugs have neither wings nor legs but they usually possess glands that secrete wax which covers and protects the eggs. They thrive in a warm and dry environment preferably with mild winters which they spend inside a protective waxy coating.

Some scale insects are of commercial value, such as Coccus cacti, which is used to make a red dye called ‘carmine’, or Laccifer lacca (Tachardia lacca) which produces Indian lac.
A possible way to control the parasite in case of small localized infestations is mechanically removing the insects or dabbing them with a toothbrush or cotton bud dipped in methylated spirit which melts their protective coating. Biological control with ladybugs (Coccinellidae) and mites (Trombidiidae) in an enclosed environment like that of a small greenhouse is not advisable as it is likely to give rise to biological balance problems. The recommendation is to use indigenous insects, not exotic ones as the latter may pose a real threat to the ecosystem. On the other hand, chemical control, usually done during spring-summer, utilizes organophosphate products containing malathion (Fenix), diazinone (Basudin), dimetoate (Rogor, Digotan), fenitrothion (Fenitan), parathion, fosalone, or endosulfan, paradichlorobenzol, acephate, carbaryl, imidacloprid (Confidor, Provado), pyrethrins while during the winter rest it is best to treat with polysulphur of Barium or Calcium. In order for the chemical compounds to be really effective, protective barriers like the fluffy mass of wax or the encrustations produced by the parasite must be eliminated. That is why the abovementioned pesticides usually contain denaturated alcohol, wetting or adhesive additives.
Mealy bugs are far more dangerous if they establish themselves below the ground feeding on the plant roots (Rhizoecus falcifer, Hypogeococcus barbarae, H. othnius, H. spinosus), favoured by dry conditions in the soil, thus making themselves undetectable until growth is arrested first and then the plant begins to show signs of illness. The remedy is to wash the plant with a jet of water until all soil is removed and then immerse it in one of the specific pesticides already mentioned; let it dry out completely then repot with new soil.

Prevention: carefully control any new plants you intend to introduce into your collection, roots included; it is also recommendable to keep it under quarantine; avoid adding peat or organic matter to soil mixes; do not leave the soil bone dry for too long; a common cause of contagion is repotting with old soil as this might be infected with parasites which are transferred in the process; clean and sterilize used pots and containers with a diluted bleach solution (1 part bleach to 10 parts water); wash and sanitize your hand thoroughly before handling other plants.

RED SPIDER MITE. This tiny phytophagous eight-legged mite (Tetranychus urticae), actually not a spider, measures 0,3 mm (0,012 in) approximately therefore can only be observed through a magnifying lens. It thrives in warm and dry conditions and it spins a fine web, preferably on the upper part of the plant stem where tissues are softer; cacti that are most likely to be attacked are Rebutia, Lobivia, Coryphantha and occasionally Melocactus, Sulcorebutia, Mammillaria, Lophophora, Turbinicarpus, Pelecyphora. Susceptible plants are also some Mesembrianthemums and caudiciforms. It sucks the plant sap after piercing the epidermis with its mouth parts allowing pathogenic fungi to penetrate; it also intoxicates the host with its gland secretions and cause great damage because of its rapid diffusion under favourable conditions. The affected part dries up and turns almost grey with a reddish tinge and later cracks pushed by new growth.
Spraying water from time to time helps to prevent the parasite from establishing itself on the plant as it notoriously keeps away from humid places. Biological control is achieved using a predatory mite, Phitoseiulus persimilis, which can be purchased from biocontrol agents producers. Chemical treatment uses acaricides like kelthane, dimetoate, organophosphate insecticides. Unfortunaltely, the massive application of these products has resulted in the development of mite strains resistant to a wide range of chemicals. A more targeted treatment consists of two applications separated by a 15-day interval, the first one aimed at adult insects with dicofol, the second one at the egg with dienochlor. It is advisable to clean and sanitize the greenhouse at the very beginning of the growing season spraying pyrethrins or burning sulphur.

ROUNDWORMS.
These invertebrate, worm-like organisms are called Nematodes; their body is elongated, unsegmented, filamentous, either cylindrical or fusiform and covered in a thick chitinous cuticle and has an internal cavity. Normally they show sexual dimorphism and most species are oviparous. After a few moults, generally four, the larvae reach maturity. The most common species are Meloidogyne javanica, Heterodera cacti and Globodera spp. They prosper in moist, fresh, rich soil; if conditions become unfavourable, nematodes enclose themselves in a cyst in which they can survive for quite a long time. Some of them can enter plant roots blocking sap circulation, causing masses of woody tissue (galls) to form and eventually plant death. This leads to infection which can be determined by viruses (blisters), insects (Diptera, Hymenoptera, Rincota), mites, bacteria (outgrowths, galls, cankers), fungi (blisters, cankers).
Symptoms: affected plants stop growing, turn yellow and production or development of flowers is arrested; That is why, if a plant looks sickly, the first thing to do is to check the health of its roots, repotting the specimen if necessary.
The best treatment is to soak the plant in water at 50°C (122°F) for 20 minutes. You might also try using halohydrocarbons and organophosphorates. Remove all infected roots cutting them as close as possible to the root collar and treat the plant as if it were a cutting. Eliminate all the dead parts of the plant as well as the soil and pot; in the most serious cases you might have to destroy the entire plant due to the high transmissibility of the disease to nearby plants. Planting marigold (Tagetes sp.) where ringworms have been spotted is useful as a biological remedy to limit their spread.

SCIARA FLY. It belongs to the order Diptera characterized by having one pare of wings only, while the hind wings (halters) are sensory organs whose function is to stabilize flight. It has piercing-sucking mouth-parts. Its larvae looks like tiny transparent worms (about 6-7 mm / 0.24-028 in long) with a black head and no legs and feed on vegetal organic substances. The adult insect is a small black fly measuring 3 to 4 mm (0.12-016 in) in length; some female flies generate males only, others females only because they lack certain chromosomes. They attack roots and plant collar causing great damage especially to seedlings. They occur mostly in peaty soils; both adults and larvae can be eliminated with a suitable insecticide.

APHIDS. These insects, belonging to the order Hemiptera, are polyphagous, green or black-coloured, and very prolific; they do not often attack cacti except sometimes the flower parts. Here are some among the most common together with the species most likely to be targeted: Aphis gossypii for Cucurbits, Aphis nerii  for Asclepiadaceae and Apocinaceae, Aphis sedii for Sedum, Macrosiphum euphorbiae for Spurges and lastly Aleophagus myersii for Aloe and Haworthia. The attack results in a general weakening of affected plants as well as a possible diffusion of viral, fungal or bacterial infections. In addition, the sugary secretion (honeydew) as it dries up favours the formation of sooty mould and the species Eriosoma lanigerum may also cause malformations. They can be eliminated manually, using soapy water (50 gr. Marseille soap per litre of water), an infusion of tobacco leaves or a chemical insecticide (rotenone) which you will need to change quite often because more resistant strains of aphids are likely to develop every few generations. 

THRIPS. Belonging to the order Thysanoptera, these tiny insects that measure around 1.5 mm (0.06 in)  have a flat body, narrow wings and are characterized by exceptional motility; they show a marked preference for cacti pollen. The thrip Frankliniella occidentalis is distinctly polyphagous and attacks not only flowers but also leaves and fruits. It injects its saliva, piercing the plant epidermis to feed but also to lay eggs. By doing so it causes flowers to dry up and is responsible for malformation of petals, leaf margin necrosis, rusty discolouration, scars and can vector TSWV (tomato spotted wilt virus). Female thrips are larger than males and can oviposit without mating, usually about fifty eggs inside petal tissues. After ten days or so, depending on the temperatures, the nymphs burrow below the ground developing into pupae first and then into adult insects. Normally the reproductive cycle is repeated 5 to 7 times a year. Two other species worth mentioning are Heliothrips haemorrhoidalis and Thrips tabaci. Recommeded as biological remedies are chromotropic traps, sticky and blue-coloured, the bioinsecticide Naturalis or a mixture of Marseille soap and quassia extract; alternatively use a pesticide (dimetoate). The most important natural enemies of thrips are the mite Amblyseius cucumeris (Phytoseiidae) and the bug Orius laevigatus (Anthocoridae).

CITRUS FLATID PLANTHOPPER. It’s a small insect in the order Homoptera, family Flatidae, introduced from North America into Europe in the early 80s. Metcalfa pruinosa is completely white after emerging from the egg and as it develops it turns light green becoming whitish again later because of the waxy coating that covers its body; the exuviae (remains of exoskeleton after moult) are quite noticeable on the stem. The adults are a greyish white and become more grey as they grow to a maximum length of 7 mm (0.28 in); they are able to jump and fly and possess a strong buccal apparatus which they use to pierce the plant and suck the sap. Nymphs develop around mid-May and live on the underside of leaves of succulents while the adults appear by June and secrete a whitish waxy substance accompanied by honeydew and sooty mould. The parasites show a gregarious behaviour and usually line up along the plant stem. They disappear as the weather turns cooler after laying eggs in bark fissures. Conventional pest management entails the use of the same insecticides recommended against mealy bugs applied three times starting in early May with a one-week interval between each application. Biological control avails itself of the natural enemy, the insect Neodryinus typhlocybae. Another natural remedy to be used early in the season (May) is to direct a forceful jet of water onto affected plants, best if mixed with potassium nitrate so as to melt the waxy coating protecting the immature stages of the insect.

WHITE FLIES. The tiny white flies are native to tropical regions but their ability to cope with cooler climates has allowed them to spread over a much wider area. They stay on the underside of leaves of succulents, caudiciforms and succulent pelargoniums and fly away when the plant is shaken. They suck sap using their mouth weakening the plant, retarding flowering, causing the formation of yellow circular spots, producing honeydew and consequently sooty mould as well as vectoring viruses and bacteria. The species which is most commonly encountered in our greenhouses is Trialeurodes vaporariorum, whose females can lay some 200 eggs on the underside of a leaf. The nymphs hatch after incubation for approximately ten days and pass through four instar stages before becoming adult insects; the whole process lasts nearly a month. Placing yellow sticky chromotropic traps ensures good biological control. Aphicides remain the most successful treatment (dimetoate, malathion, acephate, imidacloprid) although the parasite protective coating might reduce the effectiveness of the products. At least three applications should be made with a one-week interval between them so as to kill also newly hatched insects. The active principle should be changed from time to time since the parasite tend to develop a resistance to the insecticide. Good results have also been achieved alternating treatments with mineral oil and with a solution of water and 0.25% of laundry detergent. Encarsia formosa, E. tricolor and Chrysoperla caenea are their natural predators.

SNAILS and SLUGS. These gastropod molluscs belonging to the family Helicidae have a well developed foot secreting a mucous substance that enables them to glide over a surface; they are easily spotted because of the silvery trail of dried mucus they leave behind. They are hermaphrodite but most of them still reproduce by mating with another individual; they have lungs and some develop a single dorsal shell. Among the numerous species we have to mention Helix pomatia, quite common throughout central Europe as well as H. adspersa and H. pisana frequently encountered in Italy. They constitute a threat to succulents, especially once they have been moved out of the greenhouse or where they are permanently grown outdoors; they usually crawl out after dark and eat fleshy leaves, cladodes, tender shoots and seedlings spoiling the appearance of plants. They can be removed manually or use shallow containers filled with beer as baits: slugs and snails are attracted to it and will fall into the liquid and drown. In case of massive infestation sprinkle some poisonous slug pellets (acetic metaldehyde).

ANTS. Belonging to the family Formicidae, ants have both sexes but females can either be fertile or sterile. Fertile females lose their wings after having performed a nuptial flight and mated while the males soon die. The egg passes first through a larval stage then becomes a chrysalis until they finally reach maturity. Many species may choose to establish their long-lived colonies in a raised bed or large pot digging their galleries in the soil and causing damage to the roots; others are just solitary predators. They collect aphid eggs so that they can rear them inside the nest because the sugary honey-like substance excreted by aphids when palpated on the abdomen by ants with their antennae is a valuable food source. Therefore ants can be held accountable for tending harmful insects, carrying away seeds from trays and indirectly damaging roots. Use an insecticide like carbaryl if necessary.

LEAF-EATING CATERPILLARS. The definition includes the larvae of Lepidoptera (also comprising butterflies) which possess chewing mouth parts. While in the pupal stage most species spin a cocoon inside which the insect undergoes the final metamorphosis. During the larval stage many of them are phytophagous and quite deleterious, especially those characterized by nocturnal activity. These voracious pests are variable in size and colour and can appear in the spring, summer or autumn, depending on the species. The preferred food of caterpillars are young plants, succulents, fleshy leaves, flowers and seedlings. Remove manually if there are still few of them otherwise you might have to apply chemical products like Imidacloprid, Diflubenzuron, Alphamethrin, Endosulfan.

COMMON ROUGH WOODLOUSE. Porcellio scaber is the scientific name for this very common insect belonging to the order Isopoda; it has a rough shell-like exoskeleton, two tail-like appendages and a pair of short antennae. They do not directly harm plants but since they usually live in colonies they might cause disturbance to seed germination and seedling growth. They can easily be got rid of by hand or lured away using a potato which is something they really crave.


MISTAKES IN CULTIVATION
Light. Insufficient light levels during the growing period cause plants to develop spindly shoots and take on a yellowish appearance due to lack of chlorophyll. This phenomenon is known as etiolation and results in general weakening and irreparable disfigurement of plant.
On the other hand, placing a plant in full sun too suddenly without allowing it to acclimatize to increasing light levels causes the epidermis to get sunburned and permanently marred. All this can be avoided by gradually exposing them to more sunlight when moving them outdoors in the warm months at first providing some shade during the midday hours. In doing this we must take account of the conditions which our plants are used to in their habitats, protecting from intense light those that live in the tree canopy or find shelter among rocks, bushes or tall grasses. Some species, if exposed to full sun during the winter months, may take on a reddish hue but this is not in the least detrimental to the plant health.
Water. If plants are watered too sparingly their tissues lose turgor and stems appear withered and start to bend; epiphytic cacti from tropical and subtropical forests are known to be more prone to this. The remedy is watering and spraying on a more regular schedule. On the other hand, waterlogging and excess moisture often cause rot; keep in mind that succulent plants are more likely to be killed by excessive water than by lack of it. Moreover, water that has a high mineral content, particularly in calcium, causes the roots to suffocate in the long run.

Humidity.
Excess air moisture, dripping water from condensation on the greenhouse covering, poor ventilation are the main causes of rotting and mould formation, especially during the winter.
Only species that are native to forest of South East Asia require higher temperatures and humidity. During the warmer months it is advisable that such plants (Hoya, Dischidia, etc.) be placed on large pot saucers filled with gravel kept constantly moist and sprayed frequently with non-calcareous water.

Temperature.
Every species, during its evolutionary course, has adapted to surviving in a different temperature range, which is more extreme in subarctic or high-mountain regions and constantly decreases as we proceed towards the Tropics, characterized by more even temperatures. If we go below the critical point the plant juices freeze and expand thus destroying the cell walls and causing plant death. In less serious cases the plant epidermis might remain permanently blemished by dark spots. The recommendation is that you never allow the temperature to go below the critical point the plant is known to withstand in its habitat and that you also reduce watering frequency in the autumn until you stop when temperatures drop.

Excessive heat can cause scorching of leaves and stems and also sunburn which can scar plants irreversibly and subsequently expose them to fungal infections. If damage is severe and the plant loses too much moisture, this may be fatal. Such injuries are suffered by cultivated plants especially in the spring when they are moved into full sun after spending the winter in most cases under low-light conditions; of course young specimens and seedlings are even more susceptible. It is also very likely to happen inside a poorly ventilated greenhouse. Finally, cacti that have been overwintered in an unnaturally warm environment such as an heated room are also at risk of being severely damaged.
Soil. Generally, the majority of succulents do not need a type of soil identical to the one found in their habitat, instead they make do with a well-drained mix with little humus and a nearly neutral pH that can provide good air circulation around the roots. Obviously there are a few exceptions: Aztekium, Geohintonia and some Thelocacti, for instance, which require chalky soil or plants living on the forest floor or on tree branches below the canopy which prefer humus-rich soil or finally some others thriving in calcareous or quartzy soil.
Unsuitable soil may cause, in the worst case, irreparable damage as it usually happens with a peaty mix which does not allow water to drain quickly thus favouring the insurgence of rot. Iron or other micronutrients deficiency, substrates that are too alkaline or too acidic, lack or excess of minerals along with other factors may hamper or slow down plant growth, hinder flowering, result in yellowing of the epidermis and lastly impede root development. The remedy is to remove the old soil and repot the plant with a suitable mix. If roots have been irreversibly damaged the only way to save the plant is rooting it as a cutting or grafting it onto a proper rootstock.

Feeding.
Succulent plants usually occur in regions with low rainfall and this means that they are not able to absorb a large amount of minerals; if fertilizer is applied to cultivated succulents in large quantities it accumulates in the soil lowering its pH and disrupting the osmotic balance. This weakens the plant, makes it more susceptible to parasite attacks and in more serious cases it might also lead to yellowing, wilting and even rotting of the specimen.
Apply fertilizer only during the growing season and not too frequently; it should contain micronutrients and be low in nitrogen with a 1:2:4 ratio (nitrogen, phosphorus, potassium respectively). The recommended concentration is 1 gr. per litre of water (1/1000). Epiphytic cacti do well with a bit more nitrogen, a lower concentration of nutrients and more frequent applications.

Pesticides.
Usually the manufacturers of these products provide instructions for the correct use and dosage on a number of plants but fail to include succulents so unless we know for sure that a certain active principle is not harmful to our plants it is always advisable to test it on a single specimen first, sprayed or applied through watering in case it is a systemic.


SEED DISINFECTION
Most often the seeds we buy have not been disinfected and this is the most likely cause of bacterial, fungal and viral infections. The simplest way to eliminate the risk is sterilization which consists in treating the seeds with one of the following powdered products: copper carbonate, mancozeb, maneb, propamocarb, metalaxil, metiram, zineb, thiram, captan etc. Sprinkle a thin layer of product over the seeds.

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1 Phytoparasitosis 22346
2 Phytosanitary products 46555
 
Home Cultivation guide The diseases

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