Like all cryptogamous diseases, also commonly called fungal diseases, powdery mildew (more commonly known as mal white, fog or manna) develops above all in the presence of humid, mildly warm weather conditions and in case of poor ventilation.
It is a widespread disease in the northern areas, which is manifested by an inflorescence of the mycelium (fungus), which can be seen as a whitish dust with partial leaf bleaching. At these areas the leaf first turns yellow, then necrotizes (becomes dry); sometimes powdery mildew may occur with small circular perforations on the leaf page. This pest develops in warm and humid conditions, with temperatures above 6-8 ° C, but below 30 ° C, so in spring and autumn.
The mal white it affects many very different plants: in fact, oaks, roses, vines, apple trees, peach trees and many others are affected. The damage caused may consist of a slowdown in growth or loss of the crop (eg in the vine).
A good preventive remedy can be to place the affected plant in a well-ventilated area or to practice pruning to well illuminate and aerate every part of the foliage. If this is not possible, it is advisable to avoid watering in the evening hours during periods of average temperatures. It is then usually used for the repeated use of antioidic products such as sulfur, dinocap, benzimidazole derivatives, pyrimidine, triazole; in the case of use of fungicides it is advisable to perform 2-3 interventions a year, further interventions with the same active principle could cause phenomena of resistance to the product.
In biological agriculture, sulfur can be used as a prevention, or an antidone of powdery mildew, ampelomyces quisqualis, which seems to give excellent results.
Powdery mildew is a difficult disease to combat, given the resistance of the fungus to both chemical and biological treatments. The one we know as powdery mildew or mal white, in fact, is a disease caused by a genus of parasitic fungi that prefer almost all the various types of plants, both horticultural and ornamental. In particular conditions, the oidium can also attack plants in greenhouses and large-scale crops. In case of crop infection, the economic damages can be incalculable. No less important are the aesthetic damages, damages that mainly concern flowering and ornamental plants.
Mushroom morphology of oidium
The fungus of powdery mildew belongs to the vast group of ascomycetes, particular living beings that lead a parasitic life on other plants. The fungus responsible for the disease belongs to the genus Oidium. What affects our plants is the Erysiphe necator, belonging to the Erysiphaceae family. In reality, powdery mildew can also be caused by other fungi, all of which belong to the ascomycetes and the genus Oidium. Due to the diversification of the various responsible species, the disease of powdery mildew is not always easy to combat. In nature, in fact, there are powdery mildew fungi that reproduce in a sexual way and others that reproduce asexually. The former are also called "perfect" mushrooms, while the latter are called "imperfect". This different mode of reproduction also conditions the fungus's resistance to treatments against powdery mildew. The symptoms and the damages caused by the different strains of powdery mildew mushrooms are however always the same: white patches on the upper side of the leaves and then on the buds and flowers. These symptoms represent a real scourge for plants, also because, in the case of a serious and overt infection, all you have to do is eliminate the affected plant. Sometimes, perfect or imperfect mushrooms may prefer different plants, but the signs of the disease always appear with the same symptoms.
What makes oidium more frightening than other fungal diseases is the biological cycle of the fungus. This cycle, in fact, takes place practically all year, starting from winter. The fungus attacks plants, or rather it binds to them through microscopic filaments called "hyphae". The hyphae represent the cellular structure of the fungi, that is fungi. In winter, the hyphae remain hidden under the remains of organic matter (dried leaves, withered flowers and damaged branches). In spring, with rising temperatures, the hyphae release the spores, or the "offspring" of the fungus. The spores winter on the plants and to survive they need to feed on matter and vegetable substance. The activity of nourishment of the spores and the reproduction of the fungus on the plants causes the notorious white spots that represent the most evident sign of illness.
The plants affected by oidium are many and belong to almost all plant genera. Among the horticultural ones, the most affected by oidium are spinach, radicchio, chard, chicory, endive, barley, wheat, carrot, celery, parsley, fennel, rhubarb, wheat Saracen, the thistle, the apple tree, the hazel, the oak, the vine, the peach, the watermelon, the melon, the pumpkins, the courgettes and the cucumber. The list of ornamental plants is also well-fed in oidium. The rose, the maple, the laurel, the lilac, the periwinkle, the hawthorn, the hydrangea, the chrysanthemum, the plane tree, the begonia, the zinnia, the dahlia are, in fact, suffering from the disease. the name, the calendula and the mania. They resist oidium, pear and lettuce in a greenhouse. Pear tree resistance depends on sophisticated grafting techniques practiced on this plant. Greenhouse lettuce, on the other hand, is more resistant to powdery mildew due to the humidity and temperature conditions present inside the covered structures. However, in some greenhouses, there have also been cases of mildew on lettuces. In the covered crops, if the temperature is very high and if the environment is dry, the fungus cannot reproduce, on the contrary, instead, in a very humid greenhouse, the oidium can make its appearance even with temperatures close to forty. degrees. Many greenhouse lettuces are spared from powdery mildew simply because they are treated with antifungal chemicals.
The symptoms of powdery mildew are very evident on the plants, despite the invisibility of the responsible agent. Usually, these symptoms appear between spring and autumn. The activity of the fungus's ile manifests itself with white patches on the upper side of the leaves. At first, the latter may appear slightly yellowed. The step from yellowing to stains is however very short, as is the passage of the disease to the other parts of the plant. In a very short time the stains can be transmitted to the buds, the flowers and the stem. If not stopped, the oidium extends to the whole plant causing the necrosis of the plant tissues, the desiccation and then the death of the affected species. When the disease extends to all the other plant parts, the white patches become a complex and wide white-white patina that seems to cover the plant from head to toe. Unfortunately, the patch is not snow or flour, but the reproduction of the fungus that continues to feed on the substances contained in the infected plant. The ratio of the oidium fungus to the plant is called "obliged", because the fungus itself, to survive, must feed on plant matter. Without it it could not even reproduce or extend beyond measure giving life to the annoying white patina. Due to the characteristic of forming the white patina, powdery mildew is also called "white mal".
Powdery mildew is a difficult disease, indeed, very difficult to fight. In the event of extensive infection it may be necessary to eliminate and eradicate the entire plant, or worse, the entire crop. For this reason, everything should be done to prevent the disease from manifesting itself. Usually, preventing the appearance of powdery mildew is possible by using valid and effective preventive strategies. These can be applied to both crops and ornamental plants. In fruit trees, for example, it is useful to regularly prune to aerate the foliage and prevent accumulation of moisture. Plants that do not require pruning can instead be grown in sunny and well-ventilated areas. The distance between one plant and another is also very important in the prevention of powdery mildew. In the case of multiple crops, the plants should be placed at a certain distance between one and the other. This distance, called "implant distance", favors the passage of air and avoids the accumulation of heat and humidity, the main causes of oidium. The disease can also be avoided by grafting. Modern plant reproduction techniques, for example, allow the creation of cultivars resistant to various diseases, including oidium. Pear and vines are among the crops most affected by innovative grafting practices that tend to prevent the appearance of fungal diseases such as oidium. The local vine, in particular, is grafted using specimens of American vines that have proved to be very resistant to powdery mildew and downy mildew. A good preventive strategy against powdery mildew is represented by mulching, a technique which consists of covering the soil with plant material. In the case of powdery mildew, however, we must not use leaves, flowers and dry branches, because the latter, creating a moist environment, favor the proliferation of the fungus hyphae. A good anti-mildew mulch can be made with male fern leaves. In fact, these leaves have antifungal properties. For preventive purposes it is also possible to use copper to spray on the mulching material.
When the oidium is already known and present on the plant, it is possible to resort to chemical or biological control systems. Anti-cryptogamic drugs are not always effective in fighting the disease. Some of these also have some untested toxicity. The most common products used to combat powdery mildew are copper and sulfur. These can be liquid or powdered. The use of the formulation depends on the type of infected plant and the period of administration. However, the most effective preparations are liquid preparations enriched, however, by other substances. Simple liquid sulfur, for example, proved to be ineffective against powdery mildew. Liquid sulfur combined with copper or sulfur proteinate is more active against the disease. Even more effective is the colloidal sulfur, in which the active principle has the formulation of microparticles. This product should be administered in the summer and early in the morning, due to its high toxicity. The most effective fungicides against powdery mildew are the so-called "triazolics", compounds that inhibit the production of a hormone that nourishes the fungus: ergosterol. This hormone nourishes the ile of the fungus. In its absence, on the other hand, the ileum and the whole myoidium of the oidium dry up and die. To avoid resistance or adaptation of the disease, triazole compounds should be administered no more than four times a year. Furthermore, during pruning, the remains of the infected plant must be removed and burned to avoid contamination of nearby plants.
Oidium: Biological struggle
The massive use of fungicides and triazoles is not always effective against powdery mildew. Furthermore, chemical compounds have the disadvantage of being toxic, and not only for treated plants, but also for the environment and for humans. In this context it would be useful to use biological control systems, or systems based on the use of natural substances. In fact, powdery mildew can be fought with the use of an antagonist fungus always belonging to the genus Oidium: ampelomyces quisqualis. The latter works best when mixed with mineral oil (a petroleum derivative) and when given in the fall. On sick leaves, bicarbonate mixed with vinegar can also be applied. The remedy, however, does not always prove to be effective. The aromatic plants affected by oidium can instead be treated with products based on horsetail extracts, which appear to have antifungal properties.
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