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It is a fungus of the Peronosporaceae family: the Plasmopara viticola; it mainly affects the vine and some horticultural plants, it can also be found on roses and on many ornamental plants.
It strikes the leaves and manifests itself with translucent spots, which look like oil, on the upper page, which are often followed, in correspondence with the underside, by patches of yellowish mold, especially if the humidity is high.
As time goes by, the disease spreads to flower buds and buds, which are covered with white-yellowish mold. The tissues affected by downy mildew they dry up and fall.
Before the fall on the leaves the fungus releases oospores, which remain on fallen leaves, where they spend the winter, and infect the plants the following year.
It is difficult to arrive at the death of the whole plant, but obviously there is a strong deterioration of the vegetation, accompanied by poor production of flowers and fruits.
This disease is favored by high humidity and spring temperatures; the greatest infections occur in periods that present the so-called three 10 rule: 10 cm shoots, 10 mm rain in 24-48 hours, minimum 10 ° C temperature; in the presence of the three elements listed above, preventive treatments are carried out, spraying the crops with specific products against the downy mildew like the Bordeaux mixture, or copper-based products. These products are also useful to contain the infection already in place.
Since downy mildew is a very harmful disease for crops, especially for vines, in many regions the relevant bodies monitor disease development.
The term peronospora in agriculture is used referring to a disease caused by various pathogens and affecting a large number of plants. It is caused by protists belonging to the genus Peronospora, but also to some belonging to the Piziaceae family.
Generally speaking, we can say that different cultures are affected, sometimes by the same pathogen, others by different pathogens. However, the effects are very similar (depigmentation and necrosis on the roots and stems, rots and missed fruit setting on the fruits) and also the methods of prevention and care.
The crops affected are:
- Lettuce, chicory and artichoke, chard
- All crucifers
- Melons, cucumbers, watermelon, pumpkins and courgettes
- Aubergine tomato potato
- Roses, ornamental plants, strawberries
- Onion, garlic, leek, shallot
Downy mildew symptoms on different crops
Lettuce, cabbage, cauliflower, chard: round leaves appear before the leaves are green and almost transparent. Over time the margin becomes irregular and the spots reach a diameter of about 2 centimeters, taking first a yellowish, then brown color. If you look at the lower page you notice a gray mold. Finally the leaves crumple and fall.
Vine: round spots appear on the upper side, usually with a chlorotic appearance. Over time they become slightly transparent and shiny, "like wildfire": it is the moment in which the pathogen has succeeded in penetrating the most internal organs. Finally, there is the appearance of whitish mold on the underside and the necrotization of the previously affected areas. There is serious damage to the grapes. The little ones necrotize, those already big are moldy.
Cucurbitaceae: The first symptoms are the appearance of bleached areas with irregular margins on the upper page and at the same time, on the lower one the typical gray-purple mold. The most immediate consequence is the desiccation of leaves and stems, even very fast. Very common in greenhouses.
Potatoes, tomato, aubergine: in the first instance you can see discolored and then brown patches with an irregular outline. On the lower page appears the white mold. It can also affect the hypogean side causing rotting.
Agliaceae: it appears with the appearance of light-yellowish-green spots on the leaves, slightly depressed. Later, purple-colored mold develops. There is leaf desiccation and loss of productivity.
Roses: on the upper page we note the presence of roundish areas with irregular, chlorotic margins. They then turn to reddish brown, widening more and more. On the underside, white-greyish molds appear. The most serious consequences are extensive defoliation. The fungus causes the appearance, on the upper side of the leaves, of chlorotic and irregular spots, which over time turn to reddish to necrotize, sometimes involving large leaf areas. On the lower page appears a slight gray-white mold ...
The cycle of disease
In this section we will describe in detail the cycle of vine blight. This is much studied because since the mid-nineteenth century the disease seriously afflicted vineyards throughout Europe. In fact previously this disease was not known, but arrived on our continent with American vines (which were pruners, but immune since they had always lived with this pathogen).
Its cycle is quite illustrative of all kinds of downy mildew affecting crops.
1) maturation of the oospores: these are diploid eggs born in autumn in the affected tissues. Essentially they remain inside dead leaves or on the ground throughout the winter. These, since they have a double external "shell", are the only ones able to survive the winter conditions (even extreme: they can even withstand temperatures of -20 ° C). They ripen in November and the cold will only preserve their ability to germinate.
2) when spring arrives and therefore with the presence of a lot of humidity and temperatures that reach 12-13 ° C, a part of the oospores begins to produce a germinative tube inside which the central nucleus of the spore passes. Germination usually starts in March and is favored in a decisive way by rain. However the oospores can be kept for several years waiting for the right conditions to appear.
3) the nuclei multiply by mitotic division: each sporocyst can release 60 to 200 mononuclear spores called zoospores. Splashes of rain and earth caused by heavy rainfall disperse sporocysts and zoospores on all plant organs. The zoospores are endowed with two flagella and are able to move inside the thinner veil of water until they find a stoma in which to penetrate.
4) when four or five zoospores reach the same stoma they lose their flagella and emit a generative infesting siphon that penetrates the leaf wall.
5) primary infection The cytoplasm of these zoospores is transmitted to a vesicle that will rapidly emit a sucking organ towards the host cell. This primary inoculation marks the beginning of the agamic cycle. The first symptoms appear ten days later (incubation period). A mycelium develops in the intercellular spaces and is fed through the sucking organ. The surrounding tissues are rapidly parasitized: this is what causes the "wildfire" effect that appears about 4-5 days after the start of the infestation. The leaf then turns yellow due to the loss of chlorophyll: usually this symptom appears from the eighth day.
At this point there are two possibilities for reproduction
a) an asexual reproduction that contributes to the propagation of the infestation: they are released outside through the stomomata of the zoospores that will be at the origin of secondary infections.
b) in autumn, at the time of the fall of the leaves, there is a sexual reproduction to ensure the perpetuation of the species. Even if the temperatures and humidity are optimal, the oospores will not germinate before the months of January-February.
The sexual reproduction cycle lasts in practice for one year while the asexual one is much shorter and can repeat itself several times, always during the vegetative season.
Downy mildew: Prevention and cure from downy mildew
The fight against downy mildew, for every type of cultivation, is mainly based on its prevention. In fact it is possible to stop its spread, but the damage caused to the leaves becomes irremediable.
There are important precautions to be taken to prevent the creation of primary hearths deriving from the wintering oospores which, after germination, may become contaminating zoospores.
- It is certainly necessary to avoid the accumulation of water at the base of the plants and on the surrounding land. This goal can be achieved by preparing an excellent drainage at the time of planting the plants. This will facilitate the drainage of water in the most rainy periods.
- Eliminate the leaves and pruning waste at the base of the trunk and nearby.
- Always carry out patrols in order to determine the appearance of symptoms as quickly as possible.
Clearly one can also intervene with covering products (copper-based, dithiocarbamates) on healthy organs to avoid the advent of infection. To decide which is the best time to intervene and avoid unnecessary treatments (remember that copper is still harmful to the soil) you can refer to these parameters:
- The risk of germination is in the presence of a mild, rainy autumn, winter or spring.
- The 3-10 rule:
o The temperature is above 10 ° C
o The young shoots have exceeded 10 cm in length
o More than 10 mm of rain has fallen in 24-48 hours.
During the period of plant growth, the products must be sprinkled according to the growth of vegetation, the frequency of rainfall and the temperature.
The traditional fungicide treatment developed at the end of the nineteenth century was the Bordeaux mixture. It was the only product used widely and successfully until the end of the Second World War, but was quickly replaced by synthetic active ingredients. These can be mainly classified into contact products, penetrating or cytotropic and systemic products. The last two have the important advantage of being washed away to a lesser extent from the rains and are therefore able to be more persistent precisely at the moment of maximum need.
They also manage to effectively eradicate and kill spores even when they have penetrated deep into the tissues.
Some very popular endotherapics are: Dithiocarbamates, Thioftalimides, Phenylamides, Carboxylic Acid Amides, Cimoxanil.
It must however be remembered that it is always better to vary the active principles often, in particular not to use the same product more than three times during a vegetative period. In this way, in fact, resistance could be established that would make the fight against this pathogen ever more difficult.
Recently, researchers have been able to introduce the resistance gene (deriving from Amercan vines) to this disease in some vines. It could be a way to avoid the intensive use of pesticides.
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