Downy mildew, or onion downy mildew, is a widespread fungal disease that affects onions at all stages of development - both sets and turnips. The disease significantly reduces the yield, worsens the ripening of the bulbs and reduces their keeping quality.
The mushroom hibernates on post-harvest residues or in bulbs, without causing them to rot.
The disease is especially harmful in wet years. The weather with morning dew will be especially suitable for its development. A temperature of about + 15 ° C and a humidity of about 100% can be considered favorable. The incubation period after infection lasts from 5 to 15 days. During the season, it develops up to 5-6 generations of the fungus.
In dry weather, plaque may be absent. conidia die in the sun. A diseased plant is a source of infection. The spread of the disease occurs with the help of spores, which are carried by wind and raindrops over a long distance. The infection develops rapidly in rainy, cool weather, with strong shading, in beds where there is no access to fresh air.
In the spring, diseased plants develop normally at first and are no different from healthy ones. Then, after about three weeks, they take on a depressed look, and the severely affected ones seem to wither. The development of the disease begins at the tips of the leaves, then it spreads to all parts of the plant.
The feathers of such a bow develop poorly, at first they become pale green, then yellowish and curved. In dry weather, pale green oval spots appear on the leaves, and in wet weather, the leaves are completely covered with a gray-purple bloom (fungal spores).
Such diseased plants are more common in the first month of onion development. In these plants, the affected leaves turn yellow and dry out prematurely, infecting the leaves of healthy plants.
Downy mildew also affects various types of perennial onions. In this case, the causative agent of the disease can overwinter in their bulbs. The disease is not dangerous for onions with flat leaves - slime, sweet onions.
Control measures with downy mildew
- Compliance with the garden crop rotation with the return of the onion to its original place only after 3-4 years. Otherwise, the infection accumulates in the soil and the likelihood of infection of plants by the disease increases significantly.
- Planting onions should be located in sunny, open, ventilated areas with light sandy loam and loamy, fertile, non-clogged soils. The area should be well-ventilated and well-drained. All measures should contribute to the rapid drying of plants: avoid watering at night, destroy weeds.
- Good predecessors are pumpkin crops, cucumbers and cabbage, under which large amounts of organic and mineral fertilizers are applied.
- The use of healthy planting material and the spatial isolation of perennial onion plantings (batun, shallots, etc.) from onion fields reduce the intensity of infection.
- Cultivation of varieties and hybrids that are relatively resistant to the disease - Antey, Kasatik, Kachinsky, Odintsovets, Stimul, Stuttgarten Riesen, Ellan, etc.
- Disinfection of seeds, cultivation of resistant varieties.
- Planting material (sevok, turnip, samples) obtained from crops affected by peronosporosis must be warmed up. Do this in the fall before the end of drying. The mycelium of the pathogen, which is inside the bulbs, dies during heat treatment for 8 hours at a temperature of + 40 ° C.
- Warming up the onion sets before planting at a temperature of + 40 + 42 ° С for 8-10 hours.
- Avoid thickening of onion plantings and clogging of beds with weeds.
- When a disease appears, exclude fertilizing plants with nitrogen fertilizers, mullein and watering. Do not forget about potassium and phosphorus, they increase the onion's resistance to downy mildew disease.
- For the prevention of the disease with a leaf height of 10-12 cm or when the first signs of the disease appear - spraying the plantings with a 1% solution of Bordeaux liquid. You can use a suspension of chloroosine copper or "Polycarbacin" (40 g of the drug per 10 liters of water), "Arcerida" (30 g of the drug per 10 liters of water). So that the solutions used are better retained on the plants, it is advisable to add 1% skimmed milk or soap to them. The consumption rate of the working solution is 1 liter per 10 sq. M. The treatment can be repeated after one and a half to two weeks.
Attention! Bordeaux liquid can be applied no later than 2 weeks before harvesting, the rest - 20 days. Onions sprayed with Bordeaux liquid or "Polycarbacin" should not be used on a green feather.
- Dusting onion plantings with sifted wood ash (50 g per 1 sq. M). After 5-7 days, this pollination must be repeated.
To combat powdery mildew, you can use any garden weeds from which fermented grass is prepared. To do this, 1/2 bucket of finely chopped weeds must be poured to the top with hot water, mixed and infused for several days, then filtered through cheesecloth and used for spraying.
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A very good recipe is prepared on the basis of spoiled fermented milk products (sour milk, kefir or yogurt). The lactic acid bacteria that develop in these products act on the pathogen of this disease and at the same time do not harm the plants.
The preparation for spraying is prepared from the separated fermented milk whey, it is diluted in a ratio of 1:10 with cold water and stirred until a homogeneous solution is obtained. The prepared solution is poured into a sprayer and the plants are treated.
- Harvest the bulbs in dry weather at the very beginning of the lodging of the leaves, when they are still green. Cut and burn the leaves immediately.
- Drying the bulbs until completely dry and the formation of dry covering scales.
Since onion peronosporosis is a disease that has an explosive nature of distribution, the most effective tactic of protective measures is to carry out preventive (before the appearance of visual symptoms) fungicide treatments.
Read also the article Neck rot of onions and garlic.