Diseases of onions. Fight against diseases and pests of onions
Numerous diseases and pests of onions can significantly reduce the yield of this extremely useful healing vegetable. various types of rot, insect pests, bacterial infections affect the leaves, the fruit, the roots of the plant. Often, “pinpoint” countermeasures are ineffective - a complex of measures is required, including not only the processing of plantings by special means, but also strict adherence to agricultural technologies.
Consider the main diseases of onion bulb and their treatment. Onions damage about 50 species of fungi, insects and microorganisms. Most often, the plants suffer from clay, floodplain, waterlogged soils. Saturation of soil with rotted manure and mineral fertilizers, which contain a lot of nitrogen, also leads to product losses due to the accumulation of diseases.
Disease of onion peronosporosis (it is downy mildew) has long been known. The causative agent - the fungus Peronospora destructor Casp - was first discovered and described in England in 1841.Decades later, he struck Europe and the vast British colonies: from America to Australia.
For a long time, a harmful microorganism in the east of Europe met sporadically, until in 1965-66 a “epidemic” of the disease occurred. Then, peronosporosis covered half of all plantings of bulb onion in Ukraine and Kuban. A new wave of propagation swept through the 70s. Peronospora is now celebrated annually in all growing areas, especially in the southern regions.
All types of vegetables are susceptible to this onion disease: bulb, family, batun are regularly affected. Onions-Schnitt, Altai and Peking - only in the years of epiphytoties (epidemics). Mealy dew has become not only a common, but also the most harmful disease. It leads to a decrease in the size of the affected heads and delays their maturation. Attempts to remove varieties that are not susceptible to peronosporoza were unsuccessful. Among the fairly stable experts say "Skvirsky."
The causative agent of peronosporosis is a typical obligate fungus-parasite. It leads to onion disease and a decrease in yield. Mycelium of the fungus has a wintering (resting) and summer (vegetative) form.The source of infection in the initial period of the growing season are plants grown from affected bulbs. Planting diseased specimens represents the greatest danger - after two / three weeks a conidial bloom forms on the leaves of the onion, after which the plants begin to die off.
In addition to the bulbs, fungal spores can winter in plant debris, germinating under unfavorable conditions for the fungus. Bulgarian scientists found that one patient specimen within a radius of 2 meters infects 90% of the plants, and within a radius of 6 meters - 30-35% of the plants. Phenomenally: in just four incubation periods (1 period is 11-15 days), a single focus can spread over 2 kilometers!
Perinosporosis causes losses in onion storage places. The experiments of scientists have shown that the affected bulbs are poorly stored and germinate prematurely, and the loss reaches 50-60%.
Course of the disease
The spread of the disease onions in the garden is rapidly. After planting infected bulbs, the appearance of the conidial stage (spore spread) under favorable conditions may occur after a short period of time. At a humidity of 90-95% and a temperature of 15-18 ° C, after 4-9 days, the infection reaches 40-50%, and after two weeks all the plants on the plantation are infected.This period is reduced or increased depending on temperature and humidity. The formation of conidial sporulation occurs only at low temperature and high humidity. During the growing season, the fungus can produce 4-7 generations.
The development of onion disease occurs predominantly at night and in the morning, when raindrops remain on the plants, or dew falls, and the air temperature decreases. In the field, a full cycle of infection requires two wet nights. This is facilitated by the condensation of water on the leaves, the temperature of which is lower than that of air. Defeat does not always occur, but only on critical days when moisture on the leaves persists until noon. The conditions when cool nights alternate with relatively warm days are very favorable for the epiphytotics of perinosporosis - the fungus spreads with wind and heavy rain.
Signs of Powdery Mildew
The disease is accompanied by stages, which can be distinguished by the characteristic symptoms:
- The first sign of green onion disease is the appearance of a brownish-purple velvety plaque on healthy leaves.
- Over time, the spots increase in size and become at first pale yellow, and later brown.
- Over time, the spots necrotize, which leads to the death of leaf tissue. The affected arrows have a yellowish-pale color and often, like the leaves, are colonized by other mushrooms Alternaria, Stemphilium, etc.
Fusarium onion is less common than perinospora. It manifests itself both during the growing season and in storage. Fusarium wilted leaves of onions occur during the growth period, and during the storage period - the characteristic rot of bulbs ends. The disease is often fixed at the end of the growing season. Characteristic features:
- Premature yellowing, twisting and dying of leaves, starting from the top of the head.
- Subsequently, the disease spreads to the entire leaf blade, the leaves wither and rot. Infected parts of the bulb become brown in color.
- Sometimes, due to a lateral lesion, a healthy half grows, and the infected one twists and presses - the disease gives the bulb a concave-convex shape.
Cervical or gray rot
One of the most dangerous diseases of onion during storage, and sometimes during the period of growth and harvesting.The disease begins shortly after harvesting - by softening the tissue of the flakes, usually at the neck of the bulb, especially in places of mechanical damage. The affected areas have the appearance of depressed "boiled" spots and gradually cover the surface of one or several scales. They are brown, watery, over time, such areas die off and become porous, like a sponge. A gray mass appears on the infected tissue (between the scales), and a black mass of small sclerotia can form on the outer (around the neck). The area of infected tissue increases, growing down the scales.
An onion is only affected through damaged areas almost always at the cervix, so this disease is also called cervical rot. Most often, the rot penetrates the fabric during the harvest, when the leaves are cut too low in unripe and poorly dried onions. Specimens that are infected with powdery mildew are more affected by gray rot.
Bacterial rot of onion testes
The causative agent of onion disease is most often the bacterium Erwinia carotovora Bergey, which can cause vascular and parenchymal lesions.The source of infection is contaminated soil and affected post-harvest onion residues. Bacteria spread with raindrops, irrigation water and insect pests. Sometimes other bacteria cause rot of the bulbs: Pseudomonas aeruginosa, Pseudomonas gladioli and others.
Microorganisms cause diseases of soft rot of various organs. In the affected bulbs, a large bright or slightly pink spot forms around the stem. The fabric softens and becomes covered with mucus. In such heads, the first outer layer of scales is healthy, and the next ones turn yellow-brown. With a strong lesion tissue softens, emitting an unpleasant smell.
Diseases of onion and their treatment
Unfortunately, today there are no reliable mechanisms preventing the development of epiphytotics (epidemics). Scientists are constantly searching for means of suppressing the activity of microorganisms in order to obtain full onion yields. During the growing season, fungicides are applied at the first signs of infection. The effectiveness of any pesticide used, the rate of its consumption and the multiplicity of treatments depend largely on the development of the disease. In the practice of plant protection, the most common and rational way to use pesticides is the treatment or dressing of seeds and planting material.
How to treat onions from diseases? In the 20s, Bordeaux liquid was used (it is relevant even now, especially in private farms), and then substitutes appeared - zinc-containing contact fungicides. Based on them, a system of chemical protection measures against peronospora has been developed. For the protection of onions, fungicides of contact action, effective at high consumption rates, used to be used. In the 80s they began to use systemic drugs that are highly effective at the initial stage of use.
- From powdery mildew: “Kvadris”, “Ordan”, “Alirin-B”, “Revus”, “Bravo”, “Abiga-Peak” and others (according to the instructions).
- From gray and cervical rot: "Kvadris", "Bravo", "Ridomil Gold", "Switch" and others.
However, the continuous treatment of vegetative plants with systemic fungicides can lead to the emergence of resistant strains of pathogens and reduce the effectiveness of drugs.