Potato late blight is one of those diseases that annually harm the potato growing of the Russian Federation, especially in regions with increased precipitation and low average daily temperatures. It causes premature death of plants and, as a result, a shortage of crops with subsequent losses during storage. According to our data, the stem form of late blight occurs everywhere and in most cases manifests itself much earlier than the leaf one.
The main danger of late blight is plasticity and the ability to form resting structures (oospores) that persist for a long time. In addition, the pathogen is able to continue its development even at very low temperatures (3 ° C) and humidity (60%).
Many experienced farmers know that the planning of the first use of fungicides and the selection of the drug have a decisive influence on the effectiveness of protecting potatoes from late blight. Depending on the amount of precipitation from May to mid-June, the first treatment may be required before the tops are closed or much later. Spraying with fungicides should be carried out 5-7 days before the massive development of infection. In addition, for the economic efficiency of the use of drugs, it is important that their choice is adjusted depending on the variety, weather conditions and growing technology. Decision making and field inspection are important tools for the timely application of fungicides.
The purpose of the first fungicidal treatment is to reduce the spread of infection from tubers to the potato stem. On heavy soils, preparations containing systemic active substances (propamocarb hydrochloride, metalaxyl, mefenoxam) should be used.
For example, you can apply Metaxil. If the risk of infection from seed potatoes is high (high soil moisture remains), it is necessary to use the system product for the second treatment as well. Because of the danger of formation of resistance to metalaxyl, and in general to systemic products, it is necessary to alternate drugs with different substances according to the mechanism of action. Contact preparations Talant, Idol for the first treatment can be recommended in case of dry weather conditions and pure seed material.
At the stage of active growth of foliage after treatment, the plant is protected for only 4-7 days. With a strong development of late blight, the risk of infection of leaves that are not sufficiently protected is especially high. Under these conditions, it is recommended to use systemic fungicides. They penetrate the leaves and can protect new growth to a certain extent, they are also effective if the fungus has already penetrated into the tissue (protective and healing effect). But it should be borne in mind that these drugs can stop the disease only within 24-48 hours after infection, even when applied at full consumption rate. If there are visible signs of late blight development, then at this stage the disease can no longer be completely stopped.
Products containing cymoxanil - Ordan and Ordan MC - have the best healing effect ("stop effect"). Therefore, they are more suitable for curative spraying of spore-forming spots. The effect of the treatment can be enhanced by using a tank mixture of these preparations with contact group fungicides (fluazinam, mancozeb) at the full consumption rate. If the weather conditions are still favorable for the disease, the spraying should be repeated after 4-6 days. The disadvantage of cymoxanil-containing fungicides is a shorter effective period compared to other systemic active ingredients.
As soon as the intensive growth of potato plants has stopped, starting from the flowering period, at high and medium pressure infections, good control of late blight can be achieved by introducing contact preparations based on active ingredients such as chlorothalonil, fluazines, etc., for example, Talant, into the protection system.
In the case of low late blight pressure, regardless of the stage of development of potatoes, protection can be provided by economical contact fungicides based on active substances (tyram, metiram, mancozeb, tribasic copper sulfate, etc.). In contrast to flusinam and ciazofamide, these contact active substances are also highly effective against alternaria (Alternaria solani). This is important for late ripening of tubers, during periods when high temperatures and rain often alternate.
As soon as the potatoes stop growing and the lower leaves begin to turn yellow, it is necessary to protect the tubers from late infection. If there is a visible infection with late blight before harvesting, it is recommended to use fungicides with an anti-sporulent diester (fluazines, mandipropamide).
Phytophthora produces spores as long as the green parts of the potato exist. Through wind, dew and drops, they reach the soil and there they can infect the tubers of the new crop for about three weeks. The greatest risk of tuber infestation occurs during digging. To become infected, the spores must come into direct contact with the tubers, and superficial damage facilitates the entry of the pathogen. To ensure the best protection of the tubers, it is best to apply a fungicide based on fluazinam or mandipropamide 21 days before digging, and desiccate it with a diquat-based Dry Hove about two weeks before harvest. The rate and frequency of application of Dry Hove depend on the variety and the accumulated vegetative mass, as well as weather conditions.
The quality of the processing is important. According to experiments, in order to achieve maximum fungicide effectiveness, spraying must be carried out with a working solution consumption rate of 400 l / ha. Only with a weak vegetative mass (before the closure of the tier and after the start of ripening) can 300 l / ha be used. However, with strong foliage formation (the main phase of growth or varieties with highly developed foliage), a norm of 500 l / ha is recommended.