Potato late blight is one of those diseases that cause harm to potato growing in the Russian Federation every year, especially in regions with high rainfall and low daily average temperatures. It causes premature death of plants and, as a consequence, a shortage of the crop with its subsequent losses during storage. According to our data, the stem form of late blight is found everywhere and in most cases appears much earlier than the leaf.
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 tops 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 to some extent protect the new growth, they are also effective if the fungus has already penetrated into the tissue (protective and therapeutic effect). But it must be borne in mind that these drugs can stop the disease only within 24-48 hours after infection, even when applied at full rate. If there are visible signs of late blight, then at this stage the disease can no longer be completely stopped.
Products containing cymoxanil - Ordan and Ordan MC - have the best therapeutic effect (“stop effect”). Therefore, they are more suitable for therapeutic spraying of spore-forming spots. The effect of the treatment can be enhanced by using a tank mixture of these drugs with contact group fungicides (fluazines, mancozeb) in full consumption rate. If the weather is still favorable for the disease, spraying should be repeated after 4-6 days. A 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 pressure late blight, regardless of the stage of development of the potato, protection can be provided by cost-effective contact fungicides based on active substances (tyram, metira, mancozeb, tribasic copper sulfate, etc.). Unlike fluzinam and cyazofamide, these contact active ingredients are also highly effective against alternariosis (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 infection occurs during excavation. For infection, spores must come in direct contact with the tubers, and surface damage facilitates the penetration of the pathogen. To ensure the best protection of tubers, it is best to apply fungicide based on fluazinam or mandipropamide 21 days before digging, and desiccate with Sukhovey based on diquat about two weeks before harvesting. The rate and frequency of use of Sukhovey depends 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.