Potato Rhizoctonia: Methods of Control and Prevention
There are diseases of potato that cause irreparable harm to the culture and dramatically reduce its productivity. These include viral diseases, late blight, dry and wet rot. But there are diseases of potato, which are more reflected in the appearance of tubers, their marketability, than on productivity. First of all, this is a common scab and black scab (Rhizoctonia). Intensive distribution of rhizoctoniosis is promoted by snowless winters and cold springs – favorable conditions for the occurrence of putrefactive processes in the soil. Another factor provoking the disease is the planting of unheated tubers in cold soil.
The causative agent of Rhizoctonia, or black scab, is the fungus Rhizoctonia solani JG Kuhn in an imperfect mycelial stage. A characteristic feature of Rhizoctonia is black sclerotia on tubers. Often they are mistaken for pieces of adhering soil, however, unlike the earth, sclerotia are not washed off with tubers by water. If you scrape them off, you can smell a specific mushroom smell. In this form, rhizoctoniosis practically does not harm tubers (sclerotia are on the surface of the peel and do not cause necrosis or decay).
The development of the fungus begins when infected tubers are planted in damp, cool weather (soil temperature below 8 ° C). The fungus causes the main harm during the development of potato seedlings. Sclerotia located on tubers and seedlings sprout with mycelium, which penetrates the sprouts and leads to the formation of brown depressed spots on them, drying out of the tips and branching of the sprouts. Such plants can “drop out” from mass shoots.
In summer, in warm and humid weather, Rhizoctonia can occur on adult plants in the form of a “white leg”. In this case, the lower part of the stem is covered with a whitish-gray film, indicating the basidial stage of the fungus. Basidiospores can be washed away by rain from the stem into the soil and infect young tubers, forming dense black sclerotia of different sizes on them. In connection with the increase in the market for washed potatoes, the marketable appearance of tubers is becoming increasingly important.
Keep Track of the Soil
To prevent infection of the potato with rhizoctoniasis, it is necessary not only to use varieties that are relatively resistant to it but also to monitor the soil reaction (the pH should not be higher than 5.5-6.0). If the soil is acidic, it must be limy and also refuse to introduce fresh manure and straw. In order to suppress pathogenic fungi with microorganisms useful for the soil, it is recommended to use EM technology, to seed the planting tubers with protective preparations.
Please note that potato varieties resistant to rhizoctoniosis do not exist. This disease is considered non-quarantine, therefore, selection in this direction is not conducted. There are only varieties that are relatively resistant to rhizoctoniosis; the tubers of such potatoes are not covered with sclerotia so much. EM preparations designed to combat pathogenic infection in the soil, unfortunately, are also ineffective in this case.
If you want to get a crop of high-productive tubers, use the approved chemical protection means, pickling the seed material before laying it for storage and (or) before planting. Practice shows that drugs are effective against Rhizoctonia: Fitosporin-M, Fundazim, Effect” (The effect). They are used for seed dressing, both before laying for storage and before planting. However, these drugs are still not registered in Ukraine for use in vegetable gardens. They can only be used by farmers and collective farms.
The causative agent of Rhizoctonia can persist on seed tubers and in the soil. The main methods for controlling the disease are planting healthy tubers, crop rotation, optimal planting dates, early harvesting, and applying balanced doses of organic (in no case fresh manure before planting can be applied) and mineral fertilizers.
The tubers, selected for seeds, must first be washed, and noticeable lumps of sclerotia carefully, so as not to damage the peel, scraped off with a wooden sliver. If the fungus covers all the eyes, the tuber is discarded and used for other purposes. Significantly reduces the activity of the fungus in the spring gardening of planting tubers in the light before laying for storage and during germination. Partially suppresses the infection by heating the planting tubers for 20 minutes in water heated to 45 ° C.
Warm-up immediately after removing tubers from storage, provided that they have not yet sprouted. In order to disinfect and saturate tubers that have not yet sprouted with useful microelements, they are sprayed with a nutrient-disinfecting solution before and during germination. To prepare it, 15 g of boric acid, 0.2 g of copper sulfate and 0.1 g of potassium permanganate are dissolved in 1 l of water. If there are few tubers, they are soaked in nets for 20 minutes in the same solution but diluted in 5 l of water.
Apart from chemical solutions, biological products are used to spray or soak seed material, for example, Baikal EM-1, diluted in a ratio of 1: 1000. The microorganisms contained in such preparations and their metabolic products inhibit the development of the fungus. Rhizoctonia infected shoots that appeared during storage (branching, with blackened or dried tips, as well as brown pressed spots) are removed at the very beginning of germination. New shoots appearing on already warmed and disinfected tubers, as a rule, do not have signs of disease.