What are the control methods for sweet potato violet root rot?
Sweet potato violet root rot, commonly known as “red vein net” and “retained skin”.
[Pathogen scientific name] Helicobasidium mompa Tanaka
[Host crops] Sweet potato, potato, soybean, peanut, cotton, mulberry, tea, fruit trees, forest trees, etc. [Disease diagnosis] It damages the tubers and underground stems. Affected plants show yellowing and wilting of stems and leaves and stunted growth. The surface of the underground tubers is wrapped with white thread-like strands, which later gradually thicken and change in color from reddish brown to purplish brown. The affected tubers develop gradually from top to bottom and from outside to inside until the tuber flesh rots, giving off an alcoholic odor; the tuber skin, because it is covered with a fungal membrane, is tough in texture. After the skin splits, the rotted tuber flesh flows out, leaving only an empty “rigid shell.” When the underground stems and fibrous roots are infected, the cortex easily falls off. The causal fungus is Helicobasidium mompa. The hyphae are initially colorless and become purplish brown when mature. The strands formed by the hyphae are long and thread-like and irregularly branched. In the later stage, the strands gather to form fruiting bodies, on which there is a well-developed hymenium, and basidia are borne in parallel on the hymenium; the basidia are cylindrical, and the basidiospores are single-celled, colorless, and long ovoid.
[Occurrence pattern] The pathogen overwinters on diseased tubers and diseased stems as strands and sclerotia, or as pseudohyphae left in the soil. The pathogen can survive in the soil for 4 years. The following year, the overwintering strands and sclerotia produce mycelia, and the hyphal bundles formed by the gathered mycelia extend through the soil and can invade and cause damage after contacting host roots. It generally infects the soft tissue of new roots first, then spreads to the main root. Diseased and healthy roots in contact, or mycelia and sclerotia that drop from diseased roots into the soil, can also be spread by soil or by rainwater and irrigation water. Diseased residues composted as manure and applied to fields without being fully decomposed can also spread the disease. In addition, grazing cattle and sheep in diseased fields, or people entering and leaving diseased fields, can also carry the pathogen to disease-free fields. The pathogen has strong adaptability in the soil. Disease is severe in low-lying, wet, waterlogged land, sandy soil, and shallow soil layers. Plants lacking fertilizer and growing poorly have severe disease. Mixed planting or intercropping with mulberry or tea makes disease likely, and continuous cropping of sweet potato aggravates the disease. Acidic soil favors the disease.
[Control measures] (1) Strictly select seed tubers, remove diseased tubers, and raise seedlings in disease-free soil. (2) Sweet potato should not be planted in mulberry orchards, orchards, or plots where soybean or potato have been grown and violet root rot has occurred. It is best to rotate sweet potato with grain crops. Heavily diseased land should be rotated with gramineous crops for more than 4 years. (3) Apply sufficient fully decomposed manure and alkaline fertilizers, such as plant ash. (4) When diseased plants are found in the field, dig them out early, and do not wait until sclerotia have formed. The infected soil should also be dug out completely, and then disease-free soil should be filled in. Drench the soil around diseased plants with 20% lime water for disinfection. (5) In the early stage of disease, open trenches around diseased plants as a barrier to prevent mycelia, strands, and sclerotia from spreading with soil or flowing water. (6) In the early stage of disease, roots can be drenched with 36% thiophanate-methyl suspension concentrate at 500 times dilution, or 50% benomyl wettable powder at 1500 times dilution, or 40% wenkuli wettable powder at 1000 times dilution, or 60% fangmeibao water-soluble powder at 800 times dilution.