Control Methods for Downy Mildew in Isatis indigotica (Dyer's Woad) Cultivation

Symptoms: On diseased leaves, yellowish-white lesions with rather indistinct edges appear on the leaf surface, gradually enlarge, and, constrained by the leaf veins, become polygonal or irregular in shape. On the corresponding underside grows a layer of grayish-white downy mildew-like substance, namely the pathogen's sporangiophores and sporangia exposed outside the host body. In the later stage, the lesions enlarge and turn brown, the leaf color turns yellow, and the leaves wither and die. When stems and flower stalks are affected, they often swell and bend into a dragon's-head shape. The stems are black with cracks, and the diseased parts also bear a grayish-white downy mildew-like substance. Severely affected plants are stunted, with pods small and curved, often splitting before ripening or failing to set seed.

Pathogen: The downy mildew pathogen of Isatis indigotica (Peronospora isatidis) is a fungus of the subdivision Mastigomycotina, class Oomycetes, order Peronosporales, family Peronosporaceae, genus Peronospora. One to several sporangiophores arise in clusters, emerging from the stomata; the main stalk (axis) is relatively stout, swollen at the base, with dichotomous branching 2 to 6 times; the terminal branches (end branches) are curved; the sporangia are ovoid or elliptical. Infection cycle: The downy mildew pathogen overwinters as mycelium in diseased host residues. In the mycelium overwintering in diseased tissues of plants growing in the field, sporangiophores and sporangia are produced from the diseased parts after the weather warms up in the following spring, and are spread mainly by airflow, causing secondary infection and expanding the damage. After plants in the field become diseased, under suitable environmental conditions (mainly temperature and humidity), sporangia are continuously produced on the diseased parts and spread by airflow, causing repeated infection. Control methods: Pay attention to drainage and ventilation and light penetration; dispose of diseased residues after harvest to reduce overwintering inoculum sources; avoid continuous cropping or rotation with cruciferous and other crops susceptible to downy mildew; enhance the plant's own disease resistance by spraying the medicinal material "Gendaling" on the leaf surface of Isatis indigotica seedlings, promoting the transport of photosynthetic products (nutrients) from the leaf surface to the root system, improving the nutrient conversion rate and soil-loosening capacity, so that the rhizome rapidly expands and the medicinal content is greatly increased. During the disease epidemic period, use 1:1:(200-300) Bordeaux mixture spray + Xin Gao Zhi Mo (New High-Fat Film) for control. 1. Field diagnosis: On diseased leaves, yellowish-white lesions with rather indistinct edges appear on the leaf surface, gradually enlarge, and, constrained by the leaf veins, become polygonal or irregular in shape. On the corresponding underside grows a layer of grayish-white downy mildew-like substance, namely the pathogen's sporangiophores and sporangia exposed outside the host body. When humidity is high, the disease develops rapidly, with downy mildew concentrated on the leaf underside, sometimes also on the leaf surface. In the later stage, the lesions enlarge and turn brown, the leaf color turns yellow, and the leaves wither and die. When stems and flower stalks are affected, they often swell and bend into a dragon's-head shape. The stems are black with cracks, and the diseased parts also bear a grayish-white downy mildew-like substance. Severely affected plants are stunted, with pods small and curved, often splitting before ripening or failing to set seed. 2. Control measures: (1) Pay attention to drainage and ventilation and light penetration; (2) dispose of diseased residues after harvest to reduce overwintering inoculum sources; (3) avoid continuous cropping or rotation with cruciferous and other crops susceptible to downy mildew; (4) during the disease epidemic period, use 1:1:200-300 Bordeaux mixture or 65% zineb 600-fold dilution spray. 3. Chemical control: In the early stage of disease onset, apply 5% chlorothalonil dust for dusting, using 1 kg per mu, or choose 25% chlorothalonil wettable powder 500-fold dilution, or 70% mancozeb wettable powder 500-fold dilution for spraying, using 50-60 liters of spray liquid per mu, spraying once every 7-10 days, for 2-3 consecutive sprays.