Control of downy mildew of Radix Isatidis

Symptoms: yellow-white disease spots with inconspicuous edges appeared on the leaves, gradually enlarged and limited by leaf veins. Become polygonal or irregular. There is a layer of gray downy mildew on the corresponding back, that is, the pathogen cyst peduncle and sporangium exposed outside the host. In the later stage, the disease spot expanded and turned brown, the leaf color turned yellow, and the leaf withered and died. Stems and pedicels suffer, often swollen and bent into a bibcock. There are black cracks in the stem and gray-white downy mildew in the diseased part. the seriously damaged plants are dwarfed, the pods are small and curved, and they are often split or sterile before maturity.

Pathogens: Isatidis downy mildew (Peronospora isatidis) is flagellate subphylum, oomycetes, downy mildew order, downy mildew family, downy mildew fungi. Cyst peduncle one to several, tufted, protruding from stomata, main stem (axis) stout, base dilated, forked branches 2-6 times, apical branch (terminal branch) curved, sporangium oval or oval. Infection cycle: downy mildew overwintered with mycelium in host diseased tissue. The overwintering mycelium of the diseased tissue of the plant growing in the field produced sporangial peduncles and sporangia from the diseased part after the weather turned warm in the following spring, which was mainly transmitted by airflow, causing re-infection and expanding the damage. After the disease occurred in the field, sporangia were continuously produced in the diseased part under suitable environmental conditions (mainly temperature and humidity), which were transmitted through air flow, resulting in repeated infection. Control methods: pay attention to drainage, ventilation and light transmission; deal with diseased and residual plants after harvest to reduce the source of overwintering bacteria; avoid continuous cropping or rotation with crops such as Cruciferae, which are susceptible to downy mildew. To enhance the disease resistance of the plant itself, spraying Radix Isatidis on the leaves of Radix Isatidis seedlings to promote the transport of photosynthesis products (nutrients) to the roots, improve the nutrient conversion rate and the ability of loosening soil, make the rhizome expand rapidly, and the medicinal content is greatly increased. During the epidemic period, Bordeaux liquid spray + new high lipid membrane was used to control the disease at 1:1: (200,300). 1. Field diagnosis: the diseased leaves appeared yellow-white disease spots with less obvious edge on the leaf surface, which gradually enlarged and became polygonal or irregular due to the limitation of leaf veins. There is a layer of gray downy mildew on the corresponding back, that is, the pathogen cyst peduncle and sporangium exposed outside the host. When the humidity is high, the disease develops rapidly, and downy mildew is concentrated on the back of the leaves, sometimes on the leaves. In the later stage, the disease spot expanded and turned brown, the leaf color turned yellow, and the leaf withered and died. Stems and pedicels suffer, often swollen and bent into a bibcock. There are black cracks in the stem and gray-white downy mildew in the diseased part. the seriously damaged plants are dwarfed, the pods are small and curved, and they are often split or sterile before maturity. 2. Control measures: ① should pay attention to drainage, ventilation and light transmission; ② should deal with diseased and residual plants after harvest to reduce the source of overwintering bacteria; ③ should avoid continuous cropping or rotation with crops such as Cruciferae which are easy to be infected with downy mildew; during the epidemic period of ④ disease, it was sprayed with Bordeaux solution of 200-300 Bordeaux or 65% zinc. 3. Chemical control: at the initial stage of the disease, use 5% chlorothalonil dust spray, 1 kg per mu, or choose 25% chlorothalonil wettable powder 500 times liquid, or 70% mancozeb wettable powder 500 times liquid spray, 50-60 liters per mu, spray once every 7-10 days, 2-3 times.