Cucumber downy mildew people commonly known as "running horse" and "black hair", almost can make a cucumber in one week or two leaves dead, seriously affecting the yield, causing serious economic losses to vegetable farmers, must be given great attention.
Symptoms
First, from the lower leaves of the disease, there were many water-stained pale green spots along the edge of the leaves, and quickly developed into yellow-green to yellow lesions, limited by the veins, the lesions were polygonal. When the water film was formed in the morning or on the leaf surface, the outer tissues of the lesion showed dark green water stains. In humid conditions, the diseased leaves develop a purple-brown or dark brown irregular spot on the back, and the surface is covered with downy mildew. In severe cases, the lesions fuse with each other, the leaves turn dark brown, the edges curl up, and the leaves die from the bottom up.
The law of incidence
The disease is a fungal disease caused by the infestation of Cuba pseudomonas. As long as the temperature is between 16 and 20°C and the relative humidity is above 85%, the disease will develop. The optimum temperature is 20 to 26°C. The dehydration of the cucumber leaves in autumn is a necessary condition for downy mildew infection. During the infection, spores produced by sporangia, germinated tube, or sporangia directly sprouted into the tube, and were directly invaded by the leaf epidermis, causing disease epidemics. The autumn climate is cold or hot, the air is humid, and the temperature difference between day and night is large, which can cause the occurrence and prevalence of downy mildew.
Control methods
1. Agricultural control. Selection of disease-resistant varieties, such as Jin He No. 1, No. 2, No. 3, No. 4, Xia Feng No. 1, No. 2 Amaranth, Zhongnong 1101, etc. Strengthen the cultivation and management centered on water control, pouring ground water, and paying attention to ventilation and humidity. Applying basic fertilizer and adding phosphorus and potassium fertilizers will help enhance the disease resistance of plants. Reasonably arrange the planting density, remove the old leaves and diseased leaves in time, make the fields airy and transparent, drain water in time after rain, reduce the moisture in the field, and reduce the occurrence and spread of diseases. The cucumbers in the greenhouses can be kept under high temperature, and the greenhouses can be closed for 2 hours at noon so that the upper temperature of the plants can reach 44-46°C. This can kill the fungus in the shed, once every 7 days.
2. Chemical control. To be preventive, spray prevention should begin when downy mildew does not occur. When the disease occurs, it is necessary to spray the drug immediately and prevent delay. The commonly used drugs are: 58% metalaxyl MnZn WP 500 times, 64% antivirus WP 500 times, and Propex 800 times. Liquid, 80% triethylaluminum phosphate (ethylaluminophosphate, phytosanitary) 600 times, 70% diethylalumino-manganese zinc wettable powder 500 times, 72% cream urea manganese zinc wettable powder 700 times, Dupont 800 times exposed fluid, sprayed every 7 to 8 days, depending on the condition to determine the number of spraying. In order to avoid drug resistance, the above pesticides should be used alternately in order to improve the control effect.
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