Detailed Guide to Controlling the Three Major Diseases of Onions and Garlic

I. Purple blotch. It mainly affects chives, Welsh onions, garlic, onions, and Chinese chives, and is a major disease of Welsh onions and garlic, with leaves and flower stalks being the main affected parts. The lesions are initially water-soaked, grayish white, slightly sunken, and later expand into oval or spindle shapes, becoming purplish brown. When humidity is high, the diseased area produces black mold layers arranged in concentric rings. In severe cases, it causes leaves and stalks to die or break over. The disease occurs severely under warm, humid conditions or when there is a lack of fertilizer and drought. Fields with sandy soil, dry land, and severe thrips damage have severe disease incidence.

(1) Agricultural control: Apply sufficient base fertilizer, increase phosphorus and potassium fertilizer, strengthen field management, and practice crop rotation with non-onion crops. (2) Seed treatment: Before sowing, soak seeds in 300 times diluted 40% formalin solution for 3 hours, wash with water, dry, and then sow. Bulbs can be soaked in warm water at 40-45°C for 1.5 hours. (3) Chemical control: At the early stage of disease, spray 600 times diluted 70% mancozeb wettable powder, or 1000-1500 times diluted 75% chlorothalonil + 70% thiophanate-methyl wettable powder (1:1), or 500 times diluted 64% oxadixyl-mancozeb, or 1200 times diluted 50% prochloraz emulsifiable concentrate, once every 7-10 days, for a total of 2-3 sprays. Note to alternate and mix them.

II. Viral disease. Viral diseases of onions and garlic are more common in garlic and rakkyo. After garlic and rakkyo are infected, long yellow-green striped spots appear, the diseased plants are stunted, leaves are curled and distorted, bulbs become smaller and hard, tillering decreases, affecting yield and quality. Viral diseases are mainly transmitted by aphids. Generally, high temperature and drought, extensive management, large aphid populations, or excessive nitrogen fertilizer application, lack of fertilizer, and poor plant growth all lead to severe disease.

(1) Agricultural control: Clean the field to reduce sources of the virus. Sow at the right time, strengthen fertilizer and water management, apply more organic fertilizer, use more compound fertilizer or phosphorus and potassium fertilizer, and avoid excessive nitrogen fertilizer. Promptly remove and burn diseased plants. (2) Physical control: Use silver-gray plastic film to repel aphids; hang a 3-6 cm wide silver-gray plastic film every 60-100 cm above the seedbed, which has the effect of repelling aphids and preventing the virus. Set up yellow boards to trap aphids. Insert yellow boards coated with machine oil in the field (about 60 cm high) to trap and kill winged aphids. (3) Chemical control: At the early stage of disease, spray 1000 times diluted 1.5% Zhihouling emulsifiable concentrate plus 500 times diluted 20% Virus A wettable powder, or 1000 times diluted 1.5% Zhibingling emulsifiable concentrate, or 500 times diluted 20% Virus A wettable powder, or 100 times diluted 83 resistance inducer, or 300 times diluted antiviral agent No. 1, once every 7-10 days. Spray 2-3 times in total.

III. Bacterial soft rot. Bacterial soft rot of onions and garlic is more common in garlic, Welsh onions, and onions. After garlic is infected, it generally starts from the leaf margin or midrib of the basal leaves, forming yellow-white striped spots that can run through the entire leaf. When humidity is high, the diseased area becomes yellow-brown and soft-rotted, then gradually spreads to the upper leaves, causing the whole plant to wither and yellow.

(1) Agricultural control: Reasonable crop rotation. Strengthen fertilizer and water management, apply sufficient base fertilizer, and implement balanced fertilization; low-lying areas should be planted on high ridges, and standing water should be drained promptly; pay attention to field sanitation. (2) Chemical control: Spray promptly at the early stage of disease, combining spraying and drenching. You can use 250-300 times diluted 23% copper-ammonia complex aqueous solution to drench the roots; or spray 4000 times diluted 72% agricultural streptomycin, or 4000 times diluted new streptomycin, or 400-500 times diluted 77% copper hydroxide (Kocide wettable powder), alternating them, for 3-4 consecutive sprays.