Lettuce virus disease
Lettuce virus disease can affect plants throughout their reproductive phase, but the impact during the early stages has a more significant effect on yield. The symptoms typically start with the entire plant being affected, and the seedling stage is most commonly impacted at the four-leaf stage. Infected leaves show mottled patterns, wrinkled or twisted surfaces, and sometimes visible veins. In severe cases, brown necrotic spots or lesions may appear. As the disease progresses, leaves shrink, become uneven in size, and growth becomes stunted, resulting in a dwarfed appearance. Brown veins or necrotic spots are also common.
The disease is caused by various viruses, with up to ten different pathogenic viruses involved. These can infect the plant singly or in combination, including Lettuce Mosaic Virus (LMV), Dandelion Yellow Mosaic Virus (DYMV), and Cucumber Mosaic Virus (CMV). Among these, LMV and DYMV can be transmitted through seeds, while DYMV has a lower transmission rate via sap. The primary source of infection comes from infected lettuce plants and other host species in the field. Aphids are the main vectors, with peach aphids having the highest transmission rate. Carrot aphids, melon aphids, and other pests can also spread the virus.
Environmental conditions play a crucial role in the development of the disease. High temperatures above 18°C, especially during dry and hot periods, promote aphid population growth and activity, leading to rapid disease spread. Some lettuce varieties show better resistance than others. Generally, leafy types are more resistant than head-forming varieties. Heat-tolerant varieties tend to be more resilient compared to non-heat-tolerant ones. Varieties such as Emperor Bo (US), Caesar (Japan), Avant-garde 75 (China), Green Wave (Japan), Malak (imported, early maturing ball type), Taihu Lake 366, red lettuce, sweet vegetables, chicken cole lettuce, and Cantonese lettuce (also known as loose-leaf lettuce) demonstrate stronger resistance.
Control measures include selecting disease-resistant varieties suited to local conditions. During summer and autumn planting, shade nets should be used, and cooling measures implemented. During the seedling stage, proper watering is essential to prevent overgrowth. Applying foliar nutrients at the right time can help promote healthy growth. Field weeds should be removed regularly.
Aphid control is critical. When aphids are detected, they should be promptly treated using insecticides such as 50% imidacloprid diluted 2000–3000 times or 40% thiamethoxam diluted 800–1000 times. Reflective silver films or yellow sticky traps can also be used to repel aphids. For viral control, spraying with 20% Virazole wettable powder diluted 500 times, or 300 times dilution of Antiviral Agent No. 1, or 83 Agent (10% mixed fatty acid aqueous emulsion) at 100 times dilution, should begin at the onset of symptoms. Applications should be repeated 3–4 times every 10 days for effective management.
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