Lettuce virus disease
Lettuce virus disease can affect plants throughout their reproductive stage, but the impact during the early growth period is more severe and has a greater effect on yield. The symptoms typically start from the seedling stage, with the most common occurrence at the four-leaf stage. Infected leaves show mosaic patterns, wrinkled or twisted appearance, and in severe cases, brown necrotic spots may develop. As the disease progresses, leaves shrink, become uneven in size, and plant growth becomes stunted. Brown veins or necrotic lesions are also commonly observed.
The causative agent is a virus, with up to 10 different types capable of infecting lettuce. These include Lettuce Mosaic Virus (LMV), Dandelion Yellow Mosaic Virus (DYMV), and Cucumber Mosaic Virus (CMV). Infections can occur singly or in combination. Among these, LMV and DYMV can be transmitted through seeds, while DYMV has a lower transmission rate via sap. The main sources of infection come from infected lettuce fields, as well as other diseased plants. Aphids are the primary vectors, and they spread the virus efficiently. Other insects like peach aphids have the highest transmission rates, while carrot flies, melon flies, and big larvae can also act as carriers.
Environmental conditions play a significant role in the development of the disease. High temperatures above 18°C, especially when combined with drought and water shortages, promote aphid population growth and activity, leading to faster disease spread. Some lettuce varieties show better resistance than others. Generally, leafy types are more resistant compared to head-forming varieties. Heat-tolerant varieties tend to be more resistant as well. Varieties such as Emperor Bo (US), Caesar (Japanese), Avant-garde 75 (beautiful), Green Wave (Japanese), Malak (imported, early maturing ball), Taihu Lake 366, red lettuce, sweet vegetables, chicken cole lettuce, and Cantonese lettuce (also known as loose-leaf lettuce) demonstrate better resistance compared to others.
Control measures include selecting disease-resistant varieties suited to local conditions. During summer and autumn planting, shade nets should be used to provide cooling, and irrigation should be carefully managed during the seedling stage to avoid overgrowth. Applying foliar nutrients at the right time helps promote healthy growth. Weeding should be done regularly to reduce potential hosts for the virus.
Aphid control is crucial. When aphid populations appear, timely insecticide application is necessary—such as 50% imidacloprid diluted at 2000–3000 times, or 40% chlorpyrifos at 800–1000 times. Reflective silver mulch or yellow sticky traps can also help deter aphids. For viral control, early spraying with 20% virus A wettable powder diluted at 500 times, or 300 times the antiviral agent No. 1, or 83 agent (10% mixed fatty acid aqueous emulsion) diluted at 100 times, repeated 3–4 times every 10 days, is recommended.
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