Potato Late Blight Prevention and Control Technology

Potato late blight is a highly destructive fungal disease that has long plagued potato cultivation worldwide. In the 1950s, it caused severe outbreaks in certain regions of China, leading to significant crop losses. By the 1960s, the introduction of resistant potato varieties helped reduce the impact of the disease. However, in humid and foggy areas, late blight remains a serious threat to potato production. Since the 1980s, the gradual loss of resistance in these varieties, combined with changing climatic conditions, has led to an increase in the frequency and severity of late blight outbreaks. Symptoms of late blight typically appear on leaves, petioles, stems, and tubers. On the leaves, the disease begins as water-soaked lesions that quickly spread under wet conditions. These lesions have no clear boundary with healthy tissue and may develop a faint white mold around the edges, especially on the underside of the leaf. In severe cases, the infection spreads to the main veins or petioles, causing the leaves to wilt and droop, eventually leading to a "wet rot" appearance. Under dry conditions, the lesions turn brown and do not produce mold. Infected tubers show irregular light brown or gray-purple spots, which may be slightly sunken. Beneath the surface, there are brown necrotic areas of varying depth. These tubers are often prone to secondary bacterial infections, such as soft rot, which can lead to decay in the field or during storage. The stems are rarely directly infected, but lesions can spread from the petioles to the stem, forming brown streaks in the cortex. When humidity is high, white molds may appear on the lesions. The pathogen responsible for late blight is *Phytophthora infestans*, a highly specialized fungus that primarily infects potatoes and tomatoes. While it can only grow on specific hosts, it has a narrow host range, mainly affecting cultivated potatoes and tomatoes. Some strains are more virulent towards tomatoes, while others can infect various potato species, except those with high resistance. The disease cycle begins with infected seed potatoes. When planted, many diseased tubers either fail to germinate or rot before harvest. Some may produce healthy seedlings, but only a few susceptible ones develop small lesions at the base of the stem. In moist conditions, these lesions can produce sporangia, initiating the disease. Wind and rain can carry these spores to nearby plants, creating new infection sites. Overwintering spores, infected plant debris, and contaminated seed tubers can serve as initial sources of infection in the next growing season. Late blight thrives in cool, moist environments. Relative humidity must be above 85% for spore formation, and 90% or higher for optimal development. Sporangia tend to form in the evening and release when humidity fluctuates, typically between 8:00 AM and 10:00 AM on sunny days. Water films on leaves are essential for spore germination and infection. At temperatures below 13°C, spores germinate into motile zoospores, which can invade within 3–5 hours. Under favorable conditions, an infection site can expand rapidly, forming large lesions in just a few days. Control strategies include using resistant varieties, establishing disease-free seed plots, and applying chemical treatments. Breeding and promoting resistant cultivars remain key preventive measures. Creating isolation zones for seed production helps reduce initial infection sources, with recommended distances of over five miles from potato fields. In double-crop regions, autumn-planted potatoes are preferred for seed use. Chemical control involves timely application of fungicides once the first infected plants are detected. Monitoring systems are crucial for early warning. Once symptoms are observed, central infected plants should be removed and buried, or treated with copper sulfate or metsulfuron. Field-wide spraying is then conducted. Common fungicides include 25% propamocarb wettable powder (500 times), 58% mancozeb, 40% aluminum phosphite, and Bordeaux mixture (1:1200). Rotating fungicides helps prevent resistance. In areas where resistance to tomatine has developed, products like Bayer’s Silverfal offer effective control. Applications are usually spaced 7–10 days apart, with 2–3 sprays per season.

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