Symptoms and Control Methods of Bacterial Shot Hole Disease in Peach
[Distribution] It occurs throughout the country, especially in coastal areas, lakeside areas, orchards with poor drainage, and rainy years, often causing severe outbreaks. If control is not timely, it can easily cause massive leaf drop, reduce nutrient accumulation, and affect flower bud formation. It not only weakens tree vigor and reduces the yield of the current year, but also affects fruiting in the following year, resulting in poor harvests.
[Symptoms]
Water-soaked, pale yellow small spots appear on the leaves, then expand into red round spots, and subsequently turn brown, with the edge darker than the center. Around the edge there is a semi-transparent pale yellow halo. The edge is prone to forming an abscission layer, creating round shot-hole spots. When several lesions join together, the shot-hole shape becomes irregular; in severe cases, a single leaf may have dozens of lesions, and diseased leaves fall early.
After the fruit is infected, oily brown small dots appear, after which the lesions expand, the color deepens, and finally they become sunken and cracked, leading to rot. The twigs initially show water-soaked spots with a purplish-brown color, which later also become sunken and cracked. When moist, white pus appears on the lesions. When dry, cracks often develop.
After the twigs are infected, there are two different types of lesions, one called spring canker and the other summer canker. Spring canker occurs on twigs produced in the summer of the previous year (the pathogen has already invaded in the previous year). In spring, when the first batch of new leaves appears, dark brown small blisters form on the twigs, about 2 mm in diameter, later expanding to 1 to 10 cm in length, with the width mostly not exceeding half the diameter of the twig, sometimes causing twig dieback. In late spring (before and after flowering), the epidermis of the lesions ruptures, the pathogen exudes, and spread begins. Summer canker mostly occurs in late summer, on current-year young twigs, with lenticels as the center, forming water-soaked dark purple spots. The lesions later turn brown to dark brown, round or oval, slightly sunken, with water-soaked edges. Summer canker lesions do not expand easily and dry up quickly, so their role in transmission is not significant.
[Pathogen] The pathogen is a bacterium, Xanthomonas campestris pv. pruni. It belongs to the genus Xanthomonas. The bacterial cells are short rod-shaped with rounded ends, with a single polar flagellum, a capsule, and no spores. The minimum temperature for pathogen growth and development is 3°C, the optimum temperature is 24°C to 28°C, and the maximum temperature is 37°C. If exposed to sunlight for 30 to 45 minutes, it loses viability, and under dry conditions it can survive for 10 to 13 days. The pathogenic bacteria mainly overwinter in diseased tissues of twigs and are spread by wind, rain, and insects. As temperatures rise, the overwintering bacteria become active, multiply in large numbers, exude bacterial ooze, and invade through leaf stomata and twig lenticels.
[Disease Conditions]
The occurrence of this disease is related to climate, tree vigor, management, and variety.
1. Climate: Warm, frequent rain, or foggy or heavily foggy seasons favor pathogen infection and reproduction, resulting in severe disease.
2. Tree vigor: Strong tree vigor results in mild and late disease; weak tree vigor results in early and severe disease.
3. Management: Low-lying orchards, poor drainage, poor light penetration, poor ventilation, and excessive nitrogen fertilizer application result in severe disease.
4. Variety: Varieties with mild susceptibility include Lincheng peach, Okubo, Daiwa Hakuto, Zhongshan Jintao, Cangfang Zaosheng, and Canned Peach No. 2; varieties with moderate susceptibility include Mingxing, Canned Peach No. 12, Kiyomi, Nakatsu Hakuto, and Jintao; varieties with severe susceptibility include Feicheng peach, Hakuhō, Hakuto, Gaoyang Hakuto, and Nishino Hakuto. Generally, early-maturing varieties develop mild disease, while late-maturing varieties develop severe disease.
[Disease Pattern] The pathogen mainly overwinters on diseased twigs, primarily within spring canker lesions. In the following spring, as temperatures rise, bacteria latent in the tissues become active. Before and after peach flowering, the pathogen exudes from diseased tissues and is spread by wind, rain, or insects, invading through leaf stomata and twig lenticels. The incubation period is 7-14 d. Bacteria in twig canker lesions can survive for more than 1 year. Spring canker lesions are the main primary infection source of the disease. In summer, with high temperatures and low humidity, canker lesions dry easily, and the surrounding healthy tissue heals readily, so the pathogen in canker lesions dies after 10-13 d under dry conditions. Temperatures of 19-28°C and relative humidity of 70%-90% favor disease development. The bacterium prefers warm, humid environments. In Shanghai, disease begins in May, shot holes can be seen by June, disease development slows under dry, high-temperature summer conditions, and late infection occurs again during autumn rains. Its incubation period varies with temperature and tree vigor. For example, at 25°C to 26°C the incubation period is 4 to 5 days, at 20°C it is 9 days, and at 19°C it is 16 days. If tree vigor is strong, the incubation period can reach 40 days. Warm, rainy, foggy, and humid climates easily lead to severe disease; many lower sprouting twigs result in severe disease; old trees develop severe disease; poor management, abandoned peach orchards, poor ventilation and light penetration, and weakened tree vigor result in severe disease; in addition, damage by leafhoppers and aphids, as well as poor drainage at the planting site, poor ventilation, and excessive nitrogen fertilizer application, all aggravate the disease. Different varieties also differ in disease severity. It also damages plum, apricot, cherry, and other plants.
[Control Methods]
Advocate the policy of "prevention first, integrated control," and follow the principle of "treat early, treat small, treat thoroughly,"
(1) Horticultural control: Emphasis should be placed on sanitary measures. In winter, remove fallen leaves and dead twigs, prune appropriately, and promptly remove diseased branches and leaves. Bury branches and leaves deeply in a timely manner, strengthen fertilizer and water management, apply more organic fertilizer, avoid excessive nitrogen fertilizer, pay attention to drainage, ventilation, and light penetration, reduce humidity, and enhance tree vigor.
(2) Chemical control: Before peach bud break, spray 3 to 5 degrees Baumé lime sulfur mixture. Before bud break, 1:1:120 Bordeaux mixture can also be sprayed, or 77% Kocide wettable powder at 300 times dilution, or 30% Ludebao suspension concentrate at 300 times dilution. After leaf expansion, spray 50% or 80% carbendazim at 800 times dilution, 70% thiophanate-methyl or 70% mancozeb, zineb at 500 times dilution, 50% zineb at 1000 times dilution, or 50% xiaojunling at 1000 times dilution, once a month from May to August. From late May to late June, spray 65% wettable zineb at 300 to 500 times dilution once or twice. After flowering, spray 1:4:240 zinc sulfate-lime solution, or 70% thiophanate-methyl at 1500 times dilution, or other fungicides. During the disease period, timely spraying of 20% Yeqingshuang wettable powder at 600 times dilution, or 10% Yekujing wettable powder at 400 times dilution, both have good control effects. Focus control on the plum rain and typhoon seasons.
(3) Avoid mixed planting with other stone fruit trees: When establishing a peach orchard, not only should the tree species be pure and disease-resistant varieties selected, but plum, apricot, cherry, and other stone fruit trees should not be planted nearby.