Symptom Identification and Control Methods of Peach Shot Hole Disease
Peach shot hole disease is the most common leaf disease on peach trees. It occurs in peach-producing regions worldwide. It includes three types: bacterial shot hole, mildew spot shot hole, and brown spot shot hole. Among them, bacterial shot hole is the most common and is widely distributed in peach-producing regions across the country. Infection of peach trees by this disease can cause a large number of leaves to develop holes and drop off, branches and shoot tips to die, severely weakening tree vigor, affecting flower bud differentiation, and causing huge losses. In addition to harming peach trees, the three types of shot hole disease also harm stone fruit trees such as plum, apricot, and cherry.
[Symptoms]
Bacterial shot hole: harms leaves, branches, and fruit. When leaves are affected, small water-soaked spots initially form mostly on both sides of the leaf veins, gradually expanding into brown or purplish-brown nearly circular lesions, surrounded by a yellow-green halo; when humid, yellow-white gelatinous mucus oozes from the underside; in the later stage the lesions dry up and fall off, forming holes with broken, irregular edges. After the lesions fall off, holes form, or part remains connected to the leaf; the lesions are about 2 mm. When fruit is affected, the lesions are dark brown, slightly sunken, with water-soaked edges; when humid, bacterial pus appears on the lesions, and when dry, cracks develop. When shoot tips are affected, two different types of lesions form: spring cankers and summer cankers. Spring cankers occur on shoots produced in the previous summer; initially water-soaked brown blisters form, later the lesions expand, and in late spring the epidermis of the diseased part ruptures, exuding yellow mucus, which is the primary infection source for that year; summer cankers occur on current-year young shoots, centered on lenticels, forming water-soaked circular or oval dark purple lesions, later turning brown, slightly sunken, and in the later stage drying and cracking, exuding yellow-white mucus when humid.
Mildew spot shot hole: harms leaves, shoot tips, flower buds, and fruit. On leaves, the lesions are initially pale yellow-green, later becoming reddish-brown or brown spots, circular or irregular, 2-6 mm in diameter, with a red halo, and finally the necrotic central part falls off to form a hole, after which the diseased leaf drops off. The hole edges are neat, with no residual necrotic tissue. When young leaves are affected, most wither, without forming holes. When humidity is high, gray mold-like matter grows on the underside of the lesions, namely the pathogen's conidiophores and conidia, sometimes appearing only after leaf drop. When shoot tips are affected, long oval lesions form centered on the buds, with purplish-brown edges, and cracking and gumming occur. When fruit is affected, lesions form that are initially purple, gradually turning brown, with red edges and a sunken center.
Brown spot shot hole: harms leaves, new shoots, and fruit. Circular or nearly circular lesions occur on both sides of the leaves, with edges purple or reddish-brown with faint concentric rings, 1-4 mm in size; in the later stage gray-brown mold-like matter grows on the lesions, the center dries up and falls off, forming holes with neat edges, and there is often a ring of necrotic tissue around the hole. On new shoots and fruit, brown, sunken lesions with reddish-brown edges form, with gray mold-like matter growing on them.
[Pathogen]
Bacterial shot hole: the pathogen is Xanthomonas pruni (Smith) Dowson, the plum pathovar of Xanthomonas campestris. The bacterial cells are short rod-shaped. Size 1.4-1.8 um * 0.4-0.8 um. Single or in short chains. One end bears 1-6 flagella, has a capsule, no spores. Gram stain negative, aerobic. On nutrient agar medium the colonies are yellow, circular, smooth, with neat edges; liquefies gelatin, peptonizes milk. Optimum growth temperature 25 degrees.
Mildew spot shot hole: the pathogen is Clasterosporium carpophilum (Lév.) Aderh., belonging to the Deuteromycotina, genus Clasterosporium. Conidiophores are clustered, with septa. Conidia are club-shaped or spindle-shaped, with 3-6 septa, slightly curved, colorless or pale brown, size 30-56 um × 6-7 um.
Brown spot shot hole: the pathogen is Pseudocercospora circumscissa (Sacc) Liu & Guo, belonging to the Deuteromycotina, genus Pseudocercospora. A common synonym is Cercospora circumscissa Sacc. Conidiophores are clustered, unbranched, straight or slightly curved, olive-colored. With 0-2 septa, size 6.5-35.0 um × 2.5-4.0 um. Conidia are slender, whip-like or obclavate, straight or slightly curved, pale greenish-yellow, with 3-9 septa, size 25.0-80.0 × 2.0-4.0 um.
[Disease Cycle]
Bacterial shot hole disease: the pathogenic bacteria overwinter in the tissues of diseased branches and begin activity the following spring. Before and after peach blooming, the bacteria ooze from the diseased tissues, spread by wind and rain or insects, and invade through leaf stomata, shoot bud scars, and fruit lenticels, with an incubation period of 7-14 d. The bacteria in spring canker lesions die after 10-13 d under dry conditions. A temperature of 19-28 degrees and relative humidity of 70%-90% favor disease development. The disease generally appears in May and is severe in July-August. Trees with strong vigor are affected more lightly and later than trees with weak vigor; in trees with strong vigor the incubation period can reach 40 d. Orchards with low-lying terrain, poor drainage, poor ventilation and light penetration, and excessive nitrogen application have more severe disease. Early-maturing varieties are affected more lightly than late-maturing varieties.
Mildew spot shot hole disease: the pathogen overwinters as mycelium or conidia in affected leaves, shoot tips, or buds. Peach branches or buds are covered with a gelatinous layer, which helps the pathogen resist low temperatures. The following year the overwintering pathogen produces conidia, spread by wind and rain, first invading young leaves, and only after producing new conidia does it invade shoot tips or fruit. The incubation period of this disease varies greatly with temperature; the incubation period on leaves is generally 5-14 d, and on branches 7-11 d. Low temperature and abundant rain favor disease development, and the peak of disease occurrence during the year generally appears during periods of abundant rain.
Brown spot shot hole disease: the pathogen overwinters as mycelium in diseased leaves or shoot tip tissues; the following spring when temperatures rise, conidia are produced after rainfall, spread by wind and rain, and infect leaves, shoot tips, and fruit. Afterward, conidia produced in the diseased parts cause secondary infection. The temperature for disease development is 7-37°C, with an optimum of 5-28°C. Low temperature and abundant rain favor disease occurrence and epidemic.
[Disease Control]
1. Strengthen orchard cultivation management and enhance tree vigor. Fertilize rationally, apply more organic fertilizer, and avoid excessive N fertilizer; for peach orchards with a high water table or heavy clay soil, improve the soil and drain promptly; prune and train rationally, and when pruning remove dense branches, drooping branches, and branches trailing on the ground to improve ventilation and light penetration and reduce orchard temperature and humidity; apply more organic fertilizer, avoid excessive nitrogen fertilizer, promote robust tree growth, and improve disease resistance. Reduce inoculum sources during the dormant period. Combined with winter pruning, promptly prune out diseased branches, thoroughly remove diseased leaves, and burn or deeply bury them in a centralized manner to reduce overwintering inoculum sources.
2. Chemical control. Before peach bud break in early spring, spray 4-5 degrees lime sulfur mixture or 45% crystalline lime sulfur, 1:1:100 Bordeaux mixture, 30% Ludebao suspension concentrate, or 80% sodium pentachlorophenate (spraying is best done on a sunny, windless day); after leaf expansion, spray for control, with effective agents including 65% zineb, 50% carbendazim, 70% thiophanate-methyl, 40% Baijunjing, zinc sulfate lime solution, 72% agricultural streptomycin and streptomycin sulfate; during the growing season in rainy periods, spray folpet, captan, or mancozeb, etc. Between May and June, spray 75% chlorothalonil wettable powder at 650 times dilution 1-2 times, or 3% Kejunkang wettable powder at 1000 times dilution, both of which have good effects on shot hole disease. For bacterial shot hole disease, use agricultural streptomycin at 50-100 mg/L for control; for fungal mildew spot shot hole disease, use 70% thiophanate-methyl wettable powder at 1000 times dilution, or 50% carbendazim wettable powder at 800 times dilution for control. Spraying 65% zineb wettable powder at 500 times dilution 2-3 times in the early stage of disease occurrence can control multiple shot hole diseases at the same time, or spraying 45% thiabendazole emulsion at 2000 times dilution also has a relatively good effect.