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Apple anthrax
Apple anthracnose, also known as bitter rot or late rot, primarily affects apple fruits but can also infect branches, twigs, and other parts of the tree. This disease is prevalent in apple-growing regions across the country. In summer, high temperatures combined with heavy rainfall—particularly in provinces like Jiangsu, Anhui, Henan, and Hubei—can lead to severe outbreaks, significantly impacting both yield and fruit quality. The disease can cause devastating losses, especially in areas such as Shandong, Hebei, and Liaoning during rainy years. In contrast, regions like Shanxi, Gansu, and Jilin experience milder cases.
Symptoms of the disease first appear on the fruit surface as large, dark, round spots that start small and gradually expand. These lesions have clear boundaries and become soft, brown, and rotten, emitting a bitter taste. As the infection progresses, the lesions sink into the fruit, forming distinct concentric rings. When the lesions reach about 2 cm in size, black, sunken conidial discs appear, initially dark brown and later turning black, arranged in a circular pattern. Under wet conditions, bluish-red spore masses emerge from these structures. Over time, the pulp darkens, creating a gradient of brown coloration. A single lesion can cover up to one-third to one-half of the fruit's surface. Infected fruits often show multiple lesions, ranging from a few to dozens, with dry spots varying from light brown to dark brown.
In the early stages of the disease, lesions grow slowly, but as the fruit nears maturity, they expand rapidly, sometimes reaching half the fruit’s size within 7–8 days, leading to significant fruit drop. On branches, infections typically occur at the base of pests or on thin branches. Early lesions are irregular and dark brown, eventually leading to ulcers, cracking, and exposure of the xylem. Severe infections can cause the branches above the affected area to wither. Black protrusions and small particles may also appear on the diseased bark. Under indoor conditions with warmth and moisture, red conidial masses can develop within 7–9 days.
The primary pathogens responsible for anthracnose are fungal hyphae that remain latent in pests, stems, and dead fruits. The optimal temperature for mycelium growth is 28°C, with a maximum of 40°C and a minimum of 2°C. Due to the latent nature of the infection, the incubation period varies depending on the species and season, sometimes lasting up to 50–60 days. For example, in Zhengzhou, Henan, the incubation period for the most susceptible red-wringed variety is 9–11 days in May, 6–8 days in June, 4–5 days in July, and 2–3 days in August.
During the following growing season, when suitable temperature and humidity conditions arise, conidia are produced and spread by rain and insects, becoming the primary source of infection. Conidia can directly invade the fruit. High temperatures favor spore germination and propagation. Under favorable conditions, spores can penetrate the fruit flesh within 5 hours and complete the invasion process in 10 hours. The disease continues to spread throughout the growing season and can persist into late autumn. Infections occurring before harvest can continue to develop during storage, transportation, and sale, though it is not contagious during storage. The pathogen has a latent infection characteristic, meaning that early fruit invasions may only become visible during the fruit-filling stage. High temperature and humidity are key factors in the outbreak of anthracnose, which is more severe in hot and rainy regions.
Control measures include strengthening cultivation practices: deep plowing, timely drainage, increasing organic fertilizer use, avoiding excessive nitrogen application, enhancing tree vigor, and improving resistance to disease. Regular weeding and pruning help reduce humidity and improve air circulation and light penetration. Removing diseased material such as dried branches and fruits is essential. Applying fungicides before bud break helps eliminate overwintering pathogens. Experience shows that trees treated with a disinfectant spray can delay the onset of disease by 15–30 days, effectively controlling its spread.
During the growing season, protective fungicides such as Bordeaux mixture and tetramycin are widely used. In areas with severe disease, Bordeaux mixture is applied early, followed by tetramycin in mid-season and then Bordeaux mixture again in the later stages. The timing usually involves applying 1–2 sprays of 1:2–3:200 Bordeaux mixture 5–6 months before fruiting, followed by 1–2 sprays of 50% wettable powder at 600–800 times dilution from mid-June to early July. After mid-July, 1–2 more applications of 1:2–3:200 Bordeaux mixture are recommended. If heavy rains occur between May and June, fungicides should be applied earlier. During storage, infected fruits should be removed, and temperature control is crucial. As temperatures rise, regular inspections are needed to promptly identify and remove diseased fruits.