Apple rot, commonly known as "rotten skin disease" or "stink bug disease," is one of the most serious apple tree diseases in northern China. This condition primarily affects the fruit-bearing trees, leading to weakened growth, branch dieback, and even tree death in severe cases. When left unchecked, it can completely destroy orchards and cause significant economic losses.
**Symptoms**
The disease mainly targets the trunk and main branches of the tree, but it can also affect small branches, young shoots, and fruits. The symptoms are typically divided into two categories: ulcerous type and branch withering type.
The **ulcerous type** appears on the trunk and major branches. Affected areas show reddish-brown, water-soaked lesions that are round or oval in shape. The bark becomes soft and easily torn, with a sunken appearance and yellow-brown sap oozing out. A strong, unpleasant odor is often present. As the disease progresses, the affected area dries up and shrinks, forming cracks around the edges. Black spots may appear on the surface, and when wet, small dark spots may develop into yellow-brown fungal threads.
The **branch withering type** usually occurs on branches aged 2 to 5 years. The lesions have unclear borders and do not show water-soaked symptoms. Infected branches quickly dry out, and numerous small black spots form on the lesion surface. In severe cases, the disease can spread to the fruit, causing light brown, irregularly shaped rot with an unpleasant smell.
**Disease Cycle**
The disease has two peak periods annually: March to April and July to September. During early spring, as temperatures rise and sap flow begins, nutrients shift toward the growing points, leaving branches with less nourishment. This weakens the tree's resistance, making it more susceptible to infection. This sets the stage for the spring outbreak.
From May to June, the tree is vigorous, and nutrients begin to accumulate in the branches, reducing the spread of the disease. However, from July to September, as fruits grow rapidly, the tree’s branches experience a nutrient shortage, allowing the disease to re-emerge, resulting in the autumn peak.
Tree health is a key factor in determining the severity of the disease. Imbalances in nutrition, excessive fruiting, wounds, frost damage, lack of fertilizer, drought, early leaf fall, and heavy insect infestations all contribute to increased disease risk.
**Prevention and Control**
To prevent apple rot, proper cultivation and management are essential. Increase organic, phosphate, and potash fertilizer application, ensuring balanced nutrition. Thin fruit properly to avoid alternate bearing and maintain a reasonable crop load. Perform timely irrigation in spring and control water in autumn. Minimize and protect all wounds, and treat cut surfaces with 843 healing agent immediately.
Pest control should be strengthened to prevent early leaf drop. After leaves fall in early winter, prepare a whitewash using lime, 20° Baume lime sulfur, salt, animal oil, and water in a ratio of 6:1:1:0.1:10. Apply this to the tree trunks.
Promptly remove diseased branches, scrape off infected tissues and old bark, and destroy them outside the orchard. For treatment:
- **Mud Application**: Mix clay with water and apply a 3–5 cm thick layer over the lesion, extending 4–5 cm beyond the lesion edge. Cover with plastic wrap.
- **Cutting**: Make a circular cut 1 cm away from the lesion, cutting into the xylem. Apply 843 healing agent or 21% peracetic acid solution, covering 3 cm beyond the lesion. Repeat every month for 2–5 times.
- **Curettage**: Use a sharp knife to remove the lesion down to the xylem, and scrape 0.5–1 cm of healthy bark around the edge. Apply 45% dexamethasone, 5% mycorrhizal solution, or 843 healing agent.
This disease tends to recur, especially during summer and autumn. Regular inspections and timely treatments are crucial to manage it effectively.
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