Symptom Identification and Scientific Control Methods of Apple Early Leaf Drop Disease

Apples are grown in many parts of China, and there are quite a few varieties; the ones everyone is most familiar with include Red Fuji, Ugly Apple, Granny Smith, and others. During apple cultivation, if early management is poor, leaf drop disease can easily occur. What are the main symptoms of this disease? What are the scientific control methods?

1. What are the symptoms of apple early leaf drop disease?

First, when apples suffer leaf drop from brown spot disease, in the early stage there are small brown spots on the leaves, which later expand into large spots with green margins and irregular shapes. Black small dots appear on the affected areas, the lesions are dark brown, and the fruit is accompanied by dry rot and tissue necrosis.

Second, apple Alternaria leaf spot mainly damages leaves, but can also damage petioles, young twigs, and fruit. After the leaves are affected, brown round spots first appear, then gradually expand into large lesions; the lesions are brown with purple-brown edges, and the center often has a dark small dot or shows concentric rings. When the weather is humid, dark green mold-like growth can be seen on both sides of the affected area. In the middle and late stages of the disease, the lesions become grayish brown to gray, some irregular in shape, and some form holes. Sometimes secondary infection occurs in the later stage, producing small black dots on the grayish white lesions.

Third, when apples suffer leaf drop from rust disease, it mainly damages young leaves, petioles, new shoots, young fruit, and other young green tissues. Small orange-yellow, glossy round spots first appear on the leaves. They then expand, with a darker center, and many small black dots grow out, exuding transparent droplets. Afterward the droplets dry, the pycniospores turn black, and the affected tissue thickens and swells. Yellow-brown hairy structures grow on the underside of the leaves, containing large amounts of brown powder. When petioles are infected, the affected area is spindle-shaped, orange-yellow, and slightly raised. When new shoots are infected, it is initially similar to petiole damage, and later the affected area becomes sunken, cracked, and easily broken.

2. What are the scientific control methods for apple leaf drop disease?

1. Fully recognize the severity of the harm caused by apple early leaf drop disease

Apple brown spot disease mainly harms apple leaves and causes defoliation before maturity. When it occurs severely, it can cause massive leaf drop from late August to early September, seriously affecting tree vigor, causing fruit to fail to mature normally, and leading to reduced fruit sweetness and flavor and poor coloring. It is estimated that premature defoliation 7-10 days before fruit maturity can lower the fruit by one grade, seriously affecting the apple selling price. Massive leaf drop reduces photosynthetic products and nutrient accumulation, seriously affecting flower bud morphological differentiation and physiological differentiation, thereby affecting the quality and quantity of flower buds the following year; the decline in tree vigor will also aggravate the damage of apple tree valsa canker the following year. In addition to harming leaves, the brown spot pathogen can also harm apple fruit, producing dark brown spots around the stem cavity. In 2018, brown spot disease occurred severely, leading to a significant increase in brown spot disease in fruit during storage, and in some cold storage facilities the diseased fruit rate was above 30%.

2. Timely prevention is the key to successfully preventing and controlling apple early leaf drop disease

High temperature and rainfall are important meteorological conditions for the occurrence of apple early leaf drop disease. In most years in the Weibei area of Shaanxi, brown spot disease begins to occur sporadically from late June to early July, and this is the key period for preventing brown spot disease. In years with more rain, the initial occurrence of brown spot disease can be advanced to mid-June or even early June, so the time of the first pesticide application to control brown spot disease after bagging should also be appropriately advanced. Based on recent meteorological conditions and field disease occurrence, it is recommended that all regions begin pesticide application for control from early to mid-June.

3. Grasp the correct pesticide application interval and achieve scientific and rational pesticide use

The incubation period of the apple brown spot pathogen is 12-30 days, and it generally takes a relatively long time from infection to disease onset. It is recommended to apply pesticide for the first time 2 weeks after bagging, then spray once every 20 days thereafter, ending in late August, for a total of 4 applications. The application period can be appropriately adjusted according to the medium-term weather forecast. If there is continuous overcast and rainy weather, pesticide can be sprayed before the rain; if there is little rain, the application interval can be extended to 25 days.

4. Choose effective pesticide varieties and ensure spraying quality

It is recommended to use control pesticides mainly based on systemic agents. Tebuconazole, pyraclostrobin, propiconazole, trifloxystrobin·tebuconazole, and thifluzamide·tebuconazole all have good inhibitory effects on the brown spot pathogen and can be selected. Repeated use of a single pesticide can increase the resistance of the brown spot pathogen, so it is recommended to use each pesticide at most twice in one growing season. Pay attention to application technique. Spray equipment with good atomization should be selected, and spraying should be uniform to avoid omissions.