Causes and Control Methods of Sunscald in Schisandra chinensis

Sunscald on Schisandra chinensis fruit is a common physiological disorder that causes certain losses in production every year. With global climate warming, this disease tends to worsen year by year.

1. Disease symptoms Sunscald in Schisandra chinensis mainly affects the fruit. The sunscald sites often appear blistered, with sunken necrotic spots, lignification, hardened lesions, or necrotic spots in the flesh tissue. The surface of affected berries initially turns white (yellow or pink), then becomes black-yellow to brown. When sunscald is severe, sunken necrotic spots appear in the flesh tissue, localized flesh tissue becomes necrotic, and the affected areas are prone to infection by other fruit rot pathogens, causing fruit decay.

2. Disease pattern The direct causes of sunscald are heat injury and ultraviolet radiation injury, among which heat injury refers to sunscald caused by high temperature on the fruit surface and is unrelated to light; ultraviolet radiation injury is sunscald caused by ultraviolet rays, which leads to cell disintegration. The occurrence of sunscald is related to many factors such as temperature, light, relative humidity, wind speed, variety, fruit development stage and tree vigor. Among these, temperature and light are the main influencing factors. ① Temperature: air temperature is an important factor affecting sunscald in Schisandra chinensis fruit. On sunny, high-temperature summer days, the surface temperature of Schisandra chinensis fruit can reach 40-50°C, far higher than the maximum air temperature of the day. Some scholars believe that the critical air temperature for causing sunscald is between 30-32°C, and as the environmental temperature rises, the time required for sunscald to occur shortens and the severity of sunscald increases accordingly. ② Light: light intensity and ultraviolet rays are important factors in the occurrence of sunscald in Schisandra chinensis fruit. Under natural conditions, fruit exposed to light converts part of the light energy into heat, thereby raising the fruit surface temperature; together with the warming effect of high temperature on the fruit, this causes the fruit surface to reach the critical temperature for sunscald, thus inducing the occurrence of fruit sunscald.

The disease occurs from June to September, with the peak period in the hot, intensely sunny summer days of July to August. In production, it is often found that the peak period of fruit sunscald each year always coincides with the period of the highest temperatures in the year. Under the premise of relatively high temperatures, sunny days can very easily lead to sunscald, while sunny days with lower temperatures have a low incidence rate. The lower the relative humidity, the higher the incidence rate of fruit sunscald; wind speed can change fruit temperature by regulating transpiration, and a gentle breeze can lower the fruit surface temperature and thus reduce the incidence rate; different varieties differ in their sensitivity to sunscald; fruit at different development stages differs in resistance to sunscald, and as the fruit matures, its sensitivity to sunscald decreases; within the same orchard, trees with strong vigor have a low incidence rate of sunscald, while trees with weak vigor have severe incidence.

3. Control techniques ① Strengthen cultivation management to enhance tree vigor; take care to avoid excessive application of nitrogen fertilizer, apply more organic fertilizer, improve the soil's water- and fertilizer-holding capacity, promote deeper root growth, and improve the plant's drought resistance. ② When pruning, leave appropriately more branches and leaves to avoid exposing the fruit to direct sunlight; through flower and fruit thinning, regulate and determine a reasonable branch-to-fruit ratio and leaf-to-fruit ratio. ③ Before high-temperature weather arrives, cooling sprinkler irrigation can be used to lower the fruit surface temperature, which can effectively prevent the occurrence of sunscald. ④ Using fruit bagging can reduce sunlight intensity and fruit surface temperature.