What are the measures to prevent high-temperature sunburn in kiwifruit?
In recent years, due to the high economic returns of kiwifruit, fruit growers in our area have rushed to introduce and plant it, but because we previously had little research on it, we also took many detours. Kiwifruit likes water but fears waterlogging, likes light but fears scorching sun; in its natural distribution it mostly grows in shaded and moist places, and is almost absent on sunny slopes and in arid areas. Zhouzhi County has more than 20 years...
In recent years, due to the high economic returns of kiwifruit, fruit growers in our area have rushed to introduce and plant it, but because we previously had little research on it, we also took many detours. Kiwifruit likes water but fears waterlogging, likes light but fears scorching sun; in its natural distribution it mostly grows in shaded and moist places, and is almost absent on sunny slopes and in arid areas. Zhouzhi County has a kiwifruit cultivation history of more than 20 years, and the high-temperature weather every summer has a very great impact on kiwifruit. Correctly understanding the adverse effects of high temperature on kiwifruit production and adopting corresponding preventive measures is of great significance to the development of the kiwifruit industry.
I. Effects of high temperature on kiwifruit
1. Effects on leaves. Every year in June and July, when the temperature reaches above 35°C, kiwifruit leaves exposed to intense light for 5 hours lose green color at the leaf margins, which then turn brown and black. If such high temperature lasts for more than 2 days, kiwifruit shows blackened and upward-curled leaf margins, appearing scorched. When sunburn is severe, it can cause early defoliation and even tree death.
2. Effects on fruit. Sustained high temperature can cause the part of the fruit surface exposed to sunlight to turn brown and black, and the peel cells to become keratinized, thereby limiting fruit growth. Fruit with mild sunburn is smaller after ripening and has an unclean fruit surface, losing its proper commercial value. In severe cases, the fruit soft-rots and even decays.
II. Occurrence patterns of high-temperature sunburn
Sunburn caused by high temperature mainly occurs in young orchards that have just begun bearing fruit and have weaker tree vigor. According to surveys, under the same conditions, the incidence of sunburn in 3-5 year old orchards can reach 15%-30%; in mature orchards over 5 years old, the incidence is only 2%-3%. Among trees of the same age, when the number of new shoots is more than 20, the incidence is 50%-8%; when the number of new shoots is fewer than 10, the incidence can reach 20%-30% or even higher. It can thus be seen that the orchard leaf area index is the key factor affecting the degree of sunburn occurrence. In addition, orchard ground management is also an important factor in the occurrence of sunburn. Under the same conditions, orchards overgrown with weeds have much lighter sunburn than clean-cultivated orchards. Mature orchards with grass in the orchard and more than 60% coverage generally do not develop sunburn. Even within the same orchard, individual weak trees develop sunburn while robustly growing trees remain completely undamaged.
III. Causes of high-temperature sunburn
1. Related to the special structure of the leaves. Kiwifruit leaves are relatively large, and to a certain extent the leaves can provide shade, but the physiological structure of kiwifruit leaves is rather special. Because its stomata do not have two guard cells, it cannot effectively control the amount of water transpiration from the tree. When the temperature is too high and the water absorbed by the roots cannot meet transpiration needs, the water in the tree body loses balance, and the first part to show water loss is the leaf margin, where stomata are most densely distributed.
2. Related to climate and phenological period. Because Zhouzhi is located on the Loess Plateau, the climate is relatively dry, and continuous high temperatures above 35°C begin to appear every June. This period is exactly the peak growth period of kiwifruit fruit; fruit development and new shoot growth both require large amounts of nutrients and water. At this time the leaves are relatively tender and the fruit skin is relatively thin. If nutrients and water are insufficient, the tree body will become relatively weakened and its resistance will decrease. Therefore, June and July are the peak period for sunburn occurrence; entering August, the tree body has luxuriant branches and leaves, and the maturity of leaves and fruit increases, so sunburn lessens.
IV. Relationship between high temperature and other diseases
1. Sunburn and brown spot disease. The symptoms of sunburn and brown spot disease are very similar. In production, people often mistake sunburn for brown spot disease and spray pesticides for control, resulting in phytotoxicity due to excessively high temperature. In fact, as long as one observes carefully, the two can still be distinguished:
(1) Symptom expression of brown spot disease. This disease is a fungal disease. In terms of symptom expression, although brown spot disease can also cause leaves to blacken, the disease first produces black spots along the leaf veins and then on the sides, and later gradually expands. In the Zhouzhi area, it often begins to occur in August.
(2) Symptom expression of sunburn. Sunburn is a physiological disease and has no pathogenic bacteria. In terms of time, it mainly occurs in June-July. The disease develops rapidly; within one day the leaf margin can blacken, but the color is lighter than that of brown spot disease. If the leaf margin blackens and curls upward, but the area inside the leaf margin is still green, then except for pesticide damage and fertilizer damage, it is generally mostly sunburn.
2. High temperature and root rot. Root rot is a devastating disease of kiwifruit; its pathogens are mainly fungi, and also bacteria. Because the infected part is underground, symptoms often appear on the tree only when the disease is severe, and the symptoms shown on the tree are mainly due to root damage that limits the absorption and transport of nutrients and water, causing the tree to show wilting symptoms and even die. High temperature can intensify the transpiration of kiwifruit, thereby intensifying the contradiction between water supply and demand, causing the tree to repeatedly show leaves wilting during the day and unfolding at night, and finally dying.
V. Measures to prevent high-temperature damage
Adopting a grass-growing system is one of the effective ways to prevent high-temperature damage. According to comparative surveys of clean-cultivated orchards and grass-growing orchards, when the natural air temperature reaches 35°C, the air temperature in clean-cultivated orchards can reach 45-48°C. The reason the temperature in the orchard is higher than the natural temperature is that the orchard is affected by both direct sunlight and reflected light from the ground, and the air humidity in the orchard is also about 5% lower than natural humidity. In recent years, Zhouzhi has promoted the grass-growing system, advocating and guiding fruit growers to plant hairy vetch in orchards, and the incidence of sunburn has been greatly reduced. The reason is mainly that it reduces reflected light from the orchard floor, lowers the temperature, and increases the air humidity in the orchard. According to measurements, in orchards with grass grown for more than 2 years, the air humidity in the orchard is 15%-18% higher than natural humidity, and the moisture in the air compensates for part of the transpiration water. In addition, cultivating robust tree bodies, retaining branches reasonably, and scientific summer pruning are also good ways to prevent high-temperature damage.