The kiwifruit, despite its vibrant and lively nature, is actually quite sensitive and prone to stress when exposed to extreme conditions. It thrives in shaded, moist environments and struggles in sunny, dry areas. Zhouzhi County has a long history of kiwifruit cultivation, spanning over two decades. However, the summer heat, which often exceeds 35°C, poses a serious threat to the crop. Understanding the effects of high temperatures on kiwifruit and implementing effective preventive measures are crucial for sustaining and growing the local fruit industry.
First, let’s explore how high temperatures affect the kiwifruit plant.
1. **Impact on Leaves**
During June and July, when temperatures rise above 35°C, prolonged exposure to sunlight can cause leaf margins to turn yellow, then brown, and eventually black. If this condition persists for more than two days, the leaves may become completely scorched, leading to early defoliation or even tree death. This damage not only affects the appearance but also weakens the plant’s overall health.
2. **Effect on Fruits**
High temperatures can also harm the fruit directly. Prolonged exposure to the sun causes parts of the fruit surface to darken, leading to cell keratinization, which restricts growth. Mild sunburn results in smaller, unattractive fruits, reducing their market value. In severe cases, the fruit may soften, rot, or develop ulcers, making it unsellable.
Second, there's a pattern to sunburn caused by high temperatures. Orchards with weaker trees are more vulnerable. Studies show that young orchards (3–5 years old) have a sunburn incidence rate of 15% to 30%, while older ones (over 5 years) have only 2% to 3%. Within the same age group, trees with more than 20 new shoots have a lower risk, whereas those with fewer than 10 shoots are more likely to suffer from sunburn. This indicates that the density of foliage plays a key role in protecting the tree from heat. Additionally, ground management practices significantly influence the occurrence of sunburn. Orchards with well-maintained grass cover (over 60%) rarely experience sunburn, while those with overgrown or poorly managed ground are more susceptible.
Third, the reasons behind high-temperature sunburn are rooted in both the physiology of the kiwifruit and environmental factors. The leaves lack two guard cells, making it difficult to regulate water loss. When temperatures rise, the roots may not supply enough water to meet the transpiration needs, causing the leaf margins—where most stomata are located—to dry out first. Moreover, the timing of the heat wave matters. In Zhouzhi, the Loess Plateau region experiences high temperatures starting in June, coinciding with the peak fruit development period. At this stage, the leaves are tender and the fruit skin is thin, making the plant more vulnerable. By August, as the leaves mature, the risk of sunburn decreases.
Fourth, high temperatures can also interact with other diseases. For example, sunburn symptoms can resemble those of brown spot disease, leading to misdiagnosis. Brown spot is a fungal disease that typically appears in late August, with dark spots along the leaf veins. In contrast, sunburn occurs suddenly in June and July, with darker leaf edges that are still green unless caused by chemical damage. Another issue is root rot, a serious disease caused by fungi and bacteria. High temperatures increase transpiration, worsening the imbalance between water uptake and loss, which can lead to wilting and even tree death.
Finally, there are several effective strategies to prevent heat damage. One of the most successful methods is the use of a grass-cover system. In Zhouzhi, orchards with grass cover have shown a significant reduction in sunburn. This method lowers the temperature, increases humidity, and reduces reflected light. Additionally, promoting strong tree growth, proper pruning, and scientific summer management can further protect the plants from heat stress.
By understanding these factors and implementing smart agricultural practices, farmers in Zhouzhi can better protect their kiwifruit crops and ensure long-term sustainability.
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