Causes and Control Methods of Citrus Sunburn

Citrus sunburn is a common physiological disease in hot seasons, mainly affecting leaves, fruit and bark. The occurrence of this disease is mainly caused by exposure to high temperatures and scorching sun. At noon and in the afternoon during hot weather, the local temperature of orange trees exposed to direct sunlight rises sharply. When it rises above 40°C, the chlorophyll in the leaves gradually decomposes, the leaf color becomes lighter or turns yellow, and a large number of leaves fall after rain; the fruit surface shows abnormal color. If high temperatures continue for many days, the fruit surface becomes scorched yellow or turns brown, the skin at the discolored area sinks and hardens, and the bark of branches turns yellow and red, develops cracks, and later extends into the cortex, until the cortical tissue necroses.

Sunburn usually begins to occur in July, occurs most in August-September, and generally occurs on fruit on the southwesterly sun-facing side of the tree. Long sunshine duration and insufficient soil moisture can aggravate the occurrence of sunburn. Under high temperature and scorching sun conditions, using lime sulfur for citrus pest and disease control can also worsen this disease.

To control citrus sunburn, in addition to irrigation for drought resistance, the following measures can also be taken:

1. Spray fruit with limewater. In places where sunburn occurs severely, spray 1-2% limewater on the upper side of leaves on the south-facing sunny crown. After spraying limewater, it is like covering with a white film, which can reflect strong light, lower leaf temperature, and protect the leaves.

2. Whitewash tree trunks. Use 0.5 kg quicklime plus 2.5-3 kg water to make lime milk, and whitewash the main branches exposed to direct sunlight. At high temperatures, whitewashed bark is about 10°C lower in temperature than bark not whitewashed, can avoid direct sunlight on branches, and plays a protective role.

3. Cover fruit surfaces or bag fruit. For fruit at the top of the crown and on the outer southwestern side, use small pieces of newspaper 5 cm × 7 cm and stick them on the sun-exposed side of the fruit, which can effectively prevent sunburn on the fruit surface. To prevent fruit temperature from rising, fruit can also be bagged, or a water evaporation inhibitor can be sprayed onto the fruit surface to reduce water evaporation.

4. In rainy seasons, open ditches in time for drainage. Improve soil aeration, induce roots to grow deep, and enhance the water absorption capacity of citrus trees.

5. From July-September, do not use lime sulfur in orange orchards to control pests. If it must be used, lower the concentration and reduce the number of applications. A concentration of 0.1-0.2 degrees Baume is suitable, and 1-2 applications are enough. Spray evenly and do not allow too much liquid to accumulate on the fruit surface.