Orchid sunburn is a physiological disease

Sunburn usually occurs when shade-grown plants are suddenly exposed to strong sunlight, and it can also occur when new potted cuttings of tropical orchids are moved into full sunlight. After tip pruning, the leaves remaining on the stubble, which were previously shaded by the terminal leaves, also become prone to sunburn because the terminal leaves have been removed. Because the direct point of the sun moves back and forth between the Tropic of Capricorn and the Tropic of Cancer, a particular part of a plant in shade will not be exposed to scorching sunlight early in the year, but by the middle of the year it may be exposed to direct sunlight, and during this period a particularly hot day can cause sunburn on the top leaves of young or mature plants. More delicate plants such as Phalaenopsis and Vanda with small ligules, as well asCymbidium sinensesome varieties, are very susceptible to sunburn. But under especially hot conditions, allorchidsare unable to escape it. Symptoms: The usual symptom of this disease is the appearance of relatively large light brown water-soaked lesions on the top leaves of plants under sunlight. The lesions are clearly distinguishable and will not spread to other plants. In severe cases, the affected tissue will slowly turn brown, turn black, and finally split open. In general, heat only damages the epidermal cells on the upper surface of the leaf; these cells often turn grayish white to light brown, while the underside of the leaf remains green. Treatment: When sunburn occurs, there is no warning. The affected leaves do not need to be cut off. The infected leaves should be sprayed with 2 tablespoons of captan or thiram added to 2 L of water to prevent fungal infection, then move the plant to a more shaded place and spray with fungicide once every week for 2 to 3 consecutive weeks. If moving it to a more shaded place is actually impossible, then shade should be provided for the diseased plant in place to prevent sunburn from occurring again. Prevention: Plants grown in shade must not be suddenly exposed to strong sunlight. It is necessary to move shade-grown plants to a more exposed place, but the best practice is to proceed gradually and not too suddenly, so that the plants have some time to adapt to the heat. All newly plantedCymbidium sinense'Qihei' must be hardened off beforehand to prevent sunburn, and additional shading is also necessary, which can prevent sunburn caused by changes in the sun's position during a particular time of year. (Liu Zhongjian)