How to Prevent Physical Damage in Four-Season Orchids

Physical damage includes pesticide burn, fertilizer burn, sunscald, frost damage and acid rain erosion. The symptoms of these injuries closely resemble those of leaf spot disease and are often misdiagnosed as disease; in some cases, leaf spot disease or viral disease infects the plant following physical damage. With careful observation, however, there are still many differences in their characteristics. Take fertilizer burn as an example. The most common form is scorched leaf tips caused by excessive phosphorus fertilizer in winter. At first, yellow spots appear about 1 cm from the leaf tip, with water-soaked margins around the yellow spots; these then enlarge into bigger yellow spots and finally turn into black scorched tips. The cause is low winter temperatures, slow metabolism, and excess salts accumulating at the leaf tips. This phenomenon does not occur in other seasons. Although the symptoms appear at the leaf tips, unlike anthracnose they do not spread in intermittent patches. It is also different from root-rot scorched tips: root-rot scorched tips are distinctly black-green and occur on old plants, whereas fertilizer burn occurs on new plants. Another type of fertilizer burn occurs below the pseudobulb, causing hardening and necrosis of the root tips and blackening of the root base. The cause is excessive fertilization, use of incompletely decomposed organic fertilizer, or inorganic salts produced by acid-base neutralization between fertilizers and pesticides. It mostly occurs in spring and summer after rain when the sky clears. Pesticide burn mostly occurs during the new shoot stage. When new shoots have just emerged from the soil or the shoot core is opening, excessive pesticide accumulates, causing scorched black leaf sheaths or blisters and wrinkling on the leaf surface. Because the injury occurs at the base of the shoot core, when the plant matures the inner and outer leaves show pesticide spots at unequal heights. During the new shoot stage, it is most likely to occur when the room temperature is above 3O°C, and is especially severe with copper-based agents (such as Kocide, copper sulfate, copper manganese Maneb, copper hydroxide, etc.) and summer oil. Therefore, during the high-temperature period in spring and summer, doses should be reduced by 2O% to 5O% for safer use. Sunscald most often occurs at the transitions between spring and summer and between autumn and winter, when temperatures fluctuate. If the shade net is drawn back too early or put on too late, the leaf bends or leaf tips are often scorched by sunburn. For example, the white-claw variety 'Tianxiang' is prone to scorched blackening for this reason. To be safe, it is better to let it suffer insufficient sunlight and become leggy than to affect its quality through sunscald. Four-SeasonOrchid, autumn shoots usually sprout in November. The coldest months of January and February coincide with the opening period of the shoot core, when accumulated dew easily freezes into frost and causes frost damage. On highly variegated plants, black spots easily appear on the undersides of the leaves, resembling rust disease; in severe cases the leaves become scorched black and drop. The symptoms of frost damage on new shoots closely resemble pesticide burn, and the affected area is even more extensive. In regions with frost and snow damage in winter, they should not be cultivated in the opencultivation; they need to be moved indoors or protected with a double-layer film. A single-layer film easily causes water vapor to condense into ice water, which drips onto the leaf surface and causes even greater damage. The harm from acid rain has become more serious in recent years, especially in large cities and their suburbs, because afternoon thunderstorms carry sulfides in the air and form acid rain (around PH4.0). Dripping onto the leaf surface, it causes corrosive black spots of varying sizes distributed evenly; both old plants and new leaves are injured. In the early stage they appear as water-soaked spots, then turn yellow and black after two days, about the size of a soybean. The black spots do not expand, and no yellow halo forms around them. Prevention method: immediately water the plants after a brief rain shower. Besides the five common types of physical damage described above, other factors such as excessively high water pH, too high an EC value, excessive lime content, or a medium that is too loose causing root damage, as well as air pollution, excessive temperature differences, and insufficient ultraviolet light, can also cause poor leaf growth. Because these injuries are often mistaken as being caused bypests and diseases, people use every pesticide available to no effect; however, careful observation can also reveal the clues. Note: The role of ultraviolet light is to promote phototropism, making leaves thicker and more upright and increasing disease resistance. If the film turns yellow or grows moss, it will block ultraviolet light from entering. The plant can still carry out photosynthesis, but the variegation will degrade and the leaf shape will become softer and more drooping. Conversely, too much sunlight or too long a lighting period (over 16 hours) will cause leaf dwarfing and yellowing, or even premature aging with no new shoot growth. (Liu Qingyong)