Reasons for Yellowing Orchid Leaves and How to Fix Them
Many friends easily run into the problem of yellowing leaves when growing orchids. Watering is key to growing orchids, so why do yellow leaves appear? What are the reasons for yellowing orchid leaves? How do you deal with it?
The reasons for yellowing orchid leaves can be divided into 3 types: variegation (chu yi), natural leaf decline (tui cao), and pests and diseases. Variegation and natural leaf decline are yellowing caused by the physiological phenomena of the orchid. Variegation and natural leaf decline are two different directions of development in the orchid's growth process, so they need to be discussed separately. This article will introduce these 3 aspects in detail, so that orchid enthusiasts can have a more comprehensive and in-depth understanding of yellowing orchid leaves.
1. Yellowing orchid leaves caused by variegation (chu yi)
So-called variegation refers to the appearance of colored lines, patches or spots on orchid leaves. This genetic change phenomenon, in which colored lines, patches or spots appear, is called “chu yi” in the trade. It is an “evolution” that occurs during the orchid's normal growth process. Those that develop yellow lines are called “striped grass” or “silk grass”; those that develop yellow spots and patches are called “spotted grass”. Such variegated orchids are collectively called “leaf-variegated orchids” or “line-variegated orchids”. The yellow leaf variegation that appears on orchid leaves can be divided by hue into 5 common colors: yellow, white, vermilion gold, red and black.
The timing of variegation in orchids varies; some appear early and some late. Based on the different times at which orchid leaves develop variegation, this phenomenon is classified in the trade as: early-bright variegation, early-bright then dark variegation, and late-bright variegation. In plain terms, orchid leaves that come with yellow as soon as they emerge from the soil are called “early-bright variegation”; those that are yellow at first and gradually turn green later are called “early-bright then dark variegation”; those that are green at first and gradually turn yellow later are called “late-bright variegation”.
When yellow lines, patches or spots appear on orchid leaves, the leaves are still full of vitality and luster, and will not wilt. The yellow parts are very vivid and form a strong contrast with the green parts, and most have very obvious boundary lines. White is white, yellow is yellow, each going its own way. If dark, dull yellow-brown colors are mixed in, there is no luster, and the leaves wilt and turn yellow, lacking vitality, then it is not variegation, and you should consider yellowing caused by natural leaf decline or pests and diseases. If the yellowing of orchid leaves is caused by variegation, we do not need to worry and there is no need to treat it; just grow it normally.
2. Yellowing orchid leaves caused by natural leaf decline (tui cao)
Orchids are perennial herbaceous plants with a normal lifespan of about 3-5 years. Well-cultivated orchids can live more than 5 years. After reaching the end of their lifespan, the orchids will quickly wither and die. Leaf yellowing after reaching the normal lifespan is normal leaf decline in orchids.
Normal leaf decline in orchids shows as starting from the lowest and outermost leaf sheaths and leaf tips of the orchid plant, spreading from outside to inside and from top to bottom. The entire leaf rapidly yellows, and the longer the time, the darker the color, gradually turning yellow-brown, then brown, until it withers. In this situation there is nothing we can do; just wait for the leaves to wither and cut them off.
Some orchids, due to improper planting and management, fail to reach their normal growth lifespan, and the grass declines early after just 2-3 years. The more the orchids are grown, the fewer the shoots become, and the orchid leaves become sparse, failing to achieve the goals of propagation and appreciation. This situation is abnormal leaf decline.
Abnormal leaf decline in orchids is generally caused by improper cultivation methods or by nutrients and water being unable to keep up. Because the orchid's pseudobulbs are closely connected and share nutrients and water, when the nutrients and water needed for new shoot growth and flowering cannot keep up, if old grass is attached, the old grass will transfer its own nutrients to the new shoots and flower buds for their growth through leaf decline. This causes the leaves of the old orchid shoots to turn yellow, also showing as starting from the lowest and outermost leaf sheaths and leaf tips of the orchid plant, spreading from outside to inside and from top to bottom. The more nutrients the new shoots and flower buds need, the more severe the yellowing of the old shoots, and in serious cases it can directly cause the death of the entire old grass plant. If there is no old grass, and the root system is too few or too poor, nutrients and water cannot keep up, and improper cultivation leads to root rot, which can also cause yellow leaves and plant collapse.
How to handle abnormal leaf decline in orchids: When dividing plants or selecting orchids, consider the number and completeness of the new shoot's root system. The more numerous and complete the new shoot's roots, the stronger its ability to produce nutrients and absorb water, and it will not need to rely on old grass to support its growth through leaf decline. If you want to divide a new shoot as a single plant, it is best for the new shoot to have 3 or more roots. If the new shoot has too few roots, be cautious when dividing; choose to divide it together with old grass that has a well-developed root system. If the new shoot's root system is well developed and the old grass's root system is poor, the old grass should be cut off and planted separately. When planting, choose a breathable, stable granular growing medium. During the peak growth season when orchids send up shoots and bloom, apply fertilizer appropriately to the orchids, generally in spring and autumn, and be sure to apply light fertilizer frequently. After planting, place them in a ventilated, breathable location. During the orchid's peak growth season, foliar fertilizer and root fertilizer can be used to supplement nutrients.
3. Yellowing orchid leaves caused by pests and diseases
1. Yellowing orchid leaves caused by diseases
The common diseases that can cause yellowing orchid leaves are anthracnose, soft rot, and stem rot, 3 kinds. Anthracnose, also called black spot disease and brown spot disease, shows as yellow-brown lesions of varying sizes appearing on the surface of orchid leaves. The middle of the spots is yellow-brown and the edges are yellow. Some are clustered, some are scattered. The lesions can heal together into large spots, sometimes splitting longitudinally, causing perforation of the orchid leaves. The lesions vary in size, with diameters ranging from 1 to 20 mm. It is mainly caused by high temperature and high humidity and poor ventilation. It is a disease caused by fungi, is contagious, and can spread through wind, diseased seedlings, growing medium, orchid pots, and so on.
Stem rot shows as the base of the orchid leaves turning yellow, then gradually turning brown and black, starting from the heart leaf and spreading from the base of the leaf surface upward. At the same time, the pseudobulb shrinks and turns yellow until it dries up. This disease is also caused by fungi, is also contagious, and can likewise spread through wind, diseased seedlings, growing medium, orchid pots, and so on.
Soft rot shows as the base of the orchid leaves beginning to turn yellow, with water-soaked small brown spots appearing, then rapidly expanding, and finally producing foul-smelling dark brown rotting patches that fall off with a gentle pull of the hand. This is mainly caused by wound infection and water entering the leaf center without timely evaporation, and is not contagious.
Prevention and treatment methods for the above 3 diseases: For fungal diseases, pay attention to breathable growing medium and a ventilated environment, and avoid high-heat and high-humidity environments. For bacterial diseases, avoid wounds on the orchid plant; if wounds appear, disinfect and seal them promptly, and the tools used for dividing plants must also be disinfected. Common drugs for fungal causes include carbendazim, chlorothalonil, thiophanate-methyl, and so on. Common drugs for bacterial causes include streptomycin, copper hydroxide, copper oxychloride, copper succinate, and other copper formulations.
2. Yellowing orchid leaves caused by insect pests
The common insect pest that can cause yellowing orchid leaves is the mite red spider. After the red spider feeds on orchid leaves, dense small yellow spots appear on the orchid leaves, which cannot be washed off even with water. At first glance it looks like “sun spots” formed by malnutrition or overly strong light, or like thread disease, dust, or moss from long-term watering attached to the surface. Some beginners even think the orchid has mutated and produced spot variegation. The red spider is not the spider commonly seen in our daily lives, but a type of mite. It is very small, less than 1 mm, round or oval in shape, and orange-yellow or red-brown in color, and is very harmful to orchids.
Prevention and treatment methods for red spider: In the season when orchid new shoots open, as soon as any sign of red spider damage is found, it is best to spray all the orchids with an insecticide once, paying special attention to hitting the new shoots, then apply the pesticide again after an interval of 10 or 15 days. Generally, applying the pesticide 2 to 3 times in succession can effectively control the condition. You can use 10% fenbutatin oxide plus pyridaben emulsifiable concentrate at 1000 times dilution, used 2 times in a row with an interval of 7-10 days. Mite Danger 4000-5000 times (each 100 ml bottle mixed with 800-1000 jin of water), evenly sprayed; 40% dicofol emulsifiable concentrate at 1000-1500 times dilution; 20% clofentezine wettable powder at 2000 times dilution; 15% pyridaben emulsifiable concentrate at 2000 times dilution; 1.8% abamectin emulsifiable concentrate at 6000-8000 times dilution, and so on, can all achieve ideal control results.
3. Yellowing orchid leaves caused by fertilizer damage
Yellowing and blackening of the tips of new leaves is generally caused by overly frequent fertilization or too high a concentration. It shows as new leaves being thick and fleshy, with scorched yellow leaf tips, an uneven surface, old leaves scorched yellow and withered, and roots turning black and carbonized. In serious cases, the orchid roots and pseudobulbs all rot and the entire orchid dies.
Prevention and treatment methods for fertilizer damage: When potting orchids, apply base fertilizer according to the orchid's growth condition. For wild-collected orchids, weak grass, and divided old grass, it is best not to apply fertilizer. Even for mature strong grass, base fertilizer should be added in the proper amount, the base fertilizer must be fully fermented, and it should be placed in the lower part of the pot, preferably not in direct contact with the orchid roots, so as to avoid root damage. In ordinary fertilization, be sure to apply light fertilizer frequently and use less chemical fertilizer. When using chemical fertilizer, it needs to be diluted at a high ratio, preferably at a dilution ratio of 3000-5000 times. If the potting soil is found to be too rich in fertilizer and the orchid leaves show scorched tips and withered leaves, you should immediately repot, wash the orchid plant clean, cut off the rotten roots, let it dry, and replant it in plain soil.