Why Are Orchid Leaf Tips Turning Brown and Withered? Causes and Control Methods
Orchid leaf tip scorch is one of the common and frequently occurring diseases. It damages the integrity of the orchid leaves, affects their ornamental value and economic value, and also seriously affects the healthy growth of the orchid plant, and may even endanger the life of the plant. After years of practical exploration, it is believed that orchid leaf tip scorch is caused by physiological diseases and infectious diseases. The vast majority are caused by physiological diseases. They are described separately as follows:
1. Poor root growth
It can be said that wherever there is leaf tip scorch on orchid plants, there is root decay to varying degrees. And the causes of poor root growth are many-sided.
1. Waterlogging damage:
This is mainly due to the orchid pot being fine and dense in texture, so water evaporates slowly; the growing medium not being coarse enough and having poor drainage; and watering too much and too often. Orchids “like rain but fear waterlogging” and “like moisture but fear wetness”. But never understand the word “moist” as damp and soggy, and water the orchids into mud like that in a paddy field. It is best to shelter them from continuous rain and heavy rain, and to water only two or three days after the surface soil in the pot has dried. For orchid pots, it is best to choose rough-textured pottery pots, and as far as possible use specially made tall cylindrical ones with feet, with many holes in the pot wall and a large hole in the bottom. If they are fine-textured porcelain pots or purple clay pots, it is best to find a way to drill more holes; on the other hand, the bottom of the pot must never be blocked dead with tiles, and must be raised on supports. The growing medium must be as coarse as possible. For local mountain soil, crushed brick granules, burnt earth, or sterilized tree fern chips, China fir chips and other coarse planting materials should be mixed in to enhance air permeability and drainage.
2. Freezing damage:
Orchids are shade plants and cannot tolerate low temperatures and frost. When the temperature is below zero degrees, most orchids suffer freezing damage to varying degrees. If the root tips are frozen and injured, gradually dry and wrinkle, and lose the ability to absorb water and nutrients, then when the temperature rises the following spring and light intensity increases, the leaf tips cannot get nourishment and become scorched. Even the new shoots sprouted in late spring, because the plant cannot get nourishment and has no strength to supply the consumption of new shoot development, cause the new shoots to grow roots very slowly, and leaf tip scorch also appears.
There is another situation: in simple warm sheds and where plastic film is used on the spot for freeze protection, because moisture cannot be discharged and dampness cannot be absorbed, the water vapor inside the film drips back onto the orchid leaves and freezes, as if they had been scalded by boiling water. If the freezing is slight, although they may recover, by late spring and early summer large areas of scorched tips gradually appear. Therefore, for simple warm sheds and on-the-spot film covering, cloth, blankets, burlap sacks, straw curtains and the like should be hung inside the shed to absorb moisture.
3. Fertilizer damage:
“Orchids like fertilizer but fear turbidity.” When fertilizing orchids, not only should the concentration be low and the interval long, but the amount applied should also be small. Fertilizing too strongly and too frequently easily injures the orchid roots and causes leaf tip scorch.
On the other hand, planting orchids generally does not require base fertilizer; if it is to be applied, the proportion should be small, generally not exceeding one percent. Organic fertilizer that has not been fermented, decomposed and sterilized should not be used as base fertilizer for planting orchids, to prevent it from fermenting in the pot and scalding the roots.
4. High temperature damage:
In the high temperature seasons of midsummer and golden autumn, in greenhouses and plastic warm sheds, under the radiation of the blazing sun the temperature reaches more than 30 degrees, even more than 40 degrees, and it is very stuffy and hot. The limited moisture in the site is consumed as soon as it rises and cannot reach the leaf tips, thus causing the leaf ends, leaf margins and leaf tips to scorch.
There is another situation: for orchid plants grown in narrow cylindrical pots, their root tips are close against the pot wall; when the temperature is high and the pot wall becomes hot, it also scalds the root tips and causes leaf tip scorch. Therefore, in high temperature seasons, shade, ventilation, and spraying water mist to lower the temperature should be carefully strengthened, and the pot walls also need to be sprayed wet.
5. Acid damage:
For growing media not changed for more than two years, due to factors such as frequent application of acidic chemical fertilizers, the medium becomes too acidic, which can also cause root decay. The solutions are:
1) Lift the seedlings and replace with new medium;
2) Water once with a 3000-fold solution of white alkali water to neutralize it.
6. Hormone damage:
Some people, in order to dwarf orchid plants, apply dwarfing agents such as “Duoxiaoli”. Because its toxicity is too great and its residual effect period is long, it causes black rot of the roots, resulting in loss of root function. The leaves cannot get sufficient nourishment and develop scorched tips, until the whole plant dies.
7. Nutrient deficiency damage:
Some people, in order to make the leaves dark green and produce more shoots, often increase the proportion of nitrogen fertilizer, causing the three essential elements of the fertilizer to lose their relative balance. The most common is the appearance of potassium deficiency symptoms, with scorching of the leaf margins at the ends and the leaf tips. Therefore, do not overuse nitrogen fertilizer, pay attention to supplementing potassium dihydrogen phosphate through foliar feeding, and also scatter some “reed grass ash” on the pot surface.
8. Diseases:
The most common root ailments are root rot, black rot, soft rot, bacterial wilt and so on, which affect the roots, causing the roots to lose function, leading to successive leaf tip scorch and then death of the whole plant. Therefore, whenever new shoots develop slowly or the leaves lose their spirit, the roots should be examined, and if problems are found, the seedlings should be lifted, washed clean, disinfected and replanted.
For this disease, prevention is important. Generally, the growing medium should not be too wet, and should tend toward dryness rather than toward nitrogenous fertilizer. After spring warmth and rising temperatures, water the roots with an 800-1000 fold solution of “Dexon”, once every ten days, for two consecutive times, and thereafter once each season, which can mostly effectively prevent trouble before it occurs.
2. Abnormal leaf growth
1. Physiologically, there are cases caused by over-application of nitrogen, making the leaves too numerous and tender and reducing stress resistance; cases of fertilizer and pesticide damage caused by spraying medicine or fertilizer at too high a concentration and too large an amount, accumulating at the leaf tips; cases of dry scorch caused by air humidity being too low; cases of freezing injury caused by winter insulation and water vapor accumulating at the leaf tips and freezing at too low a temperature; cases of injury caused by the leaf tips being in long-term contact with the pot wall and growing medium; cases of scorched tips caused by potassium deficiency, and so on. The causes should be targeted and avoided as much as possible. After spraying medicine or foliar fertilizer, it is best to spray clear water again eight hours later to wash off the medicine and fertilizer, which is more thorough.
2. Infectious diseases:
1) Tip scorch disease: its disease characteristics are: after infection, round or irregular brown spots appear, slightly sunken, and gradually turn coin-brown and withered.
2) Rust disease: its disease characteristics are: in the early stage of onset, many iron-brown raised objects about the size of sesame seeds bulge out on the underside of the leaves. When the spot granules rupture, their spores drift with the wind and spread in all directions.
3) Iron cannon disease: its disease characteristics are: disease spots mostly appear first at the leaf ends and leaf margins; their shape goes from small to large until they densely join into patches, and their color goes from yellow to brown, finally becoming black-brown.
The control methods are to avoid over-application of nitrogen fertilizer; do not plant too densely, and do not display the orchid pots too crowded together; once disease occurs, promptly cut off the infected parts (cutting one or two cm more), and burn them in a concentrated way; for chemical control, pay attention to spraying the underside of the leaves, and spray once every other day for three consecutive times, which will control the disease relatively completely. The effective control agents include “Yili” 1000-fold solution; “Yipudan” 500-fold solution; “Dasheng 45” 500-fold solution; “Haosujing” 800-fold solution; “Shadufan” 500-fold solution.
3. How to distinguish physiological diseases from infectious diseases
The scorched parts of physiological diseases are uniform in color, with no speckled spots, and the spotted parts on the underside of the leaves do not release mold-like matter under moist conditions; infectious diseases first have spots, which gradually expand and join into patches and cause the entire section to scorch, and their spot color goes from light to dark, finally turning black, and under moist conditions, a mist-like matter is released.