What to Do When Orchids Rot and Wither? Scientific Orchid Care to Stay Away from Orchid Killers
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The so-called soil-borne infectious diseases refer to diseases whose pathogens mostly spread and infect from substrate materials. These diseases share one biggest characteristic: for orchids they are all devastating and catastrophic, in most cases fatal with no chance of survival, preventable but difficult to cure. General non-soil-borne infectious diseases are normally only damaging, such as damaging the leaf surface, causing spots, and harming the plant's integrity and ornamental beauty; such diseases can be both prevented and cured.
Phytophthora disease, whose pathogens are Phytophthora palmivora and Phytophthora cactorum of the genus Phytophthora, subphylum Mastigomycotina. The typical symptoms of the former are mostly that in the early stage the upper part of the leaf stays green while the leaf base turns yellow, and when the environment is dry the leaf base breaks off at a touch; in the latter, in the initial stage the diseased part becomes shrunken with black spots, necrosis and rot, in the middle stage the leaf withers and shrinks, and in the late stage the stem base rots, similar to soft rot, and under hot and humid conditions it is basically very hard to distinguish from soft rot. The former is common in Cymbidium sinense.
Damping-off disease, whose pathogen is Rhizoctonia solani of the subphylum Deuteromycotina. The typical symptoms are root rot first, then yellow leaves, then withered leaves. Normally the roots rot first, and finally the leaves yellow, wither and die. Sometimes in the early stage one can clearly observe that most leaves appear green and shrunken. In most cases it takes several weeks from root rot to leaf death, and some take months to one to two years. It shows the same late-stage symptoms as Fusarium wilt, and the most important difference lies in the early stage, because Rhizoctonia solani rots the roots first and then yellows and withers the leaves, so in the early stage one can sometimes observe the leaves shrinking while still green, with the leaf margins curling inward, and the leaves yellowing and dying from the leaf tip toward the leaf base; overall the leaves go from yellowing to withering from tip to base. It is yellowing and withering on one side while drying up at the same time, and the new shoots are mostly already necrotic and rotted, or no new shoots are produced.
When humidity is high, "non-" white mycelium or sclerotia can be observed (I personally understand this as brown); "non-white" is mainly to distinguish it from southern blight below. The sclerotia and mycelium of Rhizoctonia solani overwinter in the material and on diseased tissue, have strong saprophytic ability, and generally can survive in soil for two to three years. Under suitable conditions, the pathogen directly invades the roots from wounds and surfaces to cause disease, and can invade via water sources, tools, organic fertilizer and unrotted materials. High temperature and moisture favor the spread of mycelium. When choosing preventive and curative agents, they must be targeted at mycelium and sclerotia. Otherwise it is simply difficult to prevent and cure.
Fusarium wilt, whose pathogen is Fusarium oxysporum of the subphylum Deuteromycotina. The typical symptoms are yellow leaves first, then root rot, and in severe cases root death and bulb rot. The plant can wither and die rapidly in a short time, some in one to two days, some in several weeks. Fusarium infects and destroys the vascular bundles, causing the plant to die from dehydration and lack of nutrients. This is a necrosis from the inside out, which leads to the cut-off of water and nutrients. The leaf surface first turns yellow; after the leaves yellow in the early stage, we can observe on the yellow leaf surface purple-red necrotic areas, mostly on one side of the main vein, appearing on one side rather than the whole leaf, because of the necrosis of the vascular bundles inside the leaf. This phenomenon mostly starts from the marginal leaves; when the entire vascular bundle is necrotic, the leaf also dries up and necroses completely. Its difference from damping-off disease as seen on the leaf surface: damping-off is yellow on one side and withered on the other, withering from the leaf tip to the leaf base, and mostly starts from the central leaves; Fusarium yellows first, then one side turns purple-red, then the whole leaf withers without an obvious gradual dying process, and it mostly starts from the marginal leaves.
Secondly, damping-off disease rots the roots first and then shows on the leaf surface; Fusarium damages the above-ground parts first and then rots the roots. Finally, damping-off disease basically always causes necrosis of the new shoots and no new shoots are produced; basically when the leaves are dry and withered, no new shoots can be seen; Fusarium basically does not show on the new shoots, and many times when the diseased seedling has dried up and died, the new shoots can still be seen snow-white, pure green and outwardly healthy. Also, the incubation period to the manifestation period to the necrosis period of damping-off disease is a very slow process, and in many cases when we see the leaves scorched and withered, we cut while they wither, and by the end there is nothing left; Fusarium goes from leaf yellowing to death very quickly, especially once purple-red appears on the leaf surface, it basically dries up within one or two days.
Southern blight, whose pathogen in the sexual stage is Corticium centrifugum of the subphylum Basidiomycotina, and in the asexual stage is Sclerotium rolfsii of the subphylum Deuteromycotina. The typical symptoms are stem rot first, then yellow leaves, then withering from bottom to top, and in severe cases root rot. In the early stage one can first observe brown lesions with exuding fluid on the stem, and on the surface dense white mycelium can be clearly observed. With the bulb as the center, it gradually spreads upward and downward. Finally, small sclerotia the size of rapeseed, similar to those of Rhizoctonia solani, can be observed. Its differences from Rhizoctonia solani and Fusarium are, first, that white mycelium can be seen, and in terms of symptoms it starts rotting from the stem.
Nematode disease, whose pathogen is nematodes. It is nematode damage, not pathogen damage and not ordinary insect damage, so when preventing and controlling it, drugs for preventing and controlling pathogens or insect pests are basically ineffective, and can be understood as absolutely ineffective. Nematodes first pierce and suck cells with their stylet, then inject secreted liquid into the cells, liquefy the useful substances in some cells, and then suck them through the stylet into the esophagus. This special feeding method determines that the diseased parts attacked by nematodes will show a liquefied, greasy appearance. In orchids, nematodes in most cases invade the roots, and due to the destruction of the root system, the upper leaves of the plant show poor water and nutrient supply, mostly leaf tip dehydration and yellowing and withering of leaves. The typical symptoms are greasy roots with linear thin worms. Some can be seen directly on the stem, and some can only be seen by turning over the roots.
Bacterial soft rot, whose pathogen is Erwinia, with a Gram-negative staining reaction. The typical symptoms are rot and foul odor at the leaf base of new seedlings or new shoots, with the upper part of the leaf staying green. Bacterial soft rot mostly occurs on new seedlings or new shoots. Because bacteria can multiply once in twenty to thirty seconds under favorable environmental conditions, this determines its rapid onset; generally it can completely rot the base of a new seedling in two to three days. In the early stage, small water-soaked spots can be seen at the leaf base, and in about a day the lesions can be seen expanding rapidly. When the disease is severe, the cell contents at the rotting site are rapidly lost and dry up, and sometimes we can see that the upper part is mostly intact while the lower part is dry and decayed, with only part of the vascular bundles or fiber bundles at the veins remaining. On old leaves it mostly appears as bacterial brown spot disease. The symptoms are black spots or water-soaked spots in the early stage, rapidly growing in large patches, and finally perforation appears when drying up. Sometimes bacterial spot disease can also cause bacterial soft rot. The typical symptom of bacterial spot disease on new seedlings is that it mainly affects the leaf tip; generally a small black dehydrated spot first appears at the leaf tip, then rapidly expands into a short black dehydrated section, and the boundary between the lesion and healthy tissue is clear with no yellow halo transition. Under hot and humid conditions it will turn into bacterial soft rot. If observation and control are not timely, it can rapidly cause large areas of new shoots to rot.
7) Rot after fermentation, mainly appearing in deep-planted fine-soil environments. The leaf core of an orchid seedling is like a cup, which can hold water and can also hold part of the solution when fertilizing. When we water and fertilize, because the new shoots are small and tender and buried deep in fine soil, when the leaf core holds water it blocks air and creates an environment similar to fermentation, producing a large amount of harmful gas, causing the new shoots to rot. At the same time, due to water evaporation, the concentration of the solution in the leaf core gradually rises, causing reverse osmosis, which weakens cell physiology and accelerates rotting.
The above seven types are what we commonly call in orchids damping-off disease, Fusarium wilt, damping-off of seedlings, black rot, stem rot and root rot. Normally the above diseases must be observed and treated in time, but due to our experience and common sense as well as the particularity of the above pathogens, most are very difficult, because when we observe yellowing and withering of leaves, it is already too late. So in my personal view, prevention is first, and can be said to be the most important. But now, because research on orchid diseases has not yet gone deep, and also because quite a number of manufacturers do not produce orchid medicines legitimately, most are just profit-obsessed people who tear off the packaging of some ready-made medicines, change the name, exaggerate the uses and sell them to orchid growers, which also leads to the current difficulty in controlling the above six pathogens in the orchid industry.
When I go online and read various orchid magazines, the descriptions of such diseases can be said to be diverse and misattributed; some confuse fungal diseases with bacterial diseases, some confuse different fungal diseases with one another, some confuse soil-borne infectious diseases with ordinary spot diseases, and some even confuse biological infectious diseases with physiological diseases. Feeling that everyone is unfamiliar with this, I have written out what I know as a reference. I hope everyone will carefully diagnose and distinguish, clarify the pathogen, so as to facilitate the selection and use of medicines for prevention and control. Of course, there may also be orchid seedling withering and rotting caused by other pathogens. Due to limited conditions, we can only, based on the pathogens currently published by authoritative scientific research institutions, combined with our own years of observation, data investigation and test submission results, give everyone a further clear explanation to facilitate pathogen control. Due to human factors, such as understanding and description, certain differences may be caused; please forgive this.
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