How to prevent and control crop diseases in hot and humid weather

Entering midsummer, humidity and temperature both rise substantially,orchidsenter their peak reproductive season. Watching the orchid shoots grow vigorously every day brings a bit of the joy of harvest. I believe orchid enthusiasts look at them several times a day, carefully observing their growth. When disease appears, it inevitably affects one's mood. In fact, hot and humid weather conditions favor the occurrence of disease. High temperatures alter the physicochemical properties of protoplasm, reducing the dispersibility of biocolloids, causing massive exosmosis of electrolytes and non-electrolytes, transforming ester compounds into layered forms, and sometimes even leading to destruction of cell structure, making the nucleus swell, loosen and rupture. High temperatures also change respiratory intensity, blocking synthesis within the plant, so that decomposition of protoplasm exceeds synthesis; this damage is especially severe when light is insufficient while temperatures are excessively high. During management, we must always pay attention and strengthen defenses. Following the plant protection principle of emphasizing prevention over cure, treat early, treat small, treat thoroughly, so that a year of hard work is not ruined by negligence. To preventorchiddiseases, besides measures such as keeping the environment clean and moist, ensuring ventilation, watering and fertilizing lightly and evenly, applying chemicals at the right time and in the right amount, keeping the medium clean and well-draining, and providing suitable light, we must also adapt to local conditions, grasp the main contradictions and carry out targeted prevention and control. The midsummer season is hot and humid with high air humidity; it is the golden season fororchid plantsgrowth during the year, but also the peak period for various diseases. To avoid disease, we pay attention to two things: first, controlling watering; second, applying chemicals promptly. If the medium is wet, stuffy and unclean, or overly dry: when the medium is wet, stuffy and unclean, air is displaced, pathogens multiply massively, orchid roots are damaged, and various diseases arise from this, harming the orchid plants. Orchid plants struggling on the edge of life and death will, to sustain life, open their stomata wide to absorb moisture from the air, and various pathogens take the opportunity to invade, with disease following. Orchid plantsenduring the intense summer heat consume more and their resistance is relatively weakened. Some believe that in the high-temperature season, because temperatures are too high, diseases are not obvious and it is enough to do well in ventilation and shading. Actually this is not so. At present we are in a hot and humid season, which is precisely the peak period for flower diseases. The main transmission routes of pathogens are: 1. The plant suffers artificial mechanical wounds, and pathogens invade through the wounds; when encountering high temperatures of 27-30°C, combined with poor ventilation and poorly draining medium, this disease occurs and develops relatively quickly. 2. Infection through insect bites on flower plants. 3. Factors such as the flower-growing environment not having been thoroughly sterilized and disinfected, making plants susceptible to disease. If there is a long drought and high temperatures followed by sudden heavy rain causing a sharp drop in temperature, medium humidity increases and pathogens spread. In rainy seasons, attention should be paid to preventing common diseases such as black spot and anthracnose; before the mold season or after rain, thiophanate-methyl, carbendazim, Huakang No. 2 and others can be used alternately, spraying the leaf surface once every 7-10 days. If insect pests occur, apply Huakang No. 1, decis and other appropriate treatments promptly. In daily management, first, we must pay attention to ventilation, lighting and drainage to create a good environment; second, we must cultivate robust orchid seedlings to enhance their own resistance to disease; third, we must strictly quarantine and disinfect to prevent the spread of external pathogens and pests, and thereafter spray pesticides regularly to eliminate pests and diseases before damage occurs. Before planting, strictly disinfect orchid plants and potting material to reduce and eliminate mechanical wounds and contact transmission caused by insects and humans. The knives, scissors and tools we commonly use must be disinfected before use. For disinfectants, generally 0.1% potassium permanganate, 5% carbolic acid or 75% alcohol can be used for disinfection. They can also be briefly held over a flame. During the plant's growing period, if diseased leaves are found, they should be cut off promptly, and the cut waste should be cleaned up, buried deep, and preferably burned. To summarize, the diseases we commonly encounter are the following. 1.Orchidshoot rot symptoms and control measures Orchid shoot rot is a bacterial disease. This disease can destroy a year of our hard work. It mainly occurs on the shoots; initially a water-soaked spot the size of a mung bean appears at the base of the shoot, later becoming a dark green scald-like large patch extending to the outside of the shoot sheath, showing dark brown rot, and diseased leaves are easy to pull out. New shoots rot away. Control methods: ① Control soil moisture content; affected potted flowers should stop being watered, and watering resumes only after the disease is controlled; ② When new shoots emerge 4-5 cm above the surface, spraying with 0.01% boric acid solution or 1000x diquat-diquat, shakujing has a certain effect, and streptomycin can also be used for control. 2. Bacterial brown rot symptoms and control measures Bacterial brown rot has a wide distribution range and is a worldwide disease. It is more commonly seen in China, Japan and the United States. The pathogen is caused by Erwinia cypripedii, mainly parasitizing Cymbidium, Cypripedium, Paphiopedilum, Phalaenopsis, etc. Symptoms appear on leaves as water-soaked yellow small spots that later turn chestnut brown and may sunken; they often rapidly spread to successively emerging leaves, then destroy the leaves or even the entire plant. This disease is mostly seen in humid, warm environments. Control: remove the diseased parts. Then spray with 200 mg/L agricultural streptomycin, or 0.5% Bordeaux mixture, once every 10 days, with good results. 3. Symptoms and control methods of orchid rot This disease mostly occurs in the humid, rainy midsummer season; common types include black rot,soft rotdisease, etc. Black rot: caused by Phytophthora fungi. It mostly occurs on leaves, roots and growing points; the diseased part first shows translucent spots, which then enlarge, and the tissue turns black and rots.soft rotdisease: caused by Erwinia and Pseudomonas bacteria. In the early stage, tiny brown water-soaked spots appear on the leaves, which then enlarge into gray, moist patches; with a slight touch or press the epidermis ruptures and exudes foul-smelling bacterial pus. For this disease, first keep the plant ventilated and reduce humidity; in the early stage, spray the leaf surface or smear the diseased spots with 50% captan 500x solution, 50% Tuzet 1000x solution, or garlic water (mash garlic, soak in water, and take the filtrate). Orchid root rot easily induces black spot. Therefore,orchid growingpotting soil should not be too wet ordinarily. In winter, orchids are in a dormant or slow-growing stage, and the potting soil should be on the dry side; in spring and early summer, an important period for orchid root growth and leaf emergence, the potting soil should be on the moist side; but long-term excessive moisture also easily causes root rot, leading to yellowing and even death of leaves. During plum rain or heavy rain seasons, potted orchids must be sheltered from rain, because with prolonged rain and excessive moisture, black spots most easily appear on new leaves, and summer orchid flower buds often show the "rotten sheath" phenomenon, affecting flowering and ornamental value. However, if the potting soil is too dry, it easily makes the pseudobulbs shrivel or causes other tissues to lose water and impair metabolism. Improper fertilizing of orchids, such as applying too much or too concentrated fertilizer, or pouring liquid fertilizer into leaf sheaths or splashing it onto leaves, also easily causes root rot, so when fertilizing, apply along the edge of the pot. 4. Symptoms and control methods of orchid root rot The root rot pathogen belongs to Rhizoctonia solani, mainly inhabiting thegrowingmedium; in environments with suitable temperature and humidity, it harms the root tissue of Chinese cymbidiums, causing reduced ability of roots to absorb water and nutrients. Transpiration and absorption become uncoordinated, the orchid plant gradually weakens, affecting normal growth. The root rot pathogen mainly infects the root system; on affected orchid roots, brown spots, sunken areas and necrosis appear initially. In severe cases, several spots form a ring encircling the stem base and newly grown roots or root system, and finally the entire root rots. This further affects water transport and support. The damage caused by the root rot pathogen cannot be detected with the naked eye unless the orchid plant is frequently taken out for observation. When we detect through leaf observation that the orchid plant grows sluggishly, leaf tips scorch, leaves wilt, and bracts turn black, it is already too late to seek solutions! By then the roots have already rotted away. Therefore, we must usually pay more attention: acidification of the medium, permeability, application of chemical fertilizers, whether water flushing is excessive, air circulation in the orchid room, cleanliness of the medium, thoroughness of chemical control, disinfection of plants before planting, and so on all affect the growth of orchid plants and cause the occurrence of root rot. Root rot is a medium-borne disease. It mainly survives adverse environments as dormant mycelium or sclerotia. It occurs most severely in wet, poorly aerated conditions, especially in expired old medium, and sometimes the entire root system of the whole plant rots. Control methods: (1) Usually pay attention to damage by root mites, root thrips, nematodes and cockroaches and other insects. (2) Apply beneficial microbial additives for control by using bacteria against bacteria. (3) After planting, supply an appropriate amount of phosphorus fertilizer in the early stage of new shoot growth and promote rapid root growth. (4) When diseased plants are found, immediately replace the medium; before planting, the orchid plant's rotted roots must be cut off, and after planting move it to a cooler place, applying amino acid at 2000x sprayed on the leaf surface every three days, so that because of insufficient photosynthetic capacity, the nutrients needed for growing new roots come from the decomposition of stored substances in the plant cells, thereby affecting shoot sprouting. (5) Within two days after spraying, do not use flood irrigation for watering; instead use spray watering, keeping the medium as dry and refreshing as possible, so healing speeds up. 5. Symptoms and control methods of orchid nematode disease Orchidnematodes are a parasitic disease of orchidaceous plants. They belong to Nematoda, Secernentea, Tylenchidae. Orchidnematodes: adults are linear, long cylindrical, tapering and slender at both ends, unsegmented, with the body composed of three germ layers and a pseudocoelom. Nematode eggs are very small, the smallest only 1 micron. Eggs falling into the potting material absorb water and swell when the material is moist; after hatching, larvae overflow and invade the root velamen tissue, moving upward along the plant into leaves or mesophyll tissue or storage tissue of the roots to absorb nutrients from the orchid plant, stimulating cortical cells and causing the formation of galls. After the adult nematode population matures, they mate and lay eggs inside the galls; entering winter, the eggs hatch into larvae that remain in the galls, and when spring returns the following year, the larvae overflow from the gall shells, are washed by rainwater and irrigation into the potting medium, and repeat the infection. In plants with this disease, in early summer the nematodes begin migrating to the host's root velamen and leaves, invading through stomata or hydathodes, causing phosphate-yellow round dots to appear on the leaves, visible on both surfaces. After autumn, these yellow dots become thick leathery scabby galls, round in shape. Roots show localized rough bending, then form flat tumor-like small galls. When roots degenerate and dry up in winter, the living galls remain in the potting material or culture medium. When temperatures rise to 18-22°C the following spring, larvae in the galls overflow and infect other roots and new individuals in the same pot. This repeats over and over. Orchid plants parasitized by nematodes mostly have thicker, harder leaves with scab-like structures, abnormal tillering, and mostly fail to produce flower stems. For the control of orchid nematode disease, prevention should be primary; the medium can be rinsed with 1% potassium permanganate solution, then washed clean with water, and used after drying. When dividing pots in winter and spring, leaves with galls should be cut off, and old roots, gall-bearing roots and dried bract leaves should also be cut off and burned. When round small yellow dots are found on leaves, immediately apply systemic insecticides such as aldicarb; spray the leaf surface and underside with 3% citric acid piperazine solution or 800x rotenone solution, or spray with 4% santonin water agent; the above solutions can also be used to drench the potting material so that orchid roots absorb them. 6. Symptoms and control methods of orchid snow mold The pathogen of orchid snow mold is a species of the genus Thanatephorus. It parasitizes Vanda and other orchid genera without pseudobulbs. Symptoms mainly show white powder appearing on the soil, and it is even seen at the base of plant stems or extending to other parts. This pathogen does not parasitize the orchid itself, but lives on the culture soil, consuming nutrients and water and affecting air circulation, thereby harming the orchid. Therefore, it is necessary to remove the infected culture soil and mix 40% quintozene or 50% carbendazim at 5-6 g/sq m with fine soil and then incorporate it into the soil. Plants should also be rinsed. 7. Symptoms and control methods of Chinese cymbidium necrotic ringspot disease The pathogen is Chinese cymbidium ringspot virus. Hosts are Cattleya, Cymbidium, Spathoglottis, etc. Symptoms show necrotic yellow ringspots or necrotic mottling on young and old leaves, the plant weak and stunted, and new and old shoots may die. Control: controlling viral diseases mainly involves removing diseased plants, improving environmental sanitation, and disinfecting tools to prevent spread. Currently the better disinfectant solutions are: 1. 2% formalin and % sodium hydroxide aqueous solution (cannot be used again after turning green); 2. 164 g anhydrous or 377 g crystalline trisodium phosphate added to 1 L (1000 cc) of water. It can be used to disinfect tools, environments, etc. 8. Symptoms and control methods of orchid wilt disease Orchid wilt disease, also called Paphiopedilum wilt disease, has a very wide distribution and is a worldwide disease. The pathogens are Fusarium oxysporum and Fusarium oxysporum f. sp. cattleyae. Hosts: Cattleya, Cymbidium, Dendrobium and other orchid genera. The symptoms of orchid wilt disease mainly infect the rhizome, turning it purple, with the vascular tissue turning pink-purple, and it can also extend to pseudobulbs and roots. Because the vessels become blocked, leaves turn gray and the plant withers and dies. Phalaenopsis infected with this disease often has accompanying root and stem rot. Control: remove diseased plants and infected soil, and spray, soak or drench the soil with 50% captan or 50% carbendazim wettable powder 500x solution. 0.5% formalin solution can also be used. Others such as anthracnose, southern blight and leaf blight have already been introduced by many orchid enthusiasts, so I will not go on about them here. Finally, let us talk about fungicides. To prevent and control diseases, we must use fungicides. Fungicides are compounds toxic to pathogenic microorganisms. Fungicides that can kill pathogenic microorganisms have a sterilizing effect, while fungicides that can inhibit spore germination or growth of pathogens have a bacteriostatic effect; both effects can achieve the purpose of preventing and treating diseases in horticultural production. Fungicides act in two ways: first, protective fungicides; second, systemic fungicides. Protective fungicides directly contact pathogens outside or on the plant surface, killing or inhibiting pathogens so they cannot enter the plant, thereby protecting the plant from harm by pathogens. Such fungicides are called protective fungicides, and their action has two aspects: first, after spraying, the chemical contacts pathogens and directly kills them, that is, "contact fungicidal action"; second, the chemical is sprayed on the plant surface, and when pathogens land on the plant and contact the chemical, they are poisoned and killed, called "residual fungicidal action." Systemic fungicides: after being applied to a certain part of the crop, they can be absorbed by the crop and transported within the body to other parts to act; fungicides with this property are called "systemic fungicides." There are two modes of translocation: first, acropetal translocation, that is, after the chemical is absorbed into the plant, it moves with the transpiration stream toward the top of the plant to the top leaves, terminal buds, leaf classes and leaf margins. Most current systemic fungicides belong to this type. The other is basipetal translocation, that is, after the chemical is absorbed by the plant, it moves downward in the phloem along the transport of photosynthetic products. Some fungicides such as fosetyl-aluminum can translocate in both upward and downward directions. Since fungicides act in different ways, their methods of use also differ, but fundamentally, the principles of disease control by fungicides are no more than three: chemical protection, chemical therapy and chemical immunization. Chemical protection means spraying fungicides before the plant becomes diseased to prevent the occurrence of plant disease. Some fungicides have only protective measures, generally two kinds: first, applying chemicals at the source of pathogens to eliminate the source of infection; the sources of pathogens are mainly places where pathogens overwinter, intermediate hosts and soil. The purpose of eliminating or reducing the source of infection by applying fungicides is to reduce the possibility of pathogens infecting crops. Second, spraying fungicides on growing crop plants that are not yet diseased but may be infected by pathogens, to prevent pathogen infection. After the crop surface is sprayed with fungicide, it can poison and kill pathogen cells or spores that come to infect the crop. To prevent infection of crops by soil-borne pathogens, treating plants with fungicides before planting belongs to this type of measure. Chemical therapy means applying fungicides after the plant becomes diseased or infected so that they act on the protected crop or on the pathogens, changing the pathogenic process of the pathogens and achieving the purpose of reducing or eliminating the disease. Prevention is more important than cure, and the effect of disease prevention is also better than that of disease treatment. Chemical therapy can be divided into three types: one is surface chemical therapy. Some pathogens, such as powdery mildew fungi, mainly attach to the plant surface; using lime sulfur mixture can kill the pathogens and produce a surface therapeutic effect, and non-systemic fungicides can control such diseases. One is internal chemical therapy, introducing fungicides into the crop body to treat pathogens that have already invaded the plant interior. Only systemic insecticides, such as thiophanate-methyl and carbendazim, have internal chemical therapeutic effects. Internal therapeutic systemic fungicides act in two aspects: on the one hand, they directly produce toxicity to pathogens; on the other hand, they change plant metabolism, changing its response to pathogens or the pathogenic process of the pathogens. But most systemic fungicides have only one of these effects, while some fungicides have both. One is external chemical therapy, that is, scraping off the diseased spots after the external part has been infected and become diseased, then disinfecting the wound with fungicide and applying a protective or waterproof agent to prevent further expansion of the infection. Some books say that penicillin, gentamicin and such things can be effective against orchid black spot, anthracnose and southern blight. Piyanping smeared on diseased spots has a special effect in treating anthracnose. In fact, the pathogens of black spot, anthracnose and southern blight are all fungi, while penicillin, streptomycin and gentamicin "kill" bacteria, and only some kinds of bacteria at that; they are not the antibiotics with the strongest sterilizing effect. Various medical textbooks and reference books all clearly point out: penicillin is ineffective against fungi. Gentamicin and streptomycin are also ineffective against fungi. Fungi and bacteria are different kinds of microorganisms with different biological characteristics, and their sensitivity to various chemical drugs differs enormously. The main active ingredient of Piyanping is dexamethasone acetate, a corticosteroid drug; such drugs have no inhibitory or killing effect on either fungi or bacteria. When speaking of using chemicals, one cannot fail to mention the drug resistance of pathogens. I once liked to use only one kind of chemical, and only later changed. Some orchid enthusiasts ask whether there is a specific cure; I do not know, perhaps there will be in the future. But there is not now. It is a fact that long-term use of one chemical produces resistance. The term "drug resistance" contains two meanings: first, the genetic material of the pathogen changes, and the resistant trait can be stably inherited; second, the resistant mutant has a certain fitness for the environment, that is, the resistant population has survival competitiveness similar to the sensitive population. Causes of drug resistance in pathogens and pests. Long-term single use of one chemical, through repeated elimination, preserves a small number of pests and pathogens with stronger resistance, which then continuously reproduce to form resistant populations. Among pests and pathogens with short life histories, rapid reproduction, large numbers and many generations, obligate parasitic fungi are more likely to develop resistance. These pests and pathogens reproduce rapidly, have many opportunities to contact chemicals, and develop resistance quickly. Some pests themselves have detoxifying enzyme substances; when a certain pesticide is used for a long time, detoxifying enzyme activity increases, which can turn the chemical in the body from highly toxic to low-toxic or non-toxic, and their resistance naturally increases; this is the physiological detoxification effect of pests, that is, internal drug resistance. In addition, under long-term action of chemicals, some pests make it difficult for the chemical to penetrate the insect body surface, thus forming morphological protection, that is, cuticular resistance. Among fungicides, systemic fungicides generally more easily cause resistance, such as benzimidazole insecticides (carbendazim, thiophanate, etc.), antibiotic fungicides, etc. But contact fungicides are less likely to cause resistance, so the best rotation combination. For example, dithiocarbamates, inorganic sulfur formulations and copper formulations are all good rotation varieties. Drug resistance is produced through continuous contact. I feel this deeply. My hometown has many orchards, and when people apply chemicals, who bothers with protective measures? On hot days, for coolness they go around bare-backed, letting the chemicals touch their bodies. On first contact with chemicals, they feel dizzy and muddled; over time they become accustomed to it. Once, an applicator went home hungry, could not be bothered to wash his hands, picked up a steamed bun and ate it, and surprisingly was fine. (Kuai Xue Shi Qing)