What should you do about small black spots on the underside of orchid leaves?

In ourorchid growing, if we want to grow sturdy orchid seedlings free of disease spots, the first thing we must face is infectious diseases, which fall into four major categories: fungal, bacterial, viral and nematode. These have pathogenic bacteria that can spread through certain pathways, infecting one another, expanding from few to many, from a single point to a whole area, causing damage.OrchidThe pathogens of infectious diseases are mainly fungi, followed by bacteria and viruses; pests are few. Usually the fungi that harmorchidsare only a few species, such as Colletotrichum, Phytophthora, Fusarium, Rhizoctonia solani, Vermicularia, Phyllosticta, Cercospora and so on, surviving by parasitism or saprophytism. They cause disease under suitable conditions.
Pathogens infectingorchidsfirst come into contact with the infectable parts of the host plant and begin to reproduce and spread within the orchid body (mainly the above-ground tissues), then produce pathogenic effects, usually divided into three stages: invasion, incubation and onset. From the pathogen invading the host plant to the start of establishing a parasitic relationship is called the invasion stage; from the establishment of the parasitic relationship to the beginning of symptom appearance is called the incubation stage; the period after symptoms appear is called the onset stage. Pathogen, host and suitable environmental conditions, none of the three can be missing, are called the triangle relationship of plant disease. Simply put, latent in winter, onset in spring and summer, then latent again in winter. Fungi such as Fusarium and Vermicularia overwinter in diseased leaves and soil in the form of mycelium and acervuli, using acervuli as primary and secondary inoculum, completing the disease process the following year.
The invasion of pathogens is related to environmental conditions, among which humidity and temperature matter most. Within a certain range, humidity determines whether spores can germinate and invade, while temperature affects the speed of germination and invasion. In winter the temperature is low, and humidity is relatively lower than in spring and summer; at the same time, orchids enter dormancy, the invasion speed of pathogens is slow, but this does not mean they cannot harm orchids. In addition, light also has a certain relationship with pathogen invasion. For pathogenic fungi that easily invade through stomata, light can determine the opening and closing of stomata, thus affecting invasion. We have found that orchid plants with good light have strong disease resistance, while plants with poor light, because their stomata are often open, are prone to infection by pathogen invasion. Therefore, sunning orchids in winter is advocated by orchid growers.
Pathogens invading orchid tissues initially form local or spot-like infections, called local infection; local infection is relatively easy to treat. When local infection spreads and causes whole-plant infection, it is called systemic infection. At this point treatment is somewhat troublesome. Therefore, early detection and early treatment should be achieved. During the orchid growing period, most of the occasional symptoms have already been controlled by us, and the few plants withsoft rotstem rot have also been removed from the orchid house. After winter begins, the number and types of remaining pathogens that can threaten orchids are very few, so disease prevention in winter has a very ideal effect.
Another favorable factor is that in late autumn and early winter, due to the temperature, pathogens are in a suppressed state. Orchids gradually shift from vigorous reproductive growth to a dormant form, and bacteria, fungi, viruses and nematodes gradually become dormant, their resistance greatly weakened, so the efficacy of various chemicals can be fully exerted. As long as you pursue the pathogens relentlessly, the good results will be very obvious the following year, making this the easiest time of the year for orchid disease management.

What to clean in the winter garden cleanup
In winter, because the orchid house is closed, the air cannot be exchanged; this stable, sealed environment is very favorable for pathogens to overwinter. Although the winter temperature is very low, the vitality of pathogens is very strong. Moreover, because of daytime solar radiation, the temperature in the closed orchid house will still rise above 15°C, and pathogens will still slowly reproduce; after the temperature drops at night, pathogens will again be in a suppressed state, again and again, rising by day and falling by night. This year is again a rare warm winter; entering mid-November, the minimum temperature still hovers around ten-plus degrees, whereas in past years it would already be very cold at this time. If this warm winter condition continues, the growth in pathogen numbers will exceed the loss, and the accumulation will still be considerable. It should be noted that, due to the improvement of our orchid-growing facilities, even in the coldest days of the Sanjiu period, whether on balconies, in orchid rooms, in Jiangnan or beyond the northern frontiers, the temperature of our orchid-growing sites will not drop below 5°C. This is especially true for orchid houses caring for cold-sensitive orchids such asCymbidium ensifolium、Cymbidium tortisepalum、Cymbidium sinense.
Although it is said that in winter, because pathogen activity is weak, they will not cause too much harm to orchids, if chemicals are not applied regularly and in the right amount, pathogens will accumulate to a certain level and multiply rapidly in early spring. Therefore, after winter begins, spraying fungicides several times at regular intervals is very necessary.
At the same time, although the winter temperature is low, because we close the orchid house, this causes humidity to be too high. Gray mold can develop under conditions of low temperature and high humidity, causing water-soaked lesions on the sepals and petals, and in severe cases petal rot. It is said that in China it is only seen in Phalaenopsis and large-floweredCymbidium faberi, and the occurrence in Chinese orchids is unknown, but since gray mold is a common disease of flowers in winter, the author still advocates being cautious.
In spring, new orchid plants are prone to black spot disease, brown rust disease, crown rot and so on. The occurrence of these symptoms is caused by the accumulation of pathogens in winter. Black spot disease overwinters as mycelium or acervuli in dead leaves or soil; at a temperature of 10°C the spore germination rate can still reach 50%, and the following year, with the help of spring rain, it spreads in February and March. The control agent is 50% mancozeb at 1000 times dilution. Brown rust disease causes great damage to orchids and is difficult to control; only by emphasizing prevention over treatment can the disease be effectively controlled. Control with copper sulfate, jun du qing, boric acid, shi bing jing (containing triadimefon, which also controls scale insects) and others all give poor results. Once brown rust disease occurs, it spreads and expands quickly and is difficult to control, but as long as extensive management is avoided, the number of orchid flowers is appropriately controlled, and the flowering period is shortened, it helps improve the plant's disease resistance and reduces opportunities for pathogen invasion. For orchid plants without disease, spray thiophanate-methyl or triadimefon for prevention. Try to avoid using agents containing “copper” or other heavy metal ions.
Overwintering pathogens cause primary infection after the new generation of plants begins to grow in spring. In fact, once a pathogen infects successfully, it is very little affected by external factors; that is to say, contact fungicides are powerless against it, so everyone's advocacy of prevention first is very reasonable. Understanding the above principles, we can carry out proactive prevention and control. In addition to regular and measured chemical prevention during the growing period, removing pathogens during the dormant period can achieve twice the result with half the effort.

How to clean the orchid garden
1 Pruning First, prune diseased and dead leaves to eliminate places where fungi can parasitize. When cutting off dead and diseased leaves, pay attention to being gentle in movement, accurate and decisive in action, and carefully place the cut dead and diseased leaves into plastic bags, in order to prevent spores from scattering after vibration; the cut diseased leaves should be centrally destroyed.
2 Garden cleanup On sunny days, orchid growers should get to work clearing garbage, dust, weeds and so on from the orchid house, eliminating pathogens and intermediate hosts, and the dust outside the orchid pots should also be wiped clean.
3 Disinfection After cleaning the garden, first disinfect the site. Before orchids are brought into the house and placed, the orchid-growing site should be disinfected in advance. You can evenly spray the ground with 50 ml of formalin or potassium permanganate plus 10 kg of water per square meter, or spray the ground and walls with 50% carbendazim wettable powder or 36% thiophanate-methyl suspension at 500 times dilution. Then close doors and windows tightly, and after about 10 days open the windows to let the gas volatilize.
Next, disinfect the seedlings. When introducing plants in autumn, select sturdy seedlings and discard diseased and weak ones. Avoid letting the wolf into the house. Newly introduced varieties or wild-collected plants should first be treated with chemicals. Many people soak them in a 1000-800 times dilution of potassium permanganate solution for 15 minutes. Potassium permanganate is a strong oxidant; upon contact with organic matter it releases nascent oxygen, oxidizing and denaturing the active groups of proteins in the bacterial cells, thereby achieving a sterilizing effect, and it is a good fungicide. Low concentrations of potassium permanganate have an astringent effect, while high concentrations have a disinfecting and antiseptic effect, but its disinfecting action is brief and superficial.
4 Changing the potting medium For orchid plants that are managed normally yet grow very poorly, there is a great possibility that the potting medium is infected with pathogens, because a large portion of pathogens parasitize the soil. During the orchid's dormant period, we unpot it to check the root system and so on. If there are black roots or rotten roots, it shows that the root system has already been infected by pathogens and root rot has occurred; we then need to change its potting medium, disinfect it and replant it, in order to improve the growing environment of the orchid roots. At the same time, pour chemical solution into the orchid pot once or twice. What needs to be explained here is that root drenching only targets pathogens in the potting medium; if pathogens have already invaded the orchid roots, even the best chemical is ineffective, because the root cortex of orchid roots is thick and the chemical solution cannot get in. You can only use scissors to cut off the diseased roots, and you must not be soft-hearted, otherwise there will be endless trouble.