How do you deal with small black spots on the underside of orchid leaves?
In orchid growing, if you want to raise robust seedlings free of disease spots, the first thing you must face is infectious diseases, which fall into four major categories: fungal, bacterial, viral and nematode. These have pathogenic fungi and can spread and propagate through certain routes, infecting one another, expanding from few to many, from a single point to a whole area, causing damage. The pathogens of infectious orchid diseases are mainly fungi, followed by bacteria and viruses; pest damage is very rare. Usually only a few fungi harm orchids, such as Colletotrichum, Phytophthora, Fusarium, Rhizoctonia solani, Pestalotiopsis, Phyllosticta, Cercospora and so on, surviving by parasitism or saprophytism. Disease develops under suitable conditions.
The pathogen that infects orchids first comes into contact with the infectable parts of the host plant and begins to reproduce and spread within the orchid (mainly in the above-ground tissues), then produces its pathogenic effect. This is 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 first appearance of symptoms is called the incubation stage; the period after symptoms appear is called the onset stage. The pathogen, the host and suitable environmental conditions - none of the three can be missing - this is known as the disease triangle. Simply put, it lies dormant in winter, breaks out in spring and summer, and lies dormant again by winter. Fusarium, Pestalotiopsis and the like all overwinter as mycelium and acervuli in diseased leaves and soil, using the acervuli as primary and secondary inoculum to complete the disease process the following year.
The invasion of the pathogen 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, and the invasion speed of the pathogen slows, but this does not mean it cannot harm the orchids. In addition, light also has a certain relationship with pathogen invasion. For pathogenic fungi that easily invade through stomata, light can determine whether the stomata are open or closed, and thus affect invasion. We have found that orchid plants receiving good light have strong disease resistance, while plants receiving poor light, because their stomata are often open, are prone to infection by invading pathogens. That is why exposing orchids to the sun in winter is advocated by orchid growers.
When the pathogen invades orchid tissue, it first forms a local or focal infection, called local infection, which is relatively easy to treat. When local infection spreads and causes infection of the whole plant, it is called systemic infection. At this point treatment has become somewhat troublesome. Therefore, early detection and early treatment should be achieved. During the orchid's growing period, most of the occasional disease symptoms have already been brought under control by us, and the few plants with soft rot or stem rot have already been removed from the orchid room. After winter sets in, the number and variety of remaining pathogens that can threaten the orchids are very few, so carrying out disease prevention in winter gives ideal results.
Another favourable factor is that in late autumn and early winter, because of the temperature, the pathogens are in a suppressed state. Orchids gradually shift from vigorous vegetative growth to a dormant form, and bacteria, fungi, viruses and nematodes gradually go dormant, their resistance greatly weakened, so the effectiveness of various chemicals can be fully brought into play. As long as you pursue the pathogens relentlessly, the good results will be very obvious the following year, making this the easiest time of year for orchid disease management.
What to clean up in the winter garden cleanup
In winter, because the orchid room is closed and the air cannot be exchanged, this stable, sealed environment is very favourable for pathogens to overwinter. Although winter temperatures are very low, pathogens have very strong vitality; moreover, because of daytime solar radiation the temperature in a closed orchid room will still rise above 15°C, so pathogens will still slowly reproduce, and after the night temperature drops the pathogens will again be in a suppressed state - over and over like this, rising by day and falling by night. This year is also a rare warm winter; by mid-November the minimum temperature is still hovering around ten-plus degrees, whereas in past years it would already be very cold by this time. If this warm winter situation continues, the pathogens' growth will outnumber their losses, and the accumulated total will still be considerable. It should be noted that, thanks to the improvements in us orchid enthusiasts' growing facilities, even in the coldest days of winter, whether on a balcony, in an orchid room, in Jiangnan or beyond the northern frontier, the temperature of our orchid-growing sites will not drop below 5°C. This is especially true for orchid rooms keeping cold-sensitive types such as Jianlan (Cymbidium ensifolium), Lianban (Cymbidium tortisepalum) and Molan (Cymbidium sinense).
Although it is said that in winter, because the pathogens' activity is weak, they will not cause too much harm to orchids, if chemicals are not applied regularly and in the right amounts, the pathogens will accumulate to a certain level and then multiply rapidly in early spring. Therefore, after winter sets in, spraying fungicide several times at regular intervals is very necessary.
At the same time, although winter temperatures are low, because we close the orchid room, this causes humidity to be on the high side. Grey mould can develop under conditions of low temperature and high humidity, causing water-soaked lesions on the sepals and petals, and in severe cases rotting of the petals. It is said that in China it has only been seen on Phalaenopsis and Cymbidium; the situation with Chinese orchids is unknown, but since grey mould is a common disease of flowers in winter, the author still advocates caution.
In spring, new orchid growth is prone to black spot, brown rust, crown rot and so on; the occurrence of these symptoms is caused by the accumulation of pathogens in winter. Black spot overwinters as mycelium or acervuli in dead leaves or soil, and at 10°C the spore germination rate can still reach 50%; the following year it spreads with the spring rain in February and March. The control chemical is a 1000-fold dilution of 50% mancozeb. Brown rust does great damage to orchids and is difficult to control; only by emphasising prevention over cure can the disease be effectively controlled. Control with copper sulfate, Junduqing, boric acid, Shibingjing (containing triadimefon, which also controls scale insects) and the like all give poor results. Once brown rust occurs it spreads and expands quickly and is difficult to control, but as long as extensive, careless management is avoided and the number of orchid flowers is appropriately controlled and the flowering period shortened, this helps improve the plant's disease resistance and reduces the chances of pathogen invasion. For orchid plants not yet affected by disease, spray thiophanate-methyl or triadimefon as a preventive. Try to avoid using chemicals containing "copper" or other heavy metal ions.
Pathogens that overwinter cause primary infection in spring after the new generation of plants begins to grow. In fact, once a pathogen has successfully infected, 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 well founded. Understanding the above principles, we can carry out proactive control: in addition to regular, correctly dosed chemical prevention during the growing period, removing pathogens during dormancy can achieve twice the result with half the effort.
How to clean up the orchid garden
1 Pruning First prune diseased and dead leaves to eliminate places where fungi can parasitise. When cutting off dead and diseased leaves, be careful to move gently, cut accurately and decisively, and carefully place the cut dead and diseased leaves into a plastic bag, in order to prevent spores from flying off after being shaken; the cut diseased leaves should be gathered and destroyed.
2 Garden cleanup On a clear day, orchid enthusiasts set to work cleaning up the rubbish, dust and weeds in the orchid room, eliminating pathogens and intermediate hosts; the dust outside the orchid pots should also be wiped clean.
3 Disinfection After the garden cleanup, first disinfect the site. Before the orchids are brought into the room 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 metre, or spray the ground and walls with a 500-fold dilution of 50% carbendazim wettable powder or 36% thiophanate-methyl suspension concentrate. Then close doors and windows tightly to seal the room, and after about 10 days open the windows to let the gas volatilise.
Next, disinfect the seedlings. When introducing plants in autumn, select strong 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 for 15 minutes in a 1000- to 800-fold dilution of potassium permanganate solution. Potassium permanganate is a strong oxidising agent; on contact with organic matter it releases nascent oxygen, which oxidises and denatures the active groups of proteins in the fungal cells, thereby achieving a sterilising effect - it is a good fungicide. At low concentration potassium permanganate has an astringent effect, while at high concentration it has a disinfecting and antiseptic effect, but its disinfecting action is brief and superficial.
4 Changing the potting mix For orchid plants that are managed normally yet grow very poorly, there is a strong possibility that the potting mix is infected with pathogens, because a large proportion of pathogens parasitise 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 means the root system has already been infected by pathogens and root rot has occurred, so we must change its potting mix, disinfect it and repot it, in order to improve the growing environment for the orchid roots. At the same time, drench the pot with chemical solution once or twice. It should be explained here that drenching the roots only targets pathogens in the potting mix; if the pathogen has already invaded the orchid roots, no matter how good the chemical is it will be ineffective, because the orchid root's velamen is thick and the chemical solution cannot get in - the only option is to cut away the diseased roots with scissors, and you must not be soft-hearted, otherwise there will be endless trouble.