Orchid leaves have small black dots on the back.
Today's sharing: related knowledge of orchids, a member of the Orchidaceae family. The orchid leaves have small black dots on the back, and the editor will introduce them next.
In order to raise healthy orchid seedlings without disease spots, the first challenge is infectious diseases, which are divided into four categories: fungal, bacterial, viral, and nematodal. These pathogens can spread and infect each other through certain channels, expanding from a few to many, from a point to an area. The main pathogens of infectious diseases in orchids are fungi, followed by bacteria and viruses, with few pests. Usually, only a few types of fungi can harm orchids, such as Colletotrichum, Phytophthora, Fusarium, Rhizoctonia, Alternaria, Phyllosticta, and Cercospora, surviving by寄生 or saprophytism and becoming symptomatic under suitable conditions.
The pathogen that infects orchids first comes into contact with the host plant's susceptible parts and begins to multiply and expand inside the orchid (mainly above-ground tissues), then causes disease, typically divided into three stages: invasion, latency, and disease onset. The period from the invasion of the pathogen to the establishment of a parasitic relationship is called the invasion stage; the period from the establishment of the parasitic relationship to the appearance of symptoms is called the latency stage; and the period after symptoms appear is called the disease stage. Pathogen, host, and suitable environmental conditions are all essential, forming the triangle of plant diseases. Simply put, they lie dormant in winter, become symptomatic in spring and summer, and then lie dormant again in winter. For example, Fusarium and Colletotrichum overwinter in the form of mycelium and conidial plates in diseased leaves and soil, using conidial plates as the initial and secondary inoculum to complete the disease cycle the following year.
The invasion of the pathogen is related to environmental conditions, with humidity and temperature being the most significant factors. 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, while orchids enter a dormant period, the speed of pathogen invasion is slow, but this does not mean they cannot harm orchids. In addition, light is also somewhat related to pathogen invasion. For pathogenic fungi that easily enter through stomata, light can determine the opening and closing of stomata, thus affecting invasion. We have found that orchids with good lighting have strong disease resistance, while those with poor lighting are often in an open stoma state, making them more susceptible to infection. Therefore, sunbathing orchids in winter is advocated by orchid enthusiasts.
When a pathogen invades orchid tissue initially, it causes localized or spot-like infections, known as local infection, which is relatively easy to treat. When local infections spread, they cause systemic infections, which are more troublesome to treat. Therefore, early detection and treatment are essential. During the growth of orchids, most of the periodic symptoms have been controlled, and the few cases of soft rot and stem rot have been removed from the orchid house. After entering winter, the number and variety of pathogens that can pose a threat to orchids are few, so preventing diseases in winter is very effective.
Another favorable factor is that in late autumn and early winter, due to the temperature, pathogens are in a suppressed state. As orchids transition from vigorous reproductive growth to a dormant form, bacteria, fungi, viruses, and nematodes gradually become dormant, their resistance decreases significantly, and various medications can take full effect. As long as the pathogen is relentlessly pursued, the results will be very noticeable the following year, making it the easiest time for managing orchid diseases.
What to clean in the orchid garden in winter
In winter, due to the closure of the orchid house, air cannot be replaced, creating a stable and sealed environment that is very favorable for the overwintering of pathogens. Although the temperature is very low in winter, the vitality of pathogens is strong, and the temperature in a sealed orchid house can still rise to over 15°C due to sunlight radiation during the day, allowing pathogens to slowly reproduce. When the temperature drops at night, pathogens become suppressed, and this cycle continues. This year has been an unusually warm winter, with the lowest temperature still hovering around 10 degrees Celsius in mid-November, which is much warmer than usual. If this warm winter persists, the number of pathogens will exceed the number of losses, accumulating to a considerable amount. It should be noted that due to the improvement of our orchid friends' facilities, even in the coldest days of winter, whether on the balcony, in the orchid house, in the south or the north, the temperature of our orchid-growing areas will not drop below 5°C, especially in cold-sensitive orchid houses such as those for Cymbidium,莲瓣, and墨兰.
Although it is said that pathogens have weak activity in winter and will not cause significant harm to orchids, if fungicides are not applied regularly and in the right amount, the pathogens can rapidly proliferate in early spring. Therefore, it is necessary to spray fungicides periodically after entering winter.
At the same time, although the temperature is low in winter, the closure of the orchid house creates a high humidity environment, allowing gray mold to occur under low temperature and high humidity conditions, leading to water-soaked spots on the sepals and petals, and in severe cases, petal rot. It is said that this disease is only found in蝴蝶兰 and Cattleya in China, with unclear conditions for domestic orchids. However, since gray mold is a common disease in winter flowers, the author still advocates caution.
In spring, new orchid plants are prone to black spot disease, brown rust disease, and crown rot, which occur due to the accumulation of pathogens in winter. Black spot disease overwinters in the form of mycelium or conidial plates in dead leaves or soil, and the spore germination rate can reach 50% when the temperature is 10°C. The disease spreads with spring rain in February and March, and the control agent is a 50% mancozeb solution at 1000 times dilution. Brown rust disease causes significant damage to orchids and is difficult to control, with prevention being more effective than treatment. Fungicides such as copper sulfate, virucide, boric acid, and虱病净 (containing triadimefon, also treating scale insects) are all ineffective. After brown rust occurs, it spreads rapidly and is difficult to control, but avoiding粗放 management, controlling the number of orchid flowers, shortening the flowering period, and enhancing plant disease resistance can reduce the chance of pathogen invasion. Spray uninfected orchid plants with thiophanate-methyl and triadimefon for prevention. It is best to avoid using fungicides containing "copper" or other heavy metal ions.
Pathogens that overwinter cause initial infections in the new generation of plants in spring. In fact, once the pathogen successfully infects, it is less affected by external factors, meaning that contact fungicides are ineffective. Therefore, the emphasis on prevention is well-founded. Understanding these principles allows us to take proactive measures, such as regular medication prevention during the growing period and removing pathogens during the dormant period for twice the effect.
How to clean the orchid garden
1. Pruning: First, prune the diseased and dead leaves to eliminate the寄生场所 for fungi. When pruning diseased and dead leaves, be gentle, accurate, and decisive, and place the pruned leaves carefully in a plastic bag to prevent spores from scattering. The pruned leaves should be disposed of集中销毁.
2. Cleaning: On sunny days, orchid enthusiasts should clean the garbage, dust, and weeds in the orchid house to eliminate pathogens and intermediate hosts, and also wipe the dust off the outside of the orchid pots.
3. Disinfection: After cleaning, disinfect the area. Before placing orchids in the house, the growing area should be disinfected. You can spray 50 ml of formalin or potassium permanganate in 10 liters of water per square meter on the ground surface, or use a 50% wettable powder of carbendazim or a 36% methyl thiabendazole suspension at a dilution of 500 times. Then, close the doors and windows, and open the windows after about 10 days to allow the gas to dissipate.
Also, disinfect the seedlings. In autumn, select strong seedlings and eliminate weak and diseased ones to avoid introducing pathogens. New varieties or wild orchids should be treated with medication first. Many people use a 1000-800 times dilution of potassium permanganate solution to soak for 15 minutes. Potassium permanganate is a strong oxidant that releases new ecological oxygen when it encounters organic matter, oxidizing the active genes of proteins in the pathogen, thus disinfecting. Low concentrations of potassium permanganate have astringent properties, while high concentrations have disinfectant and antiseptic properties, but its disinfectant effect is temporary and superficial.
4. Change the growing medium: For orchids that are not growing well despite normal management, it is likely that the growing medium is infected with pathogens. Since a large part of the pathogens are parasitic in the soil, we turn the pots over during the dormant period to check the roots, etc. If there are black or rotting roots, it indicates that the roots have been infected by pathogens, and root rot has occurred. We need to change the growing medium, disinfect, and replant to improve the growing environment for the orchid roots. Also, apply fungicides to the pots one to two times. It should be noted that drenching is only for pathogens in the growing medium, and if the pathogens have invaded the orchid roots, even the best fungicides are ineffective because the root cortex is thick, and the fungicides cannot penetrate. You must cut off the diseased roots with scissors, as being lenient can lead to future problems.
With the detailed introduction provided above on how to deal with small black dots on the back of orchid leaves, have you understood?