By Susan Jones
A danger to orchids any time the weather is rainy or damp for extended periods, black rot can quickly destroy an entire plant if left unchecked. Caused by one or both of the fungi Pythium ultimum and Phytophthora cactorum, black rot affects a wide variety of orchids. Cattleyas seem to be particularly susceptible.
Black rot moves quickly through a plant and can decimate an orchid collection if not treated
The fungi that cause black rot in orchids can live only in environments where water is available to them. They consist of spores capable of independent movement, called zoospores, which swim through the water. If that water happens to be sitting on an orchid leaf, the zoospores can penetrate the plant tissue and begin the next stage in their life cycle. At this point, the fungus develops a vegetative part or mycelium consisting of hyphae (thin, thread-like tubes), which spread quickly through the affected plant tissues. Once this occurs, the visible signs of infection – small, watery, translucent spots – expand quickly and change to brown and then black. Left untreated, the affected plant or plants may infect others and will probably die themselves.
When the fungus attacks seedlings, it is usually at the soil line. This can cause the little plants to topple and die, and is referred to as damping off.
In mature plants, the disease can strike at any point on the plant (leaves, pseudobulbs or roots), but often starts in new leaves or growing leads, and progresses downward through the plant until reaching the roots..
As it name implies, black rot shows up as dark blackish spots or lesions on the affected plant part. The black spot or spots enlarge rapidly and can spread throughout the plant. If allowed to reach the crown of a monopodial (single-stemmed) orchid, the rot will kill the plant. Affected leaves may turn yellow around the infected area, and the lesions themselves will be soft and exude water when pressure is applied.
Unsterilized pots, media or water sources, as well as water splashed from affected plants nearby, are all excellent means of spreading the fungus. In addition, plants that spend time outdoors should be elevated 3 to 4 feet (90 to 120 cm) above the ground to avoid splash contamination. Do not allow your orchids’ leaves to remain wet for any extended periods. Good air circulation in the growing area is key, and can help plants dry quickly after watering or rains. The addition of a fan or fans to the growing area can improve the airflow and help prevent rots. At first signs of infection, segregate all affected plants to avoid contaminating healthy plants. Finally, some growers recommend the use of a high-calcium fertilizer in the spring to help avoid black rot in new growth.
To stop the spread of black rot in your orchid, begin with a sterile knife and cut out the diseased portion of the plant. Remove all infected plant material, taking out a small amount of healthy tissue bordering the infected area as a safety precaution. Examine the cut portion carefully. It is sometimes possible, especially in pseudobulbs, to see a brownish discoloration extending into the interior of tissue that appears healthy on the surface. In sympodial (many-stemmed) plants such as a cattleya, it may be necessary to cut below the leaves and into the pseudobulb to stop the spread of the disease. If the plant is badly infected, cut into the rhizome, growth by growth, until you reach healthy tissue. For a monopodial orchid such as a vanda or phalaenopsis, remove the infected portion of the leaf, or the entire leaf if necessary, to halt the spread of the fungus before it reaches the crown. Place the plant in an area that receives good air circulation to allow the cut to dry.
Once the infected plant parts have been removed, a fungicide should be applied to protect the remaining healthy tissue. Cinnamon is an excellent fungicide, and ground cinnamon may be applied straight from the spice jar directly to the exposed area where infected portions of the plant have been excised. First Rays Orchids also recommends mixing cinnamon with either casein-based glue (such as Elmer’s) or cooking oil to form a thick paste. This paste is fairly waterproof and can be used to cover the wound.
Another poultice that was developed specifically for use on orchids is StopRot. The product contains a Bordeaux mixture in lanolin. The Bordeaux, a mixture of copper sulfate and lime, has fungicidal properties. The lanolin seals the wound to keep moisture out.
Whatever method is chosen, be sure to cover all of the exposed (cut) area with the fungicide or sealant, so that no new infection can be introduced to the open wound.
Drenching the affected plant with fungicide is also an option. Drenches of a protectant fungicide such as Truban or Terrazole are recommended if the disease is diagnosed early. For more advanced cases, a systemic fungicide such as Aliette or Subdue is more effective. Captan, Dithane M-45 and Physan 20 have also been recommended by some growers for control of black rot.
To protect seedlings against damping off, water with the fungicide of choice (at the dilution rate for seedlings specified in the product’s instructions) as soon as they are deflasked and potted in compots. Afterward, treat with a preventive fungicide at two-week intervals.
By Susan Jones
Like people, orchids, especially those that prefer cooler, shadier environments, can suffer from heat stress during the hot summer months. The midday sun is intense, especially during the summer, and the high temperatures that bright sunshine brings can burn your orchids quickly, especially the more sensitive types such as phalaenopsis, pleurothallids, paphiopedilums and miltonias.
Heat stress is a condition that occurs in orchids and other plants when excessive heat causes an imbalance in transpiration, the process by which moisture evaporates from the plant’s tissues. When that rate of evaporation exceeds the pace at which moisture is being replaced through water taken up by the orchid’s roots, the plant becomes dehydrated — leaves soft and wilted, pseudobulbs wrinkled and ridged. The instinctive response in this situation is to overwater the orchid to correct the dehydration. This can instead make matters worse by rotting the plant’s root system, further hampering its ability to take in much-needed hydration. The roots can only absorb so much water at one time. In addition to watering, control of heat and humidity play a significant role in the prevention and management of heat stress.
Placing a mounted plant in an empty clay pot can help retain moisture around roots. The microclimate of higher humidity helps prevent heat stress and aids stressed plants in recovering.
These symptoms may appear singly, even in otherwise fairly healthy orchids. When they appear in combination, however, it is usually an indication of heat stress or an underlying problem (such as root loss) that has lead to heat stress.
Too much light causes a plant’s chlorophyll to deteriorate, which can turn your orchid an anemic yellow-green, and eventually cause premature loss of leaves. Most healthy orchids receiving appropriate light levels will be a robust light green.
The culprit, especially in summer weather, can be overheating. If your plant’s leaves feel warm to the touch, they could be getting too much light, and need increased air circulation and shading, or a move to a more protected area.
Initial signs of sunburn, as in humans, appear as a reddish-purple tint or freckles on leaves and pseudobulbs. In advanced stages, leaf tips and roots may brown, flowers drop, buds blast or flower spikes fail altogether.
Your orchid’s pseudobulbs serve as its water-storage organs, so shriveled pseudobulbs indicate a dehydrated orchid – it is in a stressed state, having used up its moisture reserves.
This goes beyond mere wilting, and occurs in the advanced stages of heat stress, indicating severe desiccation and possible damage on the cellular level. Depending on how long the condition has persisted, the type of orchid and its overall health aside from the leaf damage, it may or may not recover once its leaves are parched.
Mounted orchids are particularly sensitive to higher temperatures and require additional water and humidity during hot weather. This Cattleya hybrid exhibits the yellowing leaves and wrinkled pseudobulbs typical of heat-stressed plants.
Your orchids need lots of attention during the summer months, especially on hot days. With higher temperatures, orchids require more frequent watering to prevent dehydration. High temperatures quickly dry out mounted orchids, those in baskets and even potted plants.
Maintaining the balance between keeping orchids sufficiently hydrated and over-watering can be a challenge. A simple rule of thumb is that an orchid’s need for water increases and decreases with the ambient temperature, so water thoroughly and more frequently during higher temperatures. Most mounted plants and those in baskets, especially vandaceous orchids and other genera grown with little or no media, will benefit from daily watering in warmer weather.
In addition, keep humidity high and air movement continuous, as both of these factors help plants keep their cool on hot days. Adding a fan to the growing area, or locating your orchids where they receive the caress of summer’s breezes, as well as spacing your plants to allow for sufficient air circulation between orchids, can make a big difference in their environment and overall health. This will also help to keep rots from fungi and bacteria at bay.
The benefits from misting orchids during hot weather do not have much sustained effect, so unless one has a misting system in place to maintain that added humidity throughout the day, it is probably not useful to your plants. When watering or misting, always be certain that plants have ample time to dry before night falls, so as to discourage bacterial and fungal growth.
When moving orchids outside for the season, start them out in a shaded spot and gradually increase their exposure to sunlight to acclimate them to the higher outdoor light levels. Make sure to protect them from the strongest sunlight as midday summer sun has more intense UV rays that can burn tender plants. Check the amount of exposure your plants receive at different times during the day as the sun moves, and throughout the year as the earth’s orbit around the sun makes its gradual seasonal changes, which in turn changes the angle of the exposure your plants receive.
For greenhouse growers, consider using shade cloth for added protection during the summer months. Windowsill growers may want to add a sheer curtain during the summertime to keep sunlight reflected by the window glass from causing sunburn.
Finally, always have someone care for your orchids while you are away on summer vacation.
Once an orchid has suffered the effects of heat stress, it may take a long time to rebound. Apply fertilizers sparingly, using a weak solution, and flush the growing medium with clean water between applications of fertilizer, as the salts in fertilizers are difficult for dehydrated plants to assimilate, and a regular dose may further burn the plant. Be especially alert for pests on distressed orchids – insects and viruses are more likely to attack weakened plants.
Jamaica Orchid Society was established to promote awareness and disseminate knowledge of commercial and botanical aspects of orchids; to project the importance of conservation and propagation of Jamaica orchids; and to further strengthen the orchid fraternity. The membership of society is open to all persons interested in orchids.
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