I am beginning orchid grower and have only recently purchased my first orchid from the local farm and garden supply. Having heard that orchids can grow in trees, I hang the plant in a tree in the backyard. It is strange to look at, with its leathery leaves and thickened stems, but from an unfamiliar sheath at the top soon came two of the most exquisite flowers I thought I had ever seen. The plant grew in a pot of rough, arid-looking bark chips, so I watered it diligently every day.
Soon after flowering, I noticed that my orchid was turning black from the bottom up. Within the week, the base of the leaves had become yellow. I ran in terror to bring the plant indoors, only to have every single leaf break and fall from the plant when I went to move it. In my trembling hands, all that remained of my cherished first orchid was a group of oozy, blackened stumps.
Such was my rude start into the black-thumbed — and red-faced — legion of orchid growers who have suffered with rot.
FUNGAL ROTS
Almost every orchid grower has had some experience with black rots, whether they readily admit it or not! Because these rots are very much lethal to plant tissue, and spread so fast, I would venture to guess that more orchids have been destroyed by them than by any other orchid pest or disease. Beginners may be possible to have more problems with rot, because of their tendency to overwatering, but even experienced growers can be caught oil-guard.
Black rots are fungi and caused by two closely-related fungi, Pythium ultimum and Phytophthora cactorum. They are possibly to attack any part of an orchid, usually working from the bottom up or the top down. Black rots are most insidious — and therefore most possibly to kill — when infection occurs first in the roots or rhizome. From there, the infection can spread to the entire length of the rhizome.
effectively destroying the plant before the grower even knows the problem. Only when the infection then moves up the pseudobulbs, normally preceded by a characteristic band of yellow, docs it become very easy to notice — but by this time it is far too late. The only living, green tissue then remaining is in the upper pseudobulbs and their leaves, now separated and without viable eyes, or roots, at their bases. In a surprisingly short time, the pseudobulbs also will turn completely black, the leaves turn yellow and hard but easily broken. The very sad end result: a lifeless, leafless, black “skeleton” (Figure 1).
It is more possibly to be detected before a complete destruction of the plant is done, when a black rot infection appears in a leaf or a pseudobulb. Infection may appear as a purplish-black softened spot, but can similarly spread downward with alarming speed (Figure 2). New leads are known to be likely to suffer from invasion by black rot, maybe because the tissue is “unhardened”, and therefore is easy to attack, or possibly because, as their leaves expand; they can cup and collect water, making ideal conditions for infection. Pythium is often responsible for the rot, or “damping off”, of small seedlings or mericlones in community pots. In these typically restricted conditions, this decimating rot can spread quickly from plant to plant. Orchid growers should always keep an eye on for the blackening of plant tissue, yet any period during which damp conditions exist in the growing area should be a time of intensified concern. On mature orchids, pseudobulb and rhizome sheathing should be routinely got rid of as soon as it turns brown and dries so that the tissue underneath can be better observed. Once a suspicious part of an area of darkened tissue is spotted, treatment should be in at once without delay. Because these destructive rots are so fast to spread, any delay could greatly endanger the plant involved.
for treatment of a black rot we first need to remove and destroy all infected tissue. we need it to be as clean as possible, and take at least one inch of green, uninfected tissue. In this way, the rot can be separated from the plant. If the infection is detected early enough to be confined to a leaf, or the very end of a pseudobulb or rhizome, this is a fairly painless affair (for the grower). The small portion is taken away and then the plant cured with a chemical drench or dip. But for more complicated infections the grower may have to face the pain of losing whole pseudobulbs, and their connecting rhizomes, in order to give a fighting chance to what is left of the plant.
Occasionally the oldest backbulb of an orchid will die. This may be natural death, but if the darker it then turns, the more suspicious the grower should become. Leaving such a pseudobulb attached to the plant is, at the very least, giving a potential avenue for infection (FIGURE 3). It is the best to take away this non-contributing plant part and chemically treat as described below.
Truban 30 WP (Terrazole) is recommended for control of these fungal infections. It should be used as a drench to the infected plant at the rate of 1 ½ teaspoons (1/2 tablespoon) per gallon of water. In other words, you can apply this dilute mixture of the wettable powder and water to the potting medium in much the same manner as you give water alone. The roots and medium should be soaked. The next follow-up applicationshould be done within a week to fully being able to protect the plant against reinfection.
ROOT ROTS
Another troublesome fungus, Rhizoctonia solani. This is the rot which is generally limited to only affect the mature orchid roots, and then only extending as far as the leaves of smaller mericlones. Unlike rots caused by Pythium and Phytophthora, this organism makes a gradual deterioration of flowering-size orchids caused by root loss. Plants in decomposing media are especially easily to attack. Specimen plants in large pots are tended to catch this root rot as well, in their growths towards the center of the pot. On the other hand, Rhizoctonia can be a real killer, in causing the damping-off of immature plants in community pots.
If at all possible, orchids infected with Rhizoctonia should be unpotted and the old media and dead roots got rid of. Then the plant can be dipped in a fungicide solution for five minutes and afterward potted up, or repotted first and then drenched with the solution. In both cases, you should follow the manufacturer’s recommendations with regard to dilution rates. Thiophanate-methyl fungicides are marketed under a variety of product names and concentrations; each having specific handling instructions. Care should be taken afterwards to keep the treated plants somewhat dry, and out of direct light, in order to encourage the creation of new roots.
“Fusarium wilt” makes symptoms happen similar to root rot in orchids and can also be treated with Thiophanate-methyl fungicides. Infected plants similarly suffer root loss followed by a general shriveling of pseudobulbs and leaves. The living thing (animal, plant, human), Fusarium oxysporum f. cattleyae, is well known to enter orchids via the cut ends of rhizomes and appears as a slightly purple band when the infected rhizome is cut. The Handbook on Orchid Pests and Diseases provide management strategies.
BACTERIAL ROTS
Orchid rots can also be bacterial in nature. This would not have great consequence to the orchid grower, except for the fact that bacterial rots are controlled by a somewhat different set of chemicals than those recommended for fungal black rots. The problem, then, is to differentiate between the two.
Generally, the bacterial brown or soft rots — identified variably as Erwinia, Pseudomonas or Enterobacter species — are a rot that annoys your orchids with “crowns”, those with centralized growing points such as Phalaenopsis and Paphiopedilum. Hence the common name “Crown Rot”, used most frequently when Phalaenopsis are attacked. These bacteria need wet surface conditions in order to go through and infect plant tissue, and this water holding trait is thought to predispose orchids with this kind of growth habit to bacterial infection. On the other hand, the fungal black rots present more of a problem to orchids with important pseudobulbs, such as the cattleyas. With these orchids, bacterial infections cause only limited, non-lethal, black spotting (Burnett, 1974).
The difference between fungal and bacterial rots from this point on is necessarily vague. One could argue that bacterial rots are generally chestnut-brown in color (FIGURES 4, 5, 6) and fungal rots purplish-black (FIGURES 1, 2, 3). Yet this is not always true (FIGURE 7). One could suggest that bacterial rots are less dangerous, apparently slower to spread, thereby giving the grower a better opportunity to act and control the infection. This also depends on the type and condition of the host plant. Regardless, the the end result remains: once both fungal and bacterial infections reach vital meristematic issue, be it in the “eye” of a cattleya pseudobulb or in the “crown” of a phalaenopsis, the damage is serious and all or part of the plant will be lost.
Like fungal rots, bacterial rots start as a patch of dark, necrotic tissue somewhere on the plant, and are most possibly to occur during damp environments. In the middle of a leaf, the first infection may appear as a spot or a watery swelling (FIGURE 4). Very often, bacterial infections will begin at the tip of a leaf and work their way down in a nearly fluid manner (FIGURE 5). If the infection get to the crown of the plant, additional leaves will become infected (FIGURE 6) and can separate even while still largely green. This occurs because their point of attachment to the plant is the first to go. In such a case as this, it is actually true that the infected plant falls to pieces (FIGURE 7) and is damaged so badly.
Bacterial infections have also been well known to cause sheath rot (McCorkle, 1974). The symptoms are likely to those described above, and can be differentiated from the normal drying and browning of the flower sheaths of some orchids by their watery character. Flower buds will also catch the disease and die, or “blast”, when it is present inside.
Control of bacterial brown or soft rots, like their fungal counterparts, firstly involves the removal of infested plant parts prior to the infection spreads to crucial areas. Infections which are still restricted further up the leaf (or sheath) should be cut off, again along with an inch or more of healthy tissue, in the hopes of removal of the infection in its entirety. Leaves which have been infected to or from the base portion are usually all but separated from the plant, and can easily be got rid of, even without a cutting tool. Spraying with an antibiotic should follow in at once. Physan is very effective in protecting orchids against further bacterial infection, and help prevent infection if used on a regular basis (Burnett, 1974; McCorkle, 1971). At the rate of 11/2 teaspoons per gallon of water, Physan should be sprayed generously on the infected plants to the point where it collects in the crowns and the axils of the leaves. It is often helpful following up with another spraying especially powerful if it is done within the week . Copper-based fungicides like Phyton 27 show bacterial control as well however care should be taken to test plants for toxicity. Some orchid genera are likely to suffer from copper. Some growers are successfully in treating crown rot in Phalaenopsis with powdered cinnamon; an expression of its anti-bacterial/anti-fungicidal properties.
PREVENTION
If you look at most of the orchid related pests feeding on the plant’s surface,then you see the orchid rots penetrate the tissue surface to spread and consume it all from within. Because of this fact, superficial spraying after infection has started is generally less effective than it is with pest infestations. If at all possible, fungal and bacterial rots need to be physically got rid of from the infected plant and destroyed. Then a spray or soak of the suitable chemical solution can be applied, basically to protect the remaining tissue from further infection.
The trick is to prevent penetration and therefore the infection for starters. These pathogens are everywhere in our growing environments, but while on the outside of the plants, they are far more disposed to any control measure taken by the grower. One can reduce the possibility of infection by these rot diseases in several ways. Because they require wet conditions in order to first penetrate tissue, efforts should be directed towards cutting the amount of time the plants are truly wet. It is hard in watering to avoid getting water on the leaves, yet if watering is done early in the day just before or as conditions become optimum for evaporation, the length of time water remains on the plant surfaces can become shorter a lot. Naturally, good air movement, helps greatly in this regard. Efforts should still be made to cut the splashing of water from plant to plant, as this is one major ways by which diseases are spread. After watering, water which has collected in crowns and new growths should be on time cut by tipping the plants, as it can persist some time even under drying conditions. If conditions are not conducive to rapid evaporation, as they can be on a dark, winter’s day, then watering should be put back.
Keeping things less than soaking wet for rot prevention applies to below-surface roots and rhizomes as well. Potting media which is decomposed or over-watered provides most suitable conditions for an insidious infection of black rot. Repotting very carefully is a part of every good prevention program. Good habits such as always using sterilized and appropriate cutting tools in order to make clean cuts also make a lot difference. I find flaming clippers or scissors with one of the cheep, disposable lighters on the market is a quick and easy way to sterilize. Others recommend boiling water, 2% formalin or Clorox dips, or the use of disposable blades for one-time-only use. Cutting one’s orchids with the same dirty blade is a highly efficient way of spreading rots, other diseases, and virus, throughout the whole collection. The wound made in the process of cutting any plant tissue is an open invitation to infection. Many experienced orchid growers seal this most likely upcoming infection avenue with a some kind of paste fungicidal powder. Some burn these cut surfaces with a hot blade (Thomas, 1980). General sanitation involving taking away any dead or infected plant tissue from the growing area also goes far in cutting the ammunition of the enemy.
Even with all these measures diligently taken, a grower is still likely to encounter bacterial or fungal rots at one time or another. The hobbyist should be particularly careful during periods of cool, dark and damp conditions which generally happen during the winter months. As previously mentioned, these are conditions when rots can happen in greatest frequency (though of course they can and do happen during the warm times of the year). Likely rot victims are the “children of the orchid world,” those small seedlings and mericlones found in every collection. Generally, the smaller and weaker the plant, the more susceptible it is to a quick and lethal attack, though this is not to say that no healthy, flowering-sized plants have given in! Specimen plants are also possible candidates for infection, especially when they are in large pots. In the center of these pots a core of medium which is the last to dry and the first to decay can form, on top of which are normally the oldest, weakest pseudobulbs. Many a specimen plant has lost its beginnings, its size, and perhaps its life to black or root rots which take advantage of this situation!
Finally, spraying at regular times should be considered as part of a general program to prevent rot infection. Physan has been advised for both fungal “damping off’ and bacterial rot infections of seedlings. Consan Pacific, Inc. manufacturer of Physan, advises a monthly “preventative maintenance” spraying of Physan at the rate of 1-1 ½ teaspoons per gallon of water. In this method, a protective surface can be created on the plants to make infection less likely to happen before it starts.
Next month, the discussion will focus on the less life-threatening, although still very bothersome leaf-spotting diseases.