A Detailed Explanation of the Causes and Control Methods of Orchid Two-Rot Disease

Author: Guan Qi

Two-rot means soft rot and stem rot. Soft rot starts rotting from the new shoot, while stem rot starts rotting from the old shoot. In my personal opinion, there is no essential difference between soft rot and stem rot. The difference lies in the fact that a plant with stem rot is beyond saving, while a plant with soft rot sometimes does not suffer total wipeout, but this is only the surface appearance.

      The occurrence of stem rot comes down to a few causes. One: the plant itself carries pathogens, so disease is unavoidable; Two: when dividing plants, if the wounds are not treated properly, pathogens invade the pseudobulb of the old shoot; Three: new buds become infected, and if not treated in time, the infection spreads to the old shoot.

      For healthy plants, as long as division is handled properly, stem rot caused by any reason other than new bud infection can be completely prevented (the details of division will be discussed another day). In other words, apart from the two causes of diseased plants and infection from diseased new buds, stem rot can be completely prevented. Most stem rot is caused by soft rot. The focus of rot prevention should be placed on preventing soft rot.

      The causes of soft rot are very complex. Based on observations over several years, I personally think the main causes are as follows:

      One: insect pests, mainly snails. Snails gnaw on new buds and bite a hole in the new bud. The hole can sometimes be very deep. When it reaches the fleshy pseudobulb of the new bud, if not discovered in time, bud rot is unavoidable.

      Two: before the new bud opens, it grows roots, and the roots break through the leaf sheath. At this time the leaf sheath is also fleshy and very tender. If it becomes infected due to causes such as soaking in unclean fertiliser water or fungal hyphae penetrating it, bud rot is also very likely.

      Three: when the new bud has grown to five or six leaves, a strong bud will produce second-generation buds. Sometimes the second-generation bud breaks through the leaf sheath but the leaf sheath does not rot; sometimes the leaf sheath does rot. The leaf sheath of a strong plant is very thick and fleshy. During the rotting process, cell death produces a lot of "rot fluid." This liquid soaks the new bud for a long time and very easily causes the new bud to turn mushy. The bud and the mother shoot are "heart to heart." If the bud rots undetected, it very easily infects the mother shoot. This is also the main reason strong plants rot easily. Watering accelerates the speed of disease development, so many orchid growers believe that water getting into the leaf center is the main cause of soft rot, but this is not actually the case.

      Four: when dividing plants, sometimes the mother shoot must be pulled apart. If the mother shoot has relatively plump bud eyes, forceful pulling can easily crush the bud eyes without one even being aware of it. After potting, the bud eyes will rot. This also counts as a type of soft rot. In this situation, after potting, the old shoot first becomes diseased through the rotting bud, and this is mistaken for stem rot.

      Five: when dividing plants, bumping and injuring the new shoot, or breaking and injuring the old shoot, sometimes goes unnoticed, but this kind of internal injury inevitably leads to soft rot and stem rot.      

      Six: during the decomposition of some potting media, a spherical fungus grows. When this fungus grows around the pseudobulb, it very easily infects new buds.

      Only by knowing why it happens can one act with a clear target and prevent trouble before it occurs.

      Preventing insect pests is simple. If snails are found, repot in time, rinse the orchid's pseudobulbs and roots repeatedly with high-pressure water, dry them, steam the potting medium, and use pre-steamed potting medium when potting. This can basically eliminate them.Fertilising also requires attention. If organic fertiliser is mixed in, it is best to apply fertiliser along the inner wall of the pot, and avoid as much as possible letting fertiliser water reach around the pseudobulb. Where fertiliser water has been poured, green algae grow.