Botrytis (stem rot or brown rot)
Botrytis is a fungal disease that can attack any part of a begonia plant, particularly in the latter part of the season, as does mildew, since it is associated with the same conditions that favor the development of mildew.
Botrytis is evidenced by the appearance of a wet, soggy, brownish area which, if left untreated, will quickly increase in size and may eventually kill the plant. In the tuber, botrytis usually results from damage or failure to remove the scab.
Another form of botrytis that affects tubers in storage is dry rot, when they go hard for no apparent reason and die.
Botrytis usually affects plants, or parts of plants, that have either been damaged or have insufficient ventilation, or where the grower’s hygiene is poor.
Examples are a tie cutting into the stem of the plant, or the plants being too close together, or falling flowers becoming lodged in the foliage below. Excess nitrogen will also predispose a growing plant to botrytis because the new growth is soft.
Prevention: Good hygiene, adequate ventilation, and plant spacing are of paramount importance in preventing the onset of botrytis. Pay particular attention to removing stems and leaves as they drop, for they will cause the infection to start, when left lying on other parts of the plant.
If any area of rot is found, it must be dealt with immediately, especially if it is close to the tuber. The first step is to clean a knife or scalpel by dipping it in methylated spirits.
If the stem is rotten right through, then the only option is to cut it off cleanly with the sterilized knife below the infection, which may mean the end of that particular plant’s growth for the season. If the stem is only partly infected, then the area can be cut away carefully, ensuring that all the rot is removed.
The cutting instrument should be cleaned both during and after use. Once all the rot is removed, wash the wound with methylated spirits, which will both sterilize and dry it.
Then dust the wound with either flowers of sulfate or another anti-fungal powder. This will have the dual effect of drying off the wound and sealing it from possible further infection.