Begonias are probably one of the most varied and well-known houseplants. The varieties of begonias number in the thousands, and their popularity is quite apparent. Begonia blooms range from small and delicate to large and roselike. More popular than begonia blooms, the leaves of certain begonia varieties grow into mosaic patterns of varying colors. Several factors cause begonia leaves to fall off suddenly and rot.
The most common reason begonia leaves rot is too frequent watering. Begonias are succulent plants with stems that are mostly water, which makes them very sensitive to moisture and fungus. Overwatering begonias causes their leaves to turn yellow, a process called chlorosis. Eventually, those leaves rot and then fall off. If you see brown spots on the leaves as well, they have the fungal infection botrytis, which thrives in cool and wet conditions.
Temperature and Humidity
Other factors that affect begonia leaves' health are temperature and humidity. Begonias originate from tropical environs and are accustomed to growing in bright, warm and humid conditions. Even if you water your begonia properly, high humidity along with cool temperatures can increase the likelihood that the begonia will suffer from leaf rot and/or botrytis.
If your begonia's leaves begin to yellow, all hope is not lost. One of the first ways to stave off chlorosis is to give your begonia a 1/2-strength fertilizer to boost its nutrient intake. If your begonia has yellowing or rotting leaves, another treatment is to change its growing conditions by moving it to a sunnier, warmer place; if a begonia shows signs of chlorosis, it probably is not happy in its location. If your begonia has botrytis spots on its leaves, cut off and dispose of the leaves to avert the fungus' spread.
Preventing leaf rot and botrytis requires a few simple measures. When purchasing or planting a begonia, ensure that the potting mixture is light, airy and drains easily; that will prevent the begonia from sitting in water. Plant or buy a begonia in a pot that matches the begonia's size; an oversized pot holds more water in the soil than the begonia can use at one time, resulting in the plant sitting in water. Grow begonias in a sunny or partly sunny location to increase their location's temperature, which will prevent rot and fungus.