Few plants can top begonias (Begonia spp.) for attractive foliage and flowers. Many types grow as annuals or houseplants, while some can grow outdoors year-round. These include tuberous begonias (Begonia x tuberhybrida), which grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 9 through 11. Begonias are usually easy to grow, but occasionally a disease or cultural problem might cause leaves to turn yellow and eventually shriveling and falling from the plant. Identifying the cause is the first and most important step in correcting a problem and restoring the plant's health.
It's possible that yellowing of leaves on an outdoor-grown begonia signals a natural slowing of growth at the end of the season, when foliage gradually dies back and drops from the plant. If you're growing a begonia as a houseplant, it might also slow growth as fall approaches, with a few yellowing leaves that drop from the plant as it enters partial dormancy. Reduce watering during this period and, when foliage has mostly died, cut back the stems to about 3 inches, using shears you wipe with rubbing alcohol between cuts to prevent spread of plant disease. New leaves usually appears in a few weeks, indicating the plant's resuming its growth.
A number of fungal diseases can attack a begonia, causing changes that include spotting or yellowing of leaves and eventual leaf drop.
Fungal problems include gray mold, which causes soft, yellow-to-brown spots on leaves, where grayish fulffy mold accumulates, and powdery mildew, which produces puffy white spots on upper and lower leaf surfaces, with leaves eventually turning yellow and drying up. Another fungal disease called Pythium root rot can result in overall poor growth of a begonia, with leaves that wilt, turn yellow and slowly die. In this disorder, the entire plant may appear stunted, and stems can become soft and soggy; the plant might die if the problem is severe.
These fungal problems develop in overly moist conditions and are best prevented by spacing plants well apart for good air circulation, watering only when the soil feels dry at the surface and allowing a potted plant to drain well after watering.
Begonias are susceptible to several other diseases that can manifest themselves by yellowing of leaves. These include bacterial leaf spot and blight, which cause watery spots surrounded by yellow halos on leaves. Removing infected leaves may cure a minor problem, but it's best to inspect plants carefully for signs of this problem before making a purchase. Clearing debris from under the plants and keeping foliage dry when watering can also help keep the disease from spreading; for a severe infection, discard the plant.
A plant virus can also cause leaves on begonia plants to develop pale yellow rings or spots and might also lead to overall yellow mottling on leaves. No cure exists for these viral diseases, so infected plants are best discarded.
Yellowing or wilting leaves have a few other possible causes. For example, too little water can interfere with a plant's ability to carry out photosynthesis, causing leaves to turn pale green, then yellow. To prevent this, water the begonia whenever the top of its soil feels dry to the touch; for a potted begonia, never let the soil dry completely, which puts extreme stress on the plant.
Begonias also attract several pests that could cause yellowing, wilting or drying of leaves. These include thrips, tiny insects that produce reddish-brown streaks on leaf undersides, and spider mites, which produce visible webs on leaves. Control these by spraying plants with insecticidal soap, diluted at a rate of 5 tablespoons per gallon of water; repeat as needed every two weeks.