Use a fungicide that is appropriate for begonias every week or so. Water. Temperature and Humidity. Why Do Begonia Leaves Fall Off Real Suddenly & Rot? … Note the yellow “Halos” around the lesions. Lower surface of begonia leaf infected with bacter ial leaf spot. Severely affected leaves may … Control by surface or bottom watering, keep the humidity below 80 to 85 percent, space plants, and practice strict santiation. The first signs of begonia leaf spot may often be present on the underside of the plants’ foliage. Bacter ial leaf spot on the upper surfa ce of a beg onia leaf. Figu re 1. Spotted leaves must be carefully removed and burned. Propagate cuttings from healthy plants, and use sterilized soil or media with a pH of 5.5 to 5.8, a temperature of 72°F... Isolate new cutting or plug shipments for an observation period to be sure they are not diseased. The most common reason begonia leaves rot is too frequent watering. Once any signs of a begonia disease appear on a plant, the owner should remove the affected begonia and quarantine it from other plants. Other factors that affect begonia leaves' health are temperature and humidity. Xanthamonas bacteria survive on previously infected begonia leaves. Fortunately for begonias with botrytis, there are chemical controls that can be used to help infected plants. Maintain low humidity by spacing plants and venting to improve air circulation. Alternate fungicides to prevent fungi from building up resistance. Any knives used for cutting an infected begonia should be disinfected with 70 percent rubbing alcohol before it is used on another plant. They're most likely to... Foliar Nematodes. Water Spots on Begonia Leaves Bacterial Leaf Spot. Treating Begonia Bacterial Leaf Spot The best way to maintain a healthy planting of begonias is to monitor and check the overall health of the flowers before planting them into the garden. Bacterial leaf spot of begonia is caused by Xanthomonas axonopodis pv.begonia (formerly known as X. campestris pv.begonia) Symptoms include water-soaked foliar lesions and wilting.Foliar lesions are especially visible from the underside of the foliage. Closely examine the leaves of the begonia plants. You can also use biological control as begonia botrytis treatment. Managing Bacterial Leaf Spot. Begonias in the early stages of leaf-spot disease develop yellow-margined, water-soaked spots.