Note the yellow “Halos” around the lesions. Begonias in the early stages of leaf-spot disease develop yellow-margined, water-soaked spots. Bacter ial leaf spot on the upper surfa ce of a beg onia leaf. Severely affected leaves may … Water Spots on Begonia Leaves Bacterial Leaf Spot. Xanthamonas bacteria survive on previously infected begonia leaves. They're most likely to... Foliar Nematodes. Closely examine the leaves of the begonia plants. Figu re 1. 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. Control by surface or bottom watering, keep the humidity below 80 to 85 percent, space plants, and practice strict santiation. 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. Managing Bacterial Leaf Spot. Lower surface of begonia leaf infected with bacter ial leaf spot. The first signs of begonia leaf spot may often be present on the underside of the plants’ foliage.