- Damping-off: Collapse of young plants may be caused by Pythium, Rhizoctonia, or Botrytis.
- Botrytis leaf spot: Large, irregularly outlined brown leaf spots, particularly common on large plants given insufficient spacing; stems may also be invaded, leading to wilt of the cankered portion.
- Powdery mildew: Fuzzy white patches on leaves or flowers. In some cases, leaf tissue shows dark, greasy-looking spots beneath a sparse colony of powdery mildew. Begonias are highly susceptible.
- Bacterial leaf spot (Xanthomonas campestris pv. begoniae): Dark, greasy spots appear on leaves, or brown V-shaped dead areas develop at leaf margins. Spots are surrounded by a speckled or chlorotic zone of leaf tissue. With high temperatures, disease may progress until plants collapse. Certain Elatior begonias are particularly susceptible; Non- Stop begonias may show less extensive leaf spotting.
- Foliar nematodes (Aphelenchoides fragariae): Sunken gray-green blotches that turn reddish brown or black. Infected leaves may wilt, die, and hang limply from the plant. Elatior begonias are extremely susceptible to foliar nematodes.
- Impatiens necrotic spot virus (INSV): Round brown leaf spots, chlorotic mottling, brown streaking along veins, and brown patches in the leaf at the petiole end. Control of the vector, the western flower thrips, is essential.