| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Bacterial leaf spot * (Xanthomonas campestris pv. begoniae) |
Circular, necrotic spots start as small, water-soaked, blisterlike spots. Premature abscission occurs when spots are numerous. | Systemic as well as in dead begonia leaves. Favored by splashing water or overhead irrigation and high temperatures (80° to 90°F). | Keep humidity low. Avoid wetting foliage. Do not crowd plants. Remove and destroy infected plants. |
| Gray mold * (Botrytis cinerea) |
Soft, brown rot of leaves, stem, and flowers occurs. Fuzzy gray fungal spores form on decayed tissues. Fungus is common on weakened plants. Botrytis may also start in powdery mildew spots, sunburned tissues, tissues injured by other means, or in ligules. | In plant debris; common on any dead plant material on soil if moist. Resting sclerotia favored by high moisture conditions and low temperatures. | Control root rots and powdery mildew *. Pick up dead flowers and leaves. Keep humidity low. Protect plants with fungicide such as fenhexamide. Avoid overhead irrigation. |
| Powdery mildew * (Erysiphe begoniicola, Golovinomyces orontii) |
White, powdery spots develop on upper and lower leaf surfaces and small, greasy spots occur on undersides of leaves. Also may appear on flowers of some fibrous begonias. | On living begonia leaves; rarely as resistant fungal structures (chasmothecia). Spores are airborne as well as spread in water, but do not survive in free water. Favored by moderate temperatures and shade. Common on plants as they senesce in fall. | Increase air movement; some resistant cultivars available. Spray at the first sign of mildew and at 2- to 3-week intervals thereafter. Use myclobutanil, or triadimefon. Triadimefon is very effective but expect stunting of some plants. |
| Root and stem rot * (Pythium spp.) |
Plants are stunted, unthrifty, and may die. Root system is small and discolored. Also invades tubers of tuberous begonias. Stem rot phase: stems become water soaked and discolored, and collapse. Disease also causes damping-off of seedlings. Plants are predisposed to sunburning. | In soil. Spores spread in water and when infested soil is moved to uninfested areas. Favored by excess water. | Steam or chemically treat soil. Observe strict sanitary measures. Drench plants with oomycete (water mold) specific fungicide. Do not drench very young seedlings. |
| Virus or viruslike diseases | Symptoms | Host range and natural spread | Comments on control |
| Spotted wilt (Tomato spotted wilt virus) |
Rings or zoned spots develop on leaves. Plants are stunted; flowers are of poor quality. | Infects begonias, nasturtiums, callas, dahlias, and some weeds. Transmitted by thrips. | Eliminate nearby weeds and susceptible ornamental plants. Control thrips. |
| Begonias are also susceptible to Armillaria root rot (Armillaria mellea), anthracnose (Gloeosporium sp.), black root rot (Thielaviopsis basicola), cottony rot * (Sclerotinia sclerotiorum), crown gall * (Agrobacterium tumefaciens), foliar nematode** (Aphelenchoides olesistis), leaf spot (Phyllosticta sp.), Rhizoctonia stem rot (Rhizoctonia solani), root knot nematode ** (Meloidogyne spp.), soft rot (Pectobacterium (=Erwinia) carotovorum), and Verticillium wilt * (Verticillium dahliae). | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |