Most begonia problems trace back to two things: too much moisture and too little air movement. Get the water and the airflow right, and the long list of begonia troubles shrinks down to almost nothing. Nearly every issue below grows out of those same two conditions.
The pattern is easy to see on a windowsill. A begonia sitting in warm, stagnant, humid air grows a white powdery film across its leaves. A begonia left in soggy soil goes soft and collapses at the stem base. Same plant, two opposite ailments, one shared cause: stale wet conditions the roots and leaves can't dry out from.
Soggy soil is the most common killer. When your mix stays wet, begonia root rot sets in. The rot then creeps up into the crown where the stems meet the soil. The base turns brown and mushy. Your plant wilts even though the soil feels wet, and by then the roots are usually gone. Soil fungi thrive in that airless, waterlogged mix. You fix it by improving drainage and repotting into a fresh pasteurized mix.
Root and Crown Rot
- Cause: Soggy, poorly draining soil that stays wet too long, which lets soil fungi attack the roots and stem base.
- Signs: A soft, brown, mushy stem base and wilting leaves even when the soil feels damp.
- Fix: Improve drainage, let the top inch of mix dry between waterings, and repot into a fresh pasteurized mix to remove the fungi.
Powdery Mildew and Botrytis
- Cause: Damp, still air with little circulation, common when plants sit too close together.
- Signs: A white powdery coating on leaves for mildew, or fuzzy gray mold on flowers and soft growth for Botrytis blight.
- Fix: Space plants out, run a small fan for air flow, and remove any heavily coated leaves.
Bacterial Leaf Spot
- Cause: Bacteria that spread fast when leaves stay wet from overhead watering or splashing.
- Signs: Water-soaked spots with yellow halos that spread and merge across the leaf.
- Fix: Remove and discard infected leaves, keep the foliage dry, and water only at the soil line.
Still, wet air also breeds disease above the soil. Begonia powdery mildew shows up as that dusty white coating on the leaves. Botrytis blight follows in cool damp spots as fuzzy gray mold on flowers and soft stems. More spacing and steady air flow break the cycle. A small fan on a crowded shelf does more good than any spray you can buy.
Bacterial leaf spot is the one you watch for on the leaves themselves. It starts as water-soaked areas ringed by yellow halos. The spots spread fast once your foliage stays wet. There is no cure for an infected leaf. So remove infected leaves right away, and stop the splashing that carries the bacteria around.
Flower trouble usually points to stress, not disease. Begonia bud drop happens when your plant gets a sudden jolt: too much water, a heavy feeding, a draft, or a quick move into low light. The buds form and then fall off before they open. Ease off the water, hold the fertilizer, and keep your plant in steady light and temperature until it settles.
Begonias can be sensitive to insecticidal soap and oil sprays. Treat one leaf, wait two days, and check for burn before you spray the whole plant.
The day-to-day routine that prevents all of this is short. Water at the base so the leaves stay dry, never from over the top. Space your plants so air moves freely between them. And spot-test any treatment first, since a sensitive begonia can burn from a spray meant to help it. Fix the moisture and the air, and most begonia troubles never start.
Read the full article: Begonias Plants: Full Care Guide
