bacterial leaf spot & blight of begonia

Symptoms

Small, water-soaked, translucent spots on the leaf underside enlarge into brown blotches with a yellow halo, often bounded by the leaf veins (angular); spots merge and leaves brown and drop; in systemic infection the plant wilts and stems rot. A milky bacterial ooze can be seen from cut tissue.

Easily confused with

• Pythium damping-off + root rot (multi-host)

  How to tell them apart: Xanthomonas axonopodis pv. begoniae (bacterial blight) shows its first and most telling signs on the FOLIAGE: tiny water-soaked, glassy translucent dots on the leaf undersides that you can see when you hold the leaf up to light, enlarging into brown necrotic spots. Classic marker is a V-shaped chlorotic-then-brown wedge spreading inward from the leaf margin, with blackened, darkened veins as the bacterium moves systemically. The plant only wilts and collapses LATE, after the leaves are already spotted and the veins are inky-dark. Pythium ultimum (root and crown rot) leaves the upper foliage spotless at first; the trouble is at the BASE. Pull the plant and the roots are brown, soft, slimy and the outer root sheath slips off leaving bare strands; the stem at soil level turns dark, water-soaked and mushy. Seedlings simply topple (damping-off). Wilting comes from below with no leaf-margin wedges and no blackened veins, and it worsens in cold, wet, over-watered, poorly drained media.