Gummy stem blight

Stagonosporopsis cucurbitacearum

also called Black rot of cucurbits, Didymella stem blight

Tan crown lesions weeping amber gum, and a squash vine that collapses from the base out. Seed-borne, so it often arrives with the packet rather than the weather.

Fungus Serious Seed-borneRain splashAirborne sporesSoil-borne Spreads to neighbors

What you'll see

A water-soaked then tan lesion at the crown or a leaf joint, oozing a red-amber gum that dries to a bead.
Tiny black pinpoints peppering the lesion, easiest to see with a lens.
V-shaped tan patches spreading in from the leaf margin, often with a yellow halo.
Whole runners wilting and dying back while the rest of the plant looks well.
Stored winter squash and melons rotting from a firm black patch — the black rot phase of the same fungus.

Why it happens

A fungus that travels chiefly on seed and on infected debris, so a clean garden can get it from one packet. It enters through wounds and natural openings, most often at a pruning cut, a vine-borer hole or the crown where the stem meets the soil, and it fruits on the lesion within a fortnight to seed the rest of the patch.

Warm and wet is the trigger — long leaf wetness above 20 °C, so a humid spell, an overhead-watered bed, or a crowded planting that never dries. Plants already stressed by borers, mildew or drought go down first.

Treating it

Cut out and bin affected runners well below the lesion, working only when the foliage is dry.
Pull a plant with a crown lesion; a girdled crown will not recover and it is now seeding the patch.
Stop overhead watering entirely and water at the base in the morning.
Do not handle healthy plants after touching a lesion without washing your hands and tools.
Sort the stored crop carefully — one black-rotted squash will take the shelf with it.

Keeping it away

Buy treated seed, or hot-water-treat your own; the fungus rides on and in the seed coat.
Rotate cucurbits three years and clear every vine at the end of the season.
Trellis and space so the canopy dries by mid-morning.
Keep the squash vine borer and the mildew off, since their damage is the doorway.
Cure winter squash properly and inspect each fruit before it goes into store.

Who gets it

Melons, watermelons, cucumbers, squash and pumpkins — all the cucurbits, with melons worst.