Gummy stem blight identification & organic treatment
Stagonosporopsis cucurbitacearum
How to identify gummy stem blight
A cucurbit-specific fungus causing leaf spots, stem cankers that ooze amber gum, and a fruit-rotting phase confusingly also called 'black rot' on melons and squash.
Symptoms
- Tan-to-brown circular leaf spots, often starting at leaf margins
- Water-soaked stem lesions that turn tan and ooze amber-colored gummy sap
- Tiny black fungal fruiting bodies visible within lesions
- On fruit (called 'black rot' in this phase), a firm, dark, sunken rot develops, sometimes reaching deep into the flesh
Host crops
Cucumber, Winter squash, Pumpkin, Watermelon, Cantaloupe / Melon.
How it spreads
Seedborne and survives in crop debris; spreads via splashing water, wind-blown spores, and contaminated tools/hands.
Favorable conditions
Warm, humid weather (75-85°F) with prolonged leaf wetness from rain, dew, or overhead irrigation.
Progression
- Leaf spotting (Early-mid season, wet weather) — Marginal tan spots appear on leaves.
- Stem canker & gumming (As disease advances) — Cankers ooze amber gum; vines can wilt beyond the lesion.
- Fruit rot (Near harvest) — Dark, firm rot penetrates the fruit, often starting at the blossom end or a wound.
Organic treatment for gummy stem blight
- 1. Remove infected vines and fruit · medium effort
Cut out and discard cankered stems and rotting fruit promptly.
When: Ongoing
- 2. Copper fungicide · medium effort
Apply an OMRI-listed copper spray every 7-10 days once symptoms appear, especially before rain.
When: Onset through fruiting
- 3. Water at the soil line · medium effort
Switch to drip irrigation to keep foliage dry.
When: All season
Prevention
- Use certified disease-free seed.
- Rotate cucurbits out of a bed for 2 years.
- Space vines for airflow and avoid working plants when wet.
Easy to confuse with
Anthracnose (similar sunken fruit lesions, different spore color), Bacterial wilt (wilting without stem gumming).