# Sour rot

- URL: https://backyardgrapes.com/diseases/sour-rot/
- Scientific name: Yeast + acetic-acid bacteria complex (with fruit flies)
- Category: Fungal
- Affects: Fruit clusters
- Severity: moderate
- Favored by: Ripe, sugary berries with wounds or splits; warm humid weather; active fruit flies.
- Easily confused with: Botrytis bunch rot (grey mould), Ripe rot

**Summary:** Clusters that turn to vinegar-smelling mush late in the season, swarming with fruit flies.

**What to look for:**
- Berries brown, soften, and leak juice; the cluster smells sharply of vinegar.
- Clouds of fruit flies (Drosophila) around ripening clusters.
- Rot spreads berry-to-berry through tight clusters once skins split.
- Worst after rain or feeding damage cracks the ripe fruit.

**Treatment — Cultural / prevention:**
- Open the fruit-zone canopy for airflow and fast drying.
- Grow or thin toward looser clusters; tight clusters trap moisture and split.
- Remove damaged, split, and rotting berries promptly to stop berry-to-berry spread.
- Reduce the causes of skin splitting (irregular water, powdery mildew scarring, insect feeding).

**Treatment — Organic / low-toxicity:**
- Manage fruit flies, which move the microbes between wounded berries.
- Antimicrobial sprays (e.g. certain peroxide/peracetic products) on early rot.

**Treatment — Conventional (chemical):**
- Near harvest, a combined antimicrobial plus an insecticide for the fruit flies is the usual approach.

Sour rot isn't one organism — it's a late-season partnership between yeasts, acetic-acid
bacteria, and fruit flies working on ripe, wounded berries. The yeasts and bacteria turn
sugar into ethanol and then vinegar (hence the smell), and the fruit flies both feed on
and spread the microbes from berry to berry.

Because a wound is the entry point, sour rot follows anything that breaks ripe skins:
rain-driven cracking, tight clusters that split, powdery-mildew scarring, bird or insect
damage. Control is therefore as much about keeping fruit intact and dry as it is about
spraying. Once a cluster reeks of vinegar and swarms with flies, remove it — it is a
source for the rest of the vine.

**Image credits:**
- Traminette grapes with split skins and sour rot — [Purdue University, Facts for Fancy Fruit](https://fff.hort.purdue.edu/article/grapes-the-sour-rot-situation/)
- Sour rot in a Chardonnay grape cluster, with brown, collapsing berries — [Bayer Crop Science New Zealand](https://www.cropscience.bayer.co.nz/pests/diseases/sour-rot)

**Sources:**
- [Grape Sour Rot](https://extension.psu.edu/grape-sour-rot) — Penn State Extension
- [Sour Rot Disorder of Grape](https://ohioline.osu.edu/factsheet/plpath-fru-50) — Ohio State University Extension (Ohioline)
- [Management Guide for Sour Rot in North Carolina](https://content.ces.ncsu.edu/management-guide-for-sour-rot-in-north-carolina) — NC State Extension
- [Summer Bunch Rot (Sour Rot) / Grape / Agriculture: Pest Management Guidelines](https://ipm.ucanr.edu/agriculture/grape/summer-bunch-rot-sour-rot/) — UC IPM