Sour Rot

Geotrichum is present in soil and in the water and surfaces that fruit meets, so the fungus is rarely the limiting factor — access is. The skin of a sound fruit excludes it, and almost every sour rot lesion can be traced back to a break in that skin: a harvest puncture, a split from rain or irrigation, an injury at grading, or an abrasion that met contaminated soil or dump-tank water. Fruit that touched the ground, and fruit handled in wet conditions, carries a substantially higher load into the packhouse. Sour rot is therefore, like several rots, a mechanical and hygiene problem presenting as a disease.

Maturity is the second axis. Over-mature fruit is far more readily colonised, because the tissue has softened, its defences have relaxed, and its sugars are available; the same wound on a firm, properly mature fruit and on a fruit held too long are not equivalent events. This is why sour rot tends to concentrate at the end of a season, at the end of a hold, and in lots that were picked late, and why a lot can pass through packing acceptably and fail once it has aged another week. Cold holding slows the fungus, but it does not stop the fruit ageing into susceptibility.

Most post-harvest rots have a straightforward relationship to insects: damage creates wounds, wounds admit fungi. Sour rot has a tighter one, and it runs as a loop. The fermenting tissue produces volatiles that attract fruit flies and vinegar flies from a distance. Those flies land, feed on the leaking fruit, and breed in it. They then leave carrying the fungus on their bodies and in their gut, and they are drawn preferentially to exactly the damaged and over-ripe fruit that offers the fungus its next entry point. The rot recruits its own vector, and the vector finds the rot its ideal target.

The practical consequence is that a sour rot problem in a packhouse or store is not fully described as a fungal problem. A population of flies sustained by a few leaking fruit will distribute inoculum across a room continuously, and removing the decayed fruit removes both the source of spores and the breeding substrate at once. This is one of the few places in post-harvest pathology where insect management and decay management are genuinely the same intervention, and it is the reason this entry links to the fruit fly and vinegar fly pest references rather than treating them as unrelated topics.

Geotrichum is a spoilage organism rather than one discussed principally for toxin production, and it might seem that the mycotoxin question can be set aside on this page. It cannot, for a reason independent of the fungus being named. Tissue that has collapsed into a wet, sugar-rich, fermenting mass is an excellent substrate for other organisms, some of which are toxigenic, and the conditions that permitted sour rot permit them too. Whether a mycotoxin is present in a lot showing sour rot depends on which organisms were actually involved, on the substrate, and on the conditions they developed under — and the visible decay answers none of those. Visible rot does not confirm that a mycotoxin has been produced.

The more important failure is the opposite one, and it does not depend on which rot is being discussed. A lot with no visible decay and no off odour at all may still carry mycotoxin. The toxins are chemically stable and persist after the fungus that produced them has died or been discarded; they diffuse into the tissue surrounding a lesion, so cutting out the visible rot leaves contaminated material in flesh that looks perfectly sound; they are distributed unevenly through a lot, so an unaffected-looking portion is not evidence about the rest; and they are invisible, odourless, and tasteless. The sour smell that makes this rot recognisable is produced by the spoilage, not by any toxin — a mycotoxin gives no odour at all, and inferring safety from the absence of a smell inverts the logic completely.

Sour rot has the most distinctive sensory cue of any rot in this section, and it is still not a diagnosis. The odour is real and it is informative, but it fails as evidence for three reasons that compound each other. It is subjective: what one assessor calls sour another calls fermented or simply off, and there is no scale. It is unrecorded: no consignment document carries a smell, so an odour noted at intake cannot be re-examined, disputed, or audited afterwards. And it is not specific: any fermentative spoilage smells broadly similar, because the volatiles come from sugars breaking down rather than from any particular organism, and bacterial soft rots, yeast spoilage, and simple over-ripeness can all present a sour fermented note.

The absence of a dry powdery spore mass is the other cue commonly used, and it deserves a more careful statement than it usually gets. Blue and green moulds build a conspicuous surface layer of dry spores; sour rot does not produce a comparable structure, so a wet disintegrating lesion with no powdery sporulation is genuinely more consistent with sour rot than with a Penicillium rot. But this is suggestive, not confirmatory. Early Penicillium lesions have not sporulated yet and are also just soft wet decay; a lesion of any kind may be too young, too wet, or too heavily overgrown by secondary organisms to show what it otherwise would; and secondary colonisers can add sporulation to a lesion sour rot began. Naming this rot on odour plus a missing spore mass is an informed guess, and it should be recorded as one.

• Odour is subjective, unrecorded, and shared with any fermentative spoilage — it cannot separate sour rot from bacterial soft rot, yeast spoilage, or over-ripeness.
• The absence of dry powdery sporulation is suggestive of sour rot rather than of a blue or green mould, but early Penicillium lesions have not sporulated either.
• Secondary colonisers can add visible sporulation to a lesion that sour rot initiated, removing even the negative cue.
• Advanced sour rot destroys the structure of the tissue, so the lesion character that might have distinguished it is gone by the time the rot is obvious.
• Confirmation requires culture or molecular identification by a competent laboratory; a name assigned from smell and appearance is not a diagnosis.

Because sour rot needs a wound, needs a susceptible fruit, and travels with flies, the levers act on all three. Handling that avoids punctures and splits removes the entry courts. Harvesting at appropriate maturity rather than late reduces the tissue susceptibility the fungus depends on. Keeping fruit out of contact with soil, and maintaining hygiene where fruit meets water, reduces the load reaching any wound that does occur. Prompt cooling and consistent cold holding slow the rot and slow the ageing that predisposes fruit to it. And removing leaking fruit promptly acts on the fungus and its vector at the same time.

• Minimise punctures, splits, and stem-end injury at every transfer, since intact skin is the principal barrier this fungus must overcome.
• Avoid soil contact and maintain hygiene where fruit meets water, which is where the inoculum load is picked up.
• Harvest at appropriate maturity: over-mature fruit is markedly more susceptible, and cold does not stop fruit ageing into susceptibility.
• Cool promptly and hold consistently to slow both the fungus and the senescence it exploits.
• Remove leaking and decayed fruit without delay — this interrupts spread and removes fly breeding sites at once, but it is an appearance and spread measure, not a toxin measure.
• Where a mycotoxin question arises, refer it to laboratory testing under the applicable food-safety framework rather than to appearance or smell.