Grey Mould Rot

Most of what is expected of refrigeration rests on the assumption that cold stops microbial growth. For Botrytis that assumption is wrong in a specific and costly way. The fungus is adapted to cool conditions and continues to extend at the temperatures a cold store is built to hold, more slowly than it would in the warm but by no means negligibly. The consequence is arithmetic rather than biological: a rot that would express in days at ambient expresses in weeks in the cold, and a supply chain designed around weeks of storage or transit gives it precisely the interval it needs. Grey mould is thus a signature defect of the long hold, and its appearance at destination is not in itself evidence that the cold chain was breached.

Contact spread compounds this. Unlike rots that depend on airborne spores finding a fresh wound, Botrytis grows directly from a colonised unit into the sound one pressed against it, which means the packing geometry becomes part of the disease. One infected berry in a punnet can take its neighbours; a single fruit in a tray can seed the cluster around it. This produces the characteristic finding of a lot in which decay is not scattered at random but concentrated in nests, and it explains why the proportion of a consignment lost can rise sharply during transit from a starting incidence that looked tolerable at packing.

Botrytis frequently enters the fruit long before anyone handles it. In soft fruit and grapes in particular, the fungus colonises flower parts during bloom and then establishes in the developing fruit, where it stops. It does not advance, produces no symptom, and cannot be detected by looking; the fruit grows and is harvested carrying an infection that is entirely invisible. What ends the quiescence is the fruit itself: as it ripens and senesces, its defences relax and its tissue becomes hospitable, and the fungus that has been waiting resumes growth. The rot then appears in store, in transit, or on the shelf.

This has a direct bearing on attribution. Grey mould emerging in a consignment is routinely treated as a handling or storage failure, and it may not be one — the infection court may have been a flower in a field months earlier, and no packhouse operation could have found it or excluded it. Neither is the reverse safe to assume: Botrytis is also a straightforward wound coloniser, and rough handling genuinely does generate infections. The point is that the rot as observed does not distinguish between these histories, and assigning responsibility from the lesion is not possible. Establishing which applies needs the crop history, the handling record, and the temperature record, not a closer look at the fruit.

Grey mould is a decay organism rather than a fungus known primarily for toxin production, and it would be easy to conclude that the mycotoxin question does not arise here. That conclusion is not safe, for a reason that has little to do with Botrytis itself: a lot that has been rotting has not been rotting in isolation. Tissue broken down by grey mould is readily colonised by other fungi, some of which are toxigenic, and the conditions that permitted one rot commonly permit others. The presence of a grey-brown fuzzy growth therefore tells nothing useful about toxin either way — it does not establish that a mycotoxin has been produced, since that depends on which organisms were actually involved, on the substrate, and on the conditions they grew under.

The direction that matters more is the other one, and it holds regardless of which fungus is being discussed. A lot showing no visible mould at all may still carry mycotoxin. Toxins are chemically stable and remain in the commodity after the fungus that made them has died or been picked out; they diffuse into tissue around a lesion, so cutting away the visible decay leaves contaminated material behind in flesh that looks sound; they are distributed unevenly through a lot, so a clean-looking portion is not evidence about the rest; and they are invisible, odourless, and tasteless, offering no sensory cue whatever.

Blue mould and grey mould are treated as obviously different rots and are, in practice, confused constantly. Both produce soft, watery, spreading decay; both sporulate on the lesion surface as it matures; and the colour difference that supposedly separates them does not survive contact with real fruit. Sporulation colour changes as a lesion ages, varies with the substrate the fungus is growing on, and is frequently obscured or replaced entirely by secondary fungi that arrive on tissue already dying. Earlier still, before either has sporulated at all, there is simply nothing to look at: two soft watery lesions, indistinguishable. The blue mould entry in this reference makes the same statement from the other side, and it is worth stating plainly here — these two rots cannot be reliably told apart by eye, and separating them requires laboratory confirmation.

• Grey mould and blue mould cannot be reliably distinguished visually; confident separation requires culture or molecular confirmation by a competent laboratory.
• Sporulation colour shifts with lesion age and substrate and is a weak discriminator even between well-separated genera.
• Before sporulation, the lesions of many unrelated rots are simply soft watery decay and carry no distinguishing information at all.
• Secondary colonisers frequently dominate the surface of decaying tissue, so what is visible need not be what initiated the rot.
• Nesting is suggestive of contact spread but does not identify the organism — other rots also propagate between touching units.

Because Botrytis exploits wounds and senescing tissue and then travels by contact, the effective levers act on damage, on time, and on how units are held apart. Careful handling reduces the wounds available. Prompt cooling and consistent cold holding extend the interval before the rot becomes commercially significant, which for a cold-tolerant fungus is a genuine gain even though it is not a solution. Removing decayed units interrupts contact spread. Packaging that limits crushing and separates units restricts the nesting pathway, and modified or controlled atmospheres are used for some commodities as an additional brake on both fungal growth and fruit senescence.

• Handle to minimise wounding, and cool promptly — for a cold-tolerant fungus, the time bought by rapid cooling matters more than for most rots.
• Hold consistently within the commodity's own range through every link, recognising that this delays grey mould rather than preventing it.
• Remove decayed units where practical to interrupt contact spread, treating this as a spread-control and appearance measure and never as a toxin measure.
• Use packaging that limits compression and reduces unit-to-unit contact, since the packing geometry is part of how this rot propagates.
• Match the intended storage or transit duration to the commodity, because grey mould is fundamentally a defect of the long hold.
• Where a mycotoxin question arises, refer it to laboratory testing under the applicable food-safety framework rather than to visual assessment.