Storage Rot

The organisms capable of rotting a stored commodity are drawn from across the fungal and bacterial world, and they reach the lot by routes that have no relation to one another. Some are field fungi that infected the crop while it was still growing and travelled into store inside apparently sound tissue. Some are storage fungi that were never a field problem and establish only under the conditions a store creates. Some are wound pathogens that needed a handling injury to get in. Some are soft-rot bacteria that need free water and a break in the skin. And a large share of what is finally visible on a rotted unit is secondary — organisms that colonised tissue another agent had already killed.

Because these routes differ, the interventions differ. A rot that entered as a latent field infection is not prevented by store hygiene; a rot that followed a bruise is not prevented by drying; a rot driven by grain moisture is not prevented by cooling a fruit store. The label offers no guidance on which of these applies, and a decision made from the label alone is a guess dressed as a diagnosis. Establishing what actually caused a rot means investigating the lot: its history, its handling, its condition on intake, and — where it matters — laboratory identification of the organism rather than an inference from the lesion.

The most consequential error made about stored commodities is the assumption that the eye can answer the safety question. It cannot, and the reason is that the link between visible decay and mycotoxin contamination fails in both directions. In one direction the failure is merely wasteful: a conspicuously rotted lot may carry no mycotoxin at all, because whether a toxin was produced depends on which organisms were involved, on the substrate they grew in, and on the conditions the colony developed under. Many rots are caused by organisms that produce no mycotoxin whatever. Rotting is not, by itself, evidence of contamination.

The other direction is the one that causes harm. A lot that looks entirely clean — no visible mould, no off odour, nothing an inspector could point to — may still carry mycotoxin. There are four independent reasons, and each is sufficient on its own. Mycotoxins are chemically stable and persist in the commodity long after the fungus that made them has died or been removed, so a lot cleaned of mould is not thereby cleaned of toxin. Toxin diffuses into tissue surrounding a lesion, so removing the visibly affected part leaves contaminated material behind in what looks sound. Contamination is distributed unevenly through a lot, so a clean-looking sample says little about the lot it came from. And the toxins themselves are invisible, odourless, and tasteless — there is no sensory cue to find.

What follows practically is that rot control and mycotoxin control overlap without being the same programme. Managing moisture, temperature, insects, and damage suppresses the conditions under which toxigenic fungi grow, and that work is worth doing on its own merits — prevention is where the leverage genuinely is. But it does not substitute for testing, and a lot cannot be released on the basis that it looks acceptable. Sampling is itself the hard part: because contamination is patchy, an unrepresentative sample can return a reassuring result from a lot that is not acceptable, which is why sampling plans are specified rather than improvised. EFSA and FAO publish the assessment and management framework; handlers should work to that and to their national regulation, not to appearance.

Storage rot is governed by moisture, temperature, and time, and the essential point is that they interact rather than add. Moisture determines whether micro-organisms can grow at all; what matters biologically is the water available to the organism, which is a relationship between the commodity and the air around it rather than a property of either alone. Temperature sets how fast growth proceeds once moisture permits it, and also shifts which organisms are favoured — cold does not simply slow every rot equally, it selects for the ones that tolerate it. Duration is the multiplier: conditions that are harmless over a short hold become progressively less so as the hold extends, which is why a store that performed well over one season may fail over a longer one.

• Moisture and temperature cannot be assessed separately: a moisture condition that is safe when cool may not be when warm, and the reverse.
• Duration converts a marginal condition into a failing one; the intended length of hold is part of the specification, not an afterthought.
• Stored bulk is not uniform — moisture migrates and temperature gradients form, so a lot can develop a rotting pocket while average readings look acceptable.
• Insect activity and mechanical damage generate both entry wounds and localised heat and moisture, coupling pest control to rot control.
• Material entering store already infected or already damaged sets a ceiling on what any storage condition can achieve.

No safe moisture content, temperature, or holding period is given here, and none should be inferred. Those values are specific to the commodity, to the intended duration of storage, to the climate the store sits in, and to the standard being worked to; a figure lifted from one context and applied to another is a defect rather than a fact. The applicable conditions are set by sourced storage guidance for the specific commodity — consult FAO, AHDB, USDA-ARS, and the relevant national or extension references before setting a store.

The term spans two storage problems that share a name and little else. In stored grain, oilseeds, and pulses, the commodity is dry and biologically quiet, and rot is a question of whether it was dried adequately and kept dry — driven by moisture entering or migrating within the bulk, by respiring insects and their damage, and by fungi that are storage organisms rather than field ones. It is here that the mycotoxin question is most prominent, because several of the fungi that colonise stored grain are toxigenic. The relevant levers are drying, cleaning to remove damaged and infected material, sound store management, and where appropriate sealed systems.

In stored fruit, roots, tubers, and bulbs, the commodity is alive, wet, and respiring, and rot is a question of wounding, senescence, and temperature. Infection follows handling injury or arrives as a field infection that was latent at harvest; the tissue defends itself while it is sound and stops doing so as it ages. The levers are gentle handling, wound healing where the commodity allows it, prompt and consistent cooling, removal of decaying units, and appropriate packaging. Assuming that what is true of a grain store is true of a fruit store — or the reverse — is a recurring source of error, and the single label is part of why.

Every rot in this section of the reference converges on the same early appearance. Before any sporulation develops, a lesion caused by Penicillium, by Botrytis, by Geotrichum, by Colletotrichum emerging from latency, or by a soft-rot bacterium presents as soft, watery, darkening tissue, and at that stage they are not separable by eye. Sporulation, when it eventually arrives, is a weaker cue than it appears: its colour shifts as a lesion ages, varies with the substrate, and is frequently produced by an organism that arrived after the rot was already established. The most visible fungus on a decayed unit is often not the one that caused the decay.

• Early lesions of unrelated rots are indistinguishable — soft watery decay is a shared endpoint, not a signature.
• Secondary colonisers overgrow failing tissue and dominate what is seen, masking the initiating organism.
• Latent field infections express in store and are routinely misrecorded as storage failures the handler caused.
• Physiological disorders and freezing damage produce water-soaked tissue that is then colonised, so the rot obscures a non-pathological origin.
• Confident identification requires culture or molecular confirmation on appropriate material; a name assigned from a photograph, a carton, or a store floor is not a diagnosis.