Blue Mould

Penicillium spores are effectively always present in the storage environment, so the availability of inoculum is rarely the limiting factor. What limits infection is access. These fungi cannot usually penetrate sound skin, and the great majority of blue-mould lesions can be traced to a wound created at harvest, during handling, in the packhouse, or in transit. This makes blue mould substantially a mechanical-damage problem that presents as a disease problem: the visible rot is the pathological consequence of a handling failure that occurred earlier and left no obvious mark of its own.

Once established, a lesion sporulates, and those spores infect wounded neighbours within the same store or carton. Deterioration is therefore progressive rather than static, and a lot inspected as marginal early can be substantially worse later. Cold holding slows the fungus but does not stop it — blue mould is well able to develop at storage temperatures, which is why it is a characteristic rot of long-stored fruit rather than only of warm conditions.

The most damaging misconception about mould in stored produce is that appearance answers the safety question. It does not, and the failure runs in both directions. A lot with conspicuous blue-green sporulation may or may not carry a mycotoxin: toxin production depends on the species involved, on the substrate, and on the conditions the lesion developed under, and a visibly mouldy lot is not automatically a contaminated one. Conversely — and this is the direction that causes actual harm — a lot with no visible mould at all may still carry mycotoxin. Toxin can persist after the fungus that produced it is gone, it can be present in tissue adjacent to a lesion that was itself removed, and it can be distributed unevenly through a lot in a way that no visual inspection could reveal.

The practical consequence is that mould control and mycotoxin control are related but not interchangeable programmes. Good handling, cold management, and sanitation reduce the conditions under which toxigenic fungi grow, and that is worth doing on its own terms. But the question "is this lot within the applicable limit?" is answered only by testing against the framework that applies to it, and never by looking at it. EFSA and FAO publish the relevant assessment and management framework; growers and handlers should work to that and to their national regulation rather than to appearance.

Blue mould and grey mould are routinely distinguished in conversation and routinely confused in practice. Both produce soft, spreading, watery decay; both sporulate on the lesion surface; and the colour cue that supposedly separates them is unreliable in the field. Sporulation colour changes as a lesion ages, varies with the substrate, and can be obscured entirely by secondary colonisers arriving on tissue that is already rotting. On citrus the problem is sharper still: blue mould and green mould are caused by different Penicillium species that differ mainly in spore colour, a difference too subtle and too variable to carry a reliable identification.

• Sporulation colour shifts with lesion age and is a weak discriminator between Penicillium species and between blue and grey mould.
• Secondary fungi colonise decaying tissue and can dominate the visible surface, so what is seen is not necessarily what initiated the rot.
• Several unrelated rots present as soft watery decay in early lesions, before any sporulation is visible at all.
• Reliable identification to species requires culture or molecular confirmation by a competent laboratory; naming the fungus from a photograph or a carton is not diagnosis.

Because blue mould is a wound pathogen, the most effective controls act on wounding rather than on the fungus. Careful harvest and handling, well-adjusted grading equipment, appropriate packaging, and reduced drop heights all cut the number of infection courts available. Packhouse and store sanitation reduces the inoculum load that meets those wounds, and prompt cooling followed by consistent cold holding slows lesion development — without arresting it.

• Minimise wounding at every transfer point, since intact skin is the principal barrier this fungus must overcome.
• Maintain packhouse and store hygiene to reduce the spore load meeting any wound that does occur.
• Cool promptly and hold consistently: cold slows blue mould markedly but does not stop it, and long-stored fruit remains at risk.
• Remove and dispose of sporulating units to limit spread — noting that this is a spread-control and appearance measure, not a mycotoxin measure.
• Where a mycotoxin question arises, refer it to laboratory testing under the applicable food-safety framework rather than to visual assessment.