Post-harvest Anthracnose

The life cycle behind this defect is worth stating precisely, because every practical consequence follows from it. Colletotrichum spores are produced on infected plant tissue and on residues in the orchard or plantation, and are moved onto developing fruit largely by rain splash and by water running over surfaces — which is why anthracnose pressure tracks wet weather during fruit development. A spore that lands on a fruit does not need an injury to get in: it germinates and penetrates the intact skin directly. Then, having established in the outer tissue, it stops. The immature fruit is a hostile substrate — firm, acidic, and containing compounds that suppress the fungus — and the infection is held there, alive but not advancing.

It stays that way for as long as the fruit stays immature, which may be weeks or months. Nothing about the fruit reveals it. The skin is unbroken, the flesh is sound, the colour is normal, and no measurement made at the packhouse door distinguishes an infected fruit from a clean one. Ripening reverses the conditions that held the fungus: acidity falls, sugars rise, the suppressive compounds decline, the tissue softens, and the infection is released. The lesions that then appear are not new infections — they are the same infections that were present, invisibly, all along. This is why the disease seems to appear from nowhere, and why the point at which it becomes visible is set by the fruit's ripening rather than by anything the handler did.

The pattern is familiar to anyone who has received tropical fruit. The consignment was inspected and passed at origin. It was shipped under temperature control. It arrives, it ripens, and it breaks down with dark sunken lesions across a large share of the fruit. The immediate inference is that something went wrong in transit — a temperature excursion, a delay, a handling failure — and the cold chain is investigated. Frequently nothing is found, because nothing went wrong there. The lesions were determined in the field weeks or months earlier, under rain the exporter could not control, and the voyage merely provided the time and the ripening provided the trigger.

This misattribution is costly in both directions. It sends effort and investment into the part of the chain that was working, while the part that actually determines the outcome — field sanitation, canopy and orchard management, harvest timing relative to wet weather, and the disease history of the block the fruit came from — receives no attention because it is out of sight of the people seeing the loss. It also poisons commercial relationships, since the receiver has clear evidence of decay and the shipper has clear evidence of a correct inspection and a correct voyage, and both are right. The honest statement is that the arrival condition does not identify where the failure was, and that resolving it requires the crop history rather than a closer examination of the fruit.

None of this means the cold chain is irrelevant. Correct temperature management slows the fungus and, more importantly, slows the ripening that releases it, which is a real and useful delay — it is why cold-held fruit expresses anthracnose later than warm-held fruit. But delaying expression is not preventing infection. The cold chain can move the moment of failure further down the chain; it cannot remove an infection established in the field, and it should not be expected to.

Colletotrichum is not a fungus discussed principally for toxin production, and the mycotoxin question on this page arises less from the pathogen than from what happens after it. Fruit broken down by anthracnose is soft, sugar-rich, and open, and it is readily colonised by other organisms, some of which are toxigenic — the same conditions that let the lesions expand support what follows them. So visible anthracnose lesions establish nothing about toxin: whether one was produced depends on which organisms were actually involved, on the substrate, and on the conditions under which they grew, and the lesion itself carries none of that information. A lot with obvious lesions is a quality failure, not automatically a safety finding.

The opposite direction is the dangerous one, and it lands with unusual force here because this is a disease defined by invisibility. Everything about post-harvest anthracnose trains the eye to be trusted less, and the mycotoxin logic makes the same demand. A lot with no visible mould and no lesion at all may still carry mycotoxin. The toxins are chemically stable and persist after the fungus that produced them is dead or gone; they diffuse into tissue around a lesion, so cutting out the visible damage leaves contaminated material in flesh that looks sound; they are distributed unevenly through a lot, so a clean-looking fruit is not evidence about the box; and they are invisible, odourless, and tasteless. There is no cue to find, and a symptomless lot — the very thing this disease specialises in producing — is not a lot that has been shown to be safe.

A well-developed anthracnose lesion is among the more characteristic sights in post-harvest pathology — sunken, dark, sharply bounded — and it is still not a diagnosis. The character develops late: early lesions are the same soft darkening decay produced by every other rot in this section, and the stage at which a decision is usually needed is the stage at which nothing is distinctive. Several Colletotrichum species cause the disease and are not separable by eye from one another. The same genus also participates in the banana crown rot complex, so the identical fungus produces two differently named defects on the same fruit. And the sunken dark lesion is not unique to anthracnose: other fruit rots and some physiological disorders present sunken darkened areas, and once secondary organisms arrive on the collapsing tissue they can dominate what is visible entirely.

• The disease is invisible for the whole period during which it could be inspected for, so the absence of symptoms carries no information about whether the lot is infected.
• Early lesions are indistinguishable from those of unrelated rots; the characteristic sunken dark appearance develops only as the lesion matures.
• Colletotrichum species cannot be separated from one another visually, and identification to species requires culture or molecular confirmation.
• The same genus contributes to the banana crown rot complex, so a Colletotrichum finding does not by itself say which defect is being described.
• Sunken darkened lesions are also produced by other rots and by some physiological disorders, and secondary colonisers can mask the initiating organism.
• The lesion carries no record of when or where infection occurred, so it cannot distinguish a field infection from anything that happened after harvest.

Because the infection is established before the fruit is picked, the decisive interventions are in the field and are outside the scope of a post-harvest entry: orchard and plantation sanitation to reduce the inoculum available, canopy management to shorten the periods fruit surfaces stay wet, and harvest planning that accounts for wet weather during fruit development and for the disease history of the block. What post-harvest handling can do is bounded but real. Cold management slows the fungus and slows the ripening that releases it, deferring expression and buying usable market life. Careful handling avoids the additional damage that lets quiescent infections and secondary organisms advance faster. Grading removes fruit that has already begun to express, though not fruit that has not.

• Recognise that the determining decisions were made in the field: inoculum load, wetness during fruit development, and harvest timing set what the packhouse receives.
• Do not rely on visual grading to exclude this defect — it excludes fruit already expressing lesions and cannot exclude quiescent infection, which is the whole problem.
• Cool promptly and hold consistently: this defers expression by slowing both the fungus and the ripening that triggers it, without removing the infection.
• Handle to avoid further damage, which accelerates both the released infection and the organisms that follow it.
• Manage ripening deliberately where it is controlled, since ripening is the trigger and the point of expression tracks it.
• Investigate arrival breakdown against the crop history and not only the cold chain, because attributing this defect to transit sends effort to the part of the chain that was working.
• Where a mycotoxin question arises, refer it to laboratory testing under the applicable food-safety framework rather than to visual assessment.