Biological Factors Contributing to Post-harvest Losses of Fruits and Vegetables

Every year, a significant share of the fruits and vegetables grown with great effort never reach the consumer’s plate. Studies estimate that 20-25% of harvested fruits and vegetables are destroyed by pathogens during post-harvest handling, even in developed countries – and the figures are considerably worse in tropical regions with limited cold-chain infrastructure. While mechanical damage and poor storage conditions certainly play a role, biological agents – pathogens such as fungi and bacteria, along with insect pests – are among the leading drivers of this loss. Understanding exactly how these organisms damage produce, and at what stage, is the foundation of any effective post-harvest management strategy.

Why biological factors are a distinct category of post-harvest loss 🔗

Fruits and vegetables are living tissues. Even after separation from the parent plant, their cells continue to respire, age, and respond to their environment. This living quality is precisely what makes them vulnerable to biological attack. Biological processes such as respiration, ethylene production, and water loss are innate to fresh produce, but the rate of these processes is heavily influenced by pathological deterioration caused by microorganisms and pests. Unlike physical damage from rough handling, biological deterioration often begins quietly – as a latent infection in the field – and only becomes visible after harvest, when conditions favour rapid spread.

Post-harvest diseases are often associated with “latent” or “quiescent” infections, where the pathogen infects the host before harvest but enters a state of dormancy until favourable conditions occur. This makes biological losses particularly difficult to prevent purely at the storage stage; effective control requires intervention well before the crop is harvested.

Fungal pathogens: the primary biological threat 🔗

The major causes of loss in perishable produce after harvest are certain pathogenic fungi and bacteria. Of the two, fungi are responsible for the bulk of decay in fruits, primarily because the high concentration of sugars, minerals, vitamins, and amino acids, combined with the low pH of most fruits, creates ideal conditions for the growth and survival of parasitic and saprophytic fungi. These fungi produce cell wall-degrading enzymes that break down plant tissue and use it as a nutrient source, rapidly reducing shelf life and making produce inedible.

Botryodiplodia theobromae: stem-end rot in mango 🔗

One of the most economically damaging fungal diseases of mango is stem-end rot (SER), caused primarily by Botryodiplodia theobromae (now more precisely classified as Lasiodiplodia theobromae). This fungus is considered a latent pathogen capable of colonizing healthy plant tissue without exhibiting symptoms, and causes cankers, dieback, and fruit rot in more than 500 host plants across tropical and subtropical regions worldwide.

In mango, the fungus establishes itself in the necrotic tissues of the fruit’s button – the calyx and disc – while the fruit is still on the tree. After harvest, it reactivates and penetrates the fruit through the natural opening that forms at the stem during detachment. Research across popular Indian mango varieties found that physiological weight loss due to SER ranged from 9.6% to 19.0%, with varieties like Chousa and Neelam proving most susceptible. Conditions that accelerate disease development include high temperatures, high humidity, and the presence of deadwood in the orchard. In Ghana, the disease was found to cause between 6.7% and 38.7% loss of stored mango fruits, underlining its widespread economic impact.

Glomerella cingulata: anthracnose in mango 🔗

Glomerella cingulata (the sexual stage of Colletotrichum gloeosporioides) is the causal agent of anthracnose, considered the most severe mango disease globally, with stem-end rot ranking second. Like SER, anthracnose operates as a latent infection – the fungal spores infect the young fruit in the field, remain dormant until the fruit begins to ripen, and then cause characteristic dark, sunken lesions on the skin surface. These lesions coalesce and penetrate deeper into the pulp, rapidly making the fruit unmarketable. Colletotrichum species are also responsible for post-harvest anthracnose and fruit rot in apple, banana, and avocado, making this pathogen group a broad threat across tropical horticulture.

Other notable fungal pathogens 🔗

Beyond mango, several other fungi cause significant post-harvest decay across a range of crops. Botrytis cinerea is a primary post-harvest pathogen of grapes, capable of colonising the fruit cluster in the field, during storage, or in transit. Other important post-harvest fungi across crops include Penicillium expansum, Aspergillus niger, Fusarium species, and Alternaria alternata. A critical secondary concern with some of these fungi is mycotoxin production. Toxigenic fungi such as Aspergillus species can contaminate stored grains and fruits, producing mycotoxins harmful to human health.

Bacterial pathogens: soft rots and black rots 🔗

Bacteria are generally a greater threat to vegetables than to fruits, primarily because most fruits have a low pH that limits bacterial growth. Viruses and nematodes play a minor role in post-harvest losses, but certain bacteria, particularly in genera such as Erwinia and Pseudomonas, cause extensive damage.

Erwinia carotovora is among the most damaging bacterial pathogens of vegetables. It is a highly effective spoilage microbe that causes soft rot across a broad host range of vegetables and some fruits, infecting and destroying plant tissues both pre- and post-harvest. The bacterium produces enzymes that degrade pectin – the structural component of plant cell walls – resulting in the characteristic watery, translucent decay in leafy vegetables and tubers like potatoes and carrots. Entry points are typically wounds, cracks, and fresh stem scars, especially where cool produce is washed in warm water, which causes air inside the tissue to contract and draw contaminated water into the flesh.

Xanthomonas campestris is responsible for black rot in cruciferous vegetables such as cabbage and cauliflower. It causes blackening of the vascular tissue, foul odour, and rapid deterioration during storage. Other psychrotrophic bacteria capable of thriving even in cold storage conditions include Pseudomonas fluorescens, Bacillus species, and Vibrio fluvialis, making cold storage alone insufficient as a control measure against bacterial spoilage.

Insect pests: damage before and after harvest 🔗

Insects contribute to post-harvest losses through multiple mechanisms: they feed directly on produce, destroy its surface integrity, create entry wounds for pathogens, and contaminate stored commodities with excrement and metabolic byproducts. While insects are generally considered less important than fungi in post-harvest losses of perishable fruit, certain species cause severe and targeted damage that can render entire harvests unmarketable.

Fruit flies (Bactrocera dorsalis) and mango 🔗

Bactrocera dorsalis, the Oriental fruit fly, is one of the most destructive insect pests in tropical fruit production. It is considered one of the most destructive fruit flies globally, with documented losses of up to 100% if control measures are not implemented. The female fly lays eggs beneath the skin of ripening fruit. The larvae hatch and tunnel into the flesh, accelerating decay and making the fruit inedible. Even where larvae are absent, the egg-laying punctures serve as direct entry points for fungal and bacterial pathogens, compounding the biological damage. A typical fruit fly infection causes yield losses of between 25 and 30%, though it can reach up to 90% under extreme infestation conditions.

The pest’s management is complex because much of the damage cycle begins before harvest. Research conducted at the Indian Institute of Horticultural Research found that a pre-harvest integrated pest management combination of male annihilation technique using methyl eugenol as a lure, combined with sanitation practices, reduced infestation from a range of 17-66% down to 5%. Post-harvest, hot water treatment at 48°C has been developed as an effective disinfestation method that also meets quarantine requirements for mango exports.

Sweet potato weevil (Cylas formicarius) 🔗

For sweet potato, the equivalent threat is the sweet potato weevil, Cylas formicarius. Without control measures, larval tunneling by this weevil can cause crop losses ranging from 5% to 100%, depending on the level of infestation. The damage mechanism is two-fold: larvae physically tunnel through storage roots, reducing their structural integrity and marketability; and both larval and adult feeding triggers the production of defensive terpenoid compounds in the root that create a bitter, pungent taste, making the tubers completely unacceptable for human or animal consumption.

The most serious impact results from larval attack to the storage roots – the harvested commodity – and direct feeding damage is compounded by the plant producing defensive sesquiterpenes that make the roots strongly distasteful, as well as promoting secondary damage by fungi and bacteria. In Africa, losses between 73% and 100% have been reported in severe infestations. In India, farm surveys have recorded losses of 25-50% across major sweet potato producing states. The weevil is equally destructive in storage – infested tubers spread the pest rapidly if not isolated, and the problem intensifies the longer harvested roots remain without proper treatment or cold storage.

The role of pre-harvest management in controlling post-harvest biological losses 🔗

A critical insight from studying biological post-harvest losses is that many of the most damaging pathogens and pests establish themselves before the crop is ever cut from the plant. Certain fungal pathogens initiate infections during the growing season, and these can be controlled best by the timely application of fungicides prior to harvest. This means that waiting until the storage stage to address biological threats is almost always too late for the major fungal diseases.

Effective pre-harvest measures include orchard sanitation (removal of dead wood and fallen fruit to eliminate inoculum sources), timely and correctly dosed fungicide applications, physical protection such as fruit bagging to exclude fruit flies, and the use of cultural practices that minimise wounding during harvest. Post-harvest disease management strategies include sanitation of storage facilities, temperature management, use of resistant varieties, chemical treatments, and biological control.

Maintaining the physical integrity of the produce surface is particularly important. The skin of horticultural products is an effective barrier to most of the opportunistic bacteria and fungi that cause rotting of the tissues. Breaking the skin not only creates direct infection sites but also stimulates physiological deterioration and dehydration. Gentle handling, appropriate packaging, and prompt drying after washing are therefore not optional refinements – they are essential biological control measures in their own right.

Cold storage slows fungal and bacterial metabolism significantly, and controlled atmosphere storage, when combined with refrigeration, retards respiratory processes and delays yellowing, softening, quality changes, and other deteriorative processes. However, cold storage must be complemented by sound pre-harvest practices, since it cannot reverse infections that are already established before the produce enters the cold chain.

What do you think? Given that fungal pathogens like Botryodiplodia theobromae establish latent infections in mango well before harvest, do you think post-harvest storage technology alone can adequately address biological losses – or is the intervention window fundamentally in the field? And considering that weevils like Cylas formicarius can make sweet potato roots chemically unpalatable in addition to physically damaged, how should farmers prioritise their control efforts between field management and storage-stage treatments?

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