Unexplained Physiological Disorders in Fruits and Vegetables

Not every problem in fruits and vegetables comes with a clear explanation. While many postharvest issues can be traced to a specific nutrient deficiency, a pest, or a disease, some physiological disorders remain stubbornly mysterious. These are the disorders of uncertain causes – conditions where science has identified symptoms and patterns but has not pinpointed a single, definitive trigger. They arise from a tangle of environmental stress, genetic predisposition, and handling practices, making them some of the most frustrating challenges in postharvest management.

What are physiological disorders of uncertain causes? 🔗

Physiological disorders in fruits and vegetables are non-pathogenic conditions – meaning they are not caused by bacteria, fungi, or viruses. Instead, they result from disruptions in the plant’s normal metabolic processes. Many of these disorders have well-established causes: blossom end rot in tomatoes is linked to calcium deficiency, and chilling injury in bananas is a direct result of improper cold storage.

However, a subset of physiological disorders defies such neat categorization. In these cases, multiple factors seem to interact in complex ways. Researchers may observe correlations – with temperature, with nutrient levels, with specific cultivars – but cannot identify one definitive cause. These are classified as disorders of uncertain or complex causes. They pose serious economic risks because they are difficult to predict and even harder to prevent consistently.

Marginal browning in escarole 🔗

Escarole (Cichorium endivia var. latifolium) is a leafy green commonly used in salads and soups. One recurring postharvest issue is marginal browning – the development of brown to black discolouration along the edges of the leaf blades. This disorder reduces the shelf life of fresh-cut escarole and makes it commercially unacceptable.

What we know about the causes 🔗

Marginal browning in escarole shares features with tipburn in lettuce, which is associated with localized calcium deficiency in rapidly growing leaf tissues. Calcium is essential for strong cell walls, and when it cannot reach the leaf margins quickly enough – especially during periods of rapid growth or high heat – the tissue collapses and turns brown.

However, the connection to calcium in escarole is not as straightforward as it is in lettuce. Research published in Food Chemistry found that escarole and lettuce are both highly sensitive to enzymatic browning during storage as fresh-cut products. The enzyme polyphenol oxidase (PPO) plays a key role in converting phenolic compounds into brown-coloured quinones upon tissue damage. What is interesting is that the severity of browning does not always correlate neatly with PPO levels. Ascorbic acid content appears to be a protective factor – rocket salad, which resists browning, has significantly higher ascorbic acid than either lettuce or escarole.

Other contributing factors include salt accumulation at the leaf margins, disruption of laticiferous (latex-carrying) channels, poor root condition, and excessive nitrogen fertilization. Wind and high ventilation under shelters can also increase evaporation, worsening the problem. The disorder can appear both in the field and after harvest during storage, making it a concern across the entire supply chain.

Why it remains uncertain 🔗

The challenge is that no single factor consistently explains why some escarole heads develop marginal browning and others do not. Calcium, enzyme activity, phenolic compounds, environmental stress, and even microbial involvement have all been implicated. The disorder is best understood as the result of overlapping stresses, and current management involves a combination of varietal selection, careful irrigation, and optimized storage conditions rather than a targeted fix.

Waxy breakdown in garlic 🔗

Garlic (Allium sativum) is one of the most important spice crops in the world. Among its postharvest challenges, waxy breakdown is a particularly puzzling physiological disorder that can cause significant economic loss, especially in stored and shipped garlic.

Symptoms and progression 🔗

Waxy breakdown typically becomes visible after harvest, during curing or storage. According to the UC Davis Postharvest Technology Center, early symptoms appear as small, light yellow areas in the clove flesh. Over time, these areas darken to a deep yellow or amber colour. Eventually, the entire clove becomes translucent, sticky, and waxy in texture. The outer dry skins of the bulb are usually unaffected, which means the disorder may go unnoticed until cloves are peeled or the condition has advanced enough to cause visible shrinkage.

Suspected triggers 🔗

Plant pathologists have confirmed that waxy breakdown is not microbial in origin. It is a physiological condition, and researchers have been deliberately cautious about pinpointing its cause. The strongest association is with high temperatures near harvest time. Garlic left in the field under direct sunlight or cured in poorly ventilated, hot environments appears more susceptible.

The Maine Organic Farmers and Gardeners Association (MOFGA) notes that low oxygen levels and poor ventilation during curing and storage may also contribute to the development of waxy breakdown. Excessive respiration rates after harvest could play a role, as the cloves continue to metabolize without adequate airflow to remove heat and respiratory by-products.

Waxy breakdown affects cloves individually – not all cloves in a bulb will necessarily be impacted. This variability within a single bulb further complicates efforts to identify a clear mechanism. Some growers and consumers have noted that affected cloves taste milder and slightly sweeter than normal garlic, though their altered texture makes them commercially undesirable.

Management approaches 🔗

Since the exact cause remains unclear, management focuses on best practices during and after harvest. Key recommendations include harvesting in dry conditions, stopping irrigation well before harvest, ensuring excellent ventilation during curing, and keeping garlic elevated and shaded during the drying process. Hoophouses with shade cloth and open sides can be effective curing environments, provided airflow is maintained. Garlic that develops waxy breakdown will not store well and is more prone to secondary fungal infections.

Internal breakdown (spongy tissue) in ‘Alphonso’ mangoes 🔗

The ‘Alphonso’ mango is arguably India’s most prized mango variety, celebrated for its rich flavour, saffron colour, and smooth texture. Yet this premium cultivar is plagued by one of the most challenging physiological disorders in tropical horticulture – spongy tissue, also known as internal breakdown.

What spongy tissue looks like 🔗

Externally, affected fruits look perfectly normal. There are no visible signs on the skin to distinguish a healthy Alphonso from one with spongy tissue. The disorder only becomes apparent when the fruit is cut open, revealing white, corky patches in the mesocarp (pulp), sometimes with air pockets. The affected tissue remains unripe due to unhydrolyzed starch, while the rest of the fruit ripens normally. This makes the fruit unfit for consumption, and as research from the Indian Institute of Horticultural Research notes, it has severely impacted India’s Alphonso mango exports.

The competing theories 🔗

The cause of spongy tissue has been debated for decades. Early theories blamed convective heat from soil rising to mature fruit on lower branches. This idea, proposed by Katrodia in the late 1980s, suggested that soil heat inactivated ripening enzymes in the lower portion of the fruit. Mulching the orchard floor to reduce soil temperature did show some reduction in incidence, supporting this hypothesis.

A more recent and widely discussed explanation involves premature seed germination. Research published in Metabolites and studies conducted at the Indian Institute of Horticultural Research found that the seed of Alphonso mangoes begins germination-like activity when the fruit is around 70% mature. This premature germination draws moisture from the surrounding pulp into the seed, causing the mesocarp to break down. The process is genetically influenced – in Alphonso, the funiculus (the connection between seed and fruit) breaks at the peduncle end, allowing the seed to access pulp moisture, while in spongy-tissue-free varieties, it breaks at the opposite end.

Nutritional imbalance is another frequently cited factor. A comprehensive review in Plants found that high nitrogen and low calcium levels in the mesocarp correlate with increased spongy tissue incidence. Calcium is critical for cell wall stability, and its deficiency – particularly in the inner mesocarp where calcium levels are naturally lower – may predispose the tissue to breakdown.

Environmental factors such as high temperature and humidity, especially during the final stages of fruit development, contribute to higher incidence rates. Metabolomic studies have revealed disruptions in the tricarboxylic acid cycle and gamma-aminobutyric acid (GABA) shunt pathways, leading to reactive oxygen species accumulation and oxidative stress inside the mesocarp. Reduced antioxidant levels further deteriorate fruit physiology.

Why a definitive answer is elusive 🔗

The fundamental difficulty is that spongy tissue is multifactorial. Heat, seed physiology, nutrient status, respiration rates, and even microbial associations (one study found the bacterium Staphylococcus xylosus in affected tissue) all play potential roles. As the authors of one major review noted, spongy tissue is caused by various factors, and attributing an exact and specific reason remains difficult. The incidence can vary dramatically from year to year, tree to tree, and even fruit to fruit on the same tree.

Why these disorders are so difficult to manage 🔗

Disorders of uncertain causes share several features that set them apart from more straightforward postharvest problems.

Multiple interacting factors 🔗

Each of these disorders involves a web of contributing variables – genetics, environment, nutrition, and handling practices. Changing one factor (say, improving calcium supply) may reduce incidence in some years but not others, because the other contributing factors (temperature, maturity at harvest, storage conditions) also fluctuate. This makes it nearly impossible to develop a single, reliable prevention protocol.

No external symptoms 🔗

Both spongy tissue in mangoes and waxy breakdown in garlic can develop internally without any visible external signs. By the time the disorder is discovered – often after the produce has been packed and shipped – the economic damage is done. Non-destructive detection technologies like near-infrared spectroscopy and X-ray imaging are being explored for mangoes, but these are not yet widely available for commercial use.

Cultivar-specific susceptibility 🔗

These disorders often affect specific cultivars disproportionately. Spongy tissue is predominantly an Alphonso problem. Waxy breakdown tends to be more common in certain garlic types. Marginal browning is more severe in some escarole and lettuce varieties than others. This cultivar specificity suggests a genetic component, but breeding programs for resistance are slow, particularly for perennial crops like mangoes.

Current research and future directions 🔗

Advances in molecular biology, metabolomics, and non-destructive sensing are opening new avenues for understanding and managing these complex disorders.

Metabolomic profiling has been particularly useful for spongy tissue in mangoes. By identifying specific metabolites that accumulate differently in healthy versus affected tissue, researchers can build a clearer picture of which biochemical pathways are disrupted and when.

Preharvest management protocols are being developed based on the best available evidence. For Alphonso mangoes, spraying developing fruits with a nutrient mix containing macro- and micronutrients at 60-70% maturity has shown promise in reducing spongy tissue incidence to below 5%, compared to over 50% in untreated controls. For garlic, best practice curing with proper ventilation and shade remains the most reliable approach.

Systems-based approaches are gaining traction. Rather than looking for a single cause, researchers are treating fruit quality as the output of a dynamic system that integrates genotype, environment, and management across the entire preharvest-to-postharvest continuum. This holistic perspective may ultimately prove more productive than the search for individual causative agents.

The economic and practical stakes 🔗

These disorders are not merely academic curiosities. Spongy tissue alone has been a major barrier to expanding Alphonso mango exports from India to lucrative markets in the US and Europe, where the risk of rejected consignments discourages importers. Waxy breakdown in garlic can result in entire stored lots becoming unmarketable. Marginal browning reduces the shelf life and salability of fresh-cut escarole, directly affecting the ready-to-eat salad market.

For growers, the unpredictability of these disorders adds a layer of financial risk that is hard to manage. A perfectly managed crop can still develop significant losses if environmental conditions align unfavourably. This is why ongoing investment in research, detection technology, and integrated management strategies is essential for the horticultural industry.

What do you think? Given that these disorders involve so many interacting factors, do you believe a single “cure” is realistic – or should the focus remain on risk reduction through integrated management? How might emerging technologies like AI-driven crop monitoring change the way we predict and prevent these hidden quality problems?

References

1. https://www.gardeningknowhow.com/edible/vegetables/lettuce/treating-lettuce-with-tipburn.htm
2. https://www.sciencedirect.com/science/article/pii/S0308814606008910
3. http://ephytia.inra.fr/en/C/5961/Salads-Marginal-necrosis-tipburn
4. https://postharvest.ucdavis.edu/produce-facts-sheets/garlic
5. https://blogs.cornell.edu/livegpath/gallery/garlic/waxy-breakdown-on-garlic/
6. https://www.mofga.org/resources/fact-sheets/waxy-breakdown-of-garlic/
7. https://www.opensciencepublications.com/fulltextarticles/JPSR-2349-2805-1-114.html
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC6918312/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC11434751/
10. https://journals.sagepub.com/doi/10.1177/09670335241269005?icid=int.sj-full-text.similar-articles.1