Spores of decay-causing fungi are an ever-present threat to stone and pome fruit. Understanding exactly how the rot sets in can help growers prevent losses. By Anna Mouton.
A post-harvest disease is any disease where the symptoms appear after harvest. The terms disease and decay are often used interchangeably because decay is such a common symptom of post-harvest disease. Decay can take many forms, depending on the microbes responsible, but the end result is the same – fruit becomes unmarketable, and the producer loses money.
Facts about fungi
Most post-harvest diseases are caused by fungi. Fungi are complex organisms that occupy their own kingdom alongside plants and animals. Like animals, fungi do not photosynthesise, but obtain nutrients from breaking down organic matter. The fungi that cause post-harvest diseases in fruit will grow on decaying organic matter in the orchard. They also take advantage of the rich nutrients offered by flowers and fruit.
Fungi produce single-celled spores that can be spread by wind or water, and that can survive adverse environmental conditions. Spores may land on flowers or fruit at any time from flowering to harvest and beyond. If circumstances are not favourable for fungal growth, the spores may remain inactive. Alternatively, the spores may germinate, and the fungus establish, but so stealthily as to be undetectable.
The sneaky habits of fungi complicate the control of post-harvest diseases because fruit may appear flawless at harvest, even though they already carry the seeds of decay.
Spore to spore
Infections with post-harvest diseases primarily take place in the orchard. Fruits protect themselves against fungal invasion by outer defences such as their waxy cuticle and impermeable peel, and by inner defences such as antimicrobial substances and enzymes in their flesh. These mechanisms tend to decline as fruit maturity increases, especially after harvest. High sugar levels in ripe fruit encourage more aggressive fungal attack.
The transition from latent spore to active decay often starts with a wound, as wounds offer easy access to fruit tissues. In addition, wounds are usually associated with the dead or dying cells that many fungi require to gain a foothold. Once entrenched, some fungi produce enzymes that kill more fruit cells, whereas others can feed on living cells. Wounds need not be large – fungi can take advantage of microcracks that are invisible to us.
Fungi can also enter through lenticels and stomata. Some fungi are carried into the calyx by water or even by mites. Once fungi are actively growing in its tissues, the game is over for a fruit – although we should remember that decay is a natural process that facilitates the release of seeds from fruit.
For the fungus to complete its life cycle, it has to produce spores. Spore formation is often responsible for the colourful mouldy appearance of spoiled fruit. Sporulation can be triggered by environmental conditions such as warmer temperatures – this is why a break in the cold chain can spell disaster.
Mouldy fruit are a concentrated source of spores that infect other fruit during handling, storage, and transport. Mouldy fruit or mummified fruit that remain on the tree will likewise spread spores in the orchard.
Factors that promote post-harvest diseases
Post-harvest diseases result from an interaction between the disease-causing agent, the fruit, and the environment. As mentioned above, the agents are usually fungi, but may sometimes be bacteria or other microbes.
Certain fungi tend to target certain fruit types, and within fruit types, certain cultivars are more prone to certain diseases. For example, apple cultivars that have an open calyx, such as Red Delicious and its relatives, are more likely to develop core rot, while bull’s-eye rot tends to affect Cripps Pink. This suggests the possibility of developing cultivars that are better able to resist post-harvest diseases.
General fruit factors that increase the risk of post-harvest diseases include increasing maturity, stress, damage, wounds, and extreme temperatures. Besides impacting fruit physiology, temperatures also affect fungal growth and sporulation.
The most important environmental factor that drives fungal infection is moisture. The risk of many post-harvest diseases skyrocket with rainy weather during harvest. One reason is that moisture promotes germination of fungal spores. Another is that rain splashes spores onto fruit, and washes spores into cavities and wounds. But wet conditions can also cause fruit to swell, resulting in small gaps in their peels that open the door to infections.
Five common fungal felons
- Penicillium species cause decay of both pome and stone fruit.
- Penicillium often causes a soft rot, but symptoms vary according to which species of Penicillium is attacking which type and cultivar of fruit.
- Penicillium expansum and other Penicillium species are associated with wet core rot of apples in South Africa.
- Infections at the fruit surface may produce furry white fungal mycelium, followed by blue or green spores, hence the term blue mould for Penicillium infections.
- Several species of Alternaria cause pre- and post-harvest diseases of fruit and vegetables.
- Alternaria infections commonly occur at flowering and fruit set.
- Alternaria alternata is associated with dry core rot and mouldy core of apples in South Africa.
- Alternaria rots in pome and stone fruit tend to be firm.
- Both the mycelium and the spores of Alternaria are usually dark.
- Botrytis cinerea causes grey mould on more than 200 plant species, from hemp to hops. Other species of Botrytis also affect plants, but less commonly.
- Botrytis spores are ubiquitous. Botrytis also produces resistant structures called sclerotia to survive harsh environmental conditions.
- Primary Botrytis infections frequently occur as early as flowering, and continue throughout the season. Secondary infections during storage
- Infections eventually produce furry grey fungal mycelium, followed by powdery grey spores. The skin of infected fruit becomes loose and easily pushed off.
- Monilinia infection is more commonly seen in stone than in pome fruit.
- Monilinia laxa is associated with brown rot of stone fruit in South Africa.
- As for many other post-harvest diseases, Monilinia infections start as early as flowering and continue until harvest. Monilinia can enter fruit through stomata.
- Brown rots in stone fruits are firm. The fungal growth is light-coloured. Spores are light brown and often form concentric rings as the infection spreads.
- Phlyctema (Neofabraea) vagabunda is the only known cause of bull’s-eye rot of apples in South Africa.
- The fungus infects the lenticels of apples while still in the orchard, and can remain dormant for several months.
- Symptoms usually appear after 3-5 months in storage. The disease is named for its characteristic round lesions that have a dark outer ring and a lighter centre. Fruit may have several lesions.
- The fungus may eventually form cream-coloured spores at the centre of the lesions.