How infection occurs
Most organisms that cause postharvest disease are weak pathogens. Only a few fungi (such as Colletotrichum) can directly invade through healthy skin. Most fungi enter vegetables through wounds in the skin, or infect products already damaged by unsuitable storage conditions.
Pre-harvest infection can occur by direct penetration of the skin, entry through natural openings (stomata and lenticels), transmission through the parent plant, and through damaged areas. Plant defence responses are able to hold such infections in check while the plant is actively growing. However, once removed from sources of water and nutrient, the disease can take hold.
Infection of some fruiting vegetables (eg. internal rots in capsicums) actually occurs during flowering. Fungi such as Alternaria and Botrytis are able to infect flower petals. However, they only start to develop once the vegetable has matured.
Many plant pathogens are present in the soil or on plant debris. Vegetables that are grown in contact with the soil are likely to be carrying spores or mycelium that cause infection, even though these are not apparent at harvest.
In the example shown below, pak choy grown hydroponically on raised benches appeared similar at harvest to the same product grown in the ground. However, the hydroponic product had consistently longer shelf life, largely because of postharvest development of leaf spots and rots on the field-grown product. The pathogens responsible for these diseases are present in the soil but transfer to the leaves by water splash during rain or irrigation.
Fungal spores are also carried on the wind. Nearby crops infected with fungal pathogens such as white blister (Albugo candida), or blue mould (Penicillium spp.) release huge numbers of spores, which can land on the intact vegetables. These spores are unable to germinate while the product remains dry. However, condensation during cooling or storage can allow germination, growth and infection.
Fungal spores are present in the air, on equipment, on containers and on the hands of harvest workers and packers. However, for those spores to infect the vegetables there must be suitable conditions for spore germination and growth as well as openings that allow them to penetrate the plant tissue.
Moisture is essential for spores to germinate. While most can germinate in pure water, if nutrients are present then germination is increased and growth is more vigorous. For example, studies have found that spores of Rhizopus spp. and Botrytis cinerea need both water and nutrients to allow spores to germinate and penetrate a host. Bacteria also need water plus nutrients for cells to divide and multiply.
Wounding allows cell contents to leak out. Broken cells provide both water and nutrients, in the form of sugars, acids and other substrates. Wounds therefore provide ideal environments for spores to germinate and bacteria to grow.
Moreover, while waxy epidermal (skin) cells represent a significant barrier to infection, the cells underneath are less protected. Once established in a wound site, pathogens can directly attack these cells and spread through the plant tissue.