Factors affecting postharvest disease development
Low temperature storage cannot prevent disease, but can limit the rate at which disease develops. The optimum temperature for spore germination of most fungal pathogens is 20–25°C. Although germination is possible at a much wider range, the further the temperature is from this optimum the slower germination will be. Cold temperatures also delay deterioration of the product, reducing susceptibility to decay.
Fungi differ in their ability to grow at low temperatures;
- Aspergillus niger is unable to grow below 11°C,
- Colletotrichum spp. cannot grow below 9°C and
- Rhizopus stolonifer cannot grow below 2°C.
- Botrytis cinerea can continue to grow down to –2°C and
- Alternaria alternata still develops at –3°C, making these diseases hard to control in storage.
High relative humidity and free moisture on produce both increase opportunities for disease development. Dry conditions mean that fungal spores are unable to germinate and grow, and bacteria cannot multiply. Even if fungal germination does occur in a small amount of moisture, low humidity can inhibit further growth.
It is impossible to remove every fungal spore or bacterial cell. However, reducing the number that is present—the microbial load—reduces the chance that infection will occur, even if other conditions are suitable. Reducing the microbial load generally involves pre-harvest hygiene and/or postharvest removal by washing.
Pre-harvest hygiene involves removing sources of inoculum. This may mean removing or rotary hoeing diseased crop residues, using pre-harvest fungicides and ensuring good plant health; healthy, well grown crops are less susceptible to disease infection and development than those grown under more challenging conditions.
Vigorous washing and/or brushing can reduce microbial load by 99% or more. Factors that affect how effectively the washing process removes bacteria and fungal spores include:
- Temperature of the water—high water temperatures can increase the effectiveness of washing. For example, short hot water rinsing and brushing treatments are used commercially to reduce postharvest disease on fruit vegetables such as capsicums and tomatoes. Treatments typically range from 50 to 60°C and last for 10 to 30 seconds. This method has been shown to remove 99.9 to 99.99% of pathogens on the product surface.
- Vegetable structure—products with a smooth surface will be easier to clean than those with an irregular surface or complex structure, like cabbage.
- Presence and concentration of a sanitiser and pH of the water.
- Number of washes and amount of agitation—multiple washes are more effective than one.
- Cleanliness of the water—if the water already contains a large amount of organic matter then sanitisers will be ineffective and the washing process may deposit more microbes than it removes.