Origins of Postharvest Diseases
Symptoms of postharvest diseases develop during storage, but infection of fruit by decay-causing pathogens may occur prior to harvest, at harvest, or during the postharvest handling process and storage. Based on the nature of infection, postharvest diseases of pome fruits can be divided into two categories: those that originate from latent fungal infection of fruit in the orchard and those that originate from infection of wounds, such as stem punctures and bruises, by fungi at harvest or during the postharvest handling and packing process. The former category is exemplified by Sphaeropsis rot caused by Sphaeropsis pyriputrescens, Phacidiopycnis rot caused by Potebniamyces pyri, and Bull’s eye rot caused by Neofabraea spp. The latter category is exemplified by blue mold caused by Penicillium spp., mainly P. expansum, and Mucor rot caused by Mucor piriformis. Understanding when infection occurs is an essential step for developing and implementing control measures to reduce storage losses due to decay.
1. Infection prior to harvest
• Infection of fruit by Sphaeropsis pyriputrescens (Sphaeropsis rot), Potebniamyces pyri (Phacidiopycnis rot) and Neofabraea spp. (Bull’s eye rot) in the orchard. These three pathogens are commonly associated with dead bark and twigs and cankers on trees. Stem-end rot and calyx-end rot are common symptoms of Sphaeropsis rot on apples and Phacidiopycnis rot on d’Anjou pears. Infection of the stem and calyx of the fruit occurs before harvest in the orchard, and symptoms develop after a period of time during storage. Previous reports indicated that the Bull’s eye rot fungus may infect fruit anytime between petal fall and harvest, but symptoms appear only after a few months of storage, and that fruit susceptibility to infection by the fungus increases as the growing season progresses.
• Infection of fruit by Phytophthora cactorum. P. cactorum is a soilborne and waterborne pathogen. Fruit are infected by pathogen-contaminated irrigation water. Fruit infected early by Phytophthora spp. may develop symptoms before harvest. Fruit infected by P. cactorum near harvest may develop symptoms in storage.
• Blossom infection early in the season. During the bloom period, the gray mold fungus Botrytis cinerea may colonize floral parts and cause latent infections. The fungus resumes growth when fruit become senescent after a period of time in storage and develop calyx-end rot. This has been reported on pears, but calyx-end gray mold is not commonly seen on fruit grown in the semi-arid climate of eastern Washington.
2. Infection at harvest
Some decay-causing organisms are essentially wound pathogens. Damage on the fruit skin such as stem punctures, limb rubs and bruises may be created at harvest or during the fruit handling process. Decay-causing pathogens, particularly those airborne pathogens such as gray mold fungus, Botrytis cinerea, and blue mold fungus, Penicillium expansum, may contaminate wounds and later cause decay symptoms in storage.
3. Infection after harvest
• Infection due to contamination of wounds by decay-causing pathogens during the drenching and packing processes. During the drenching process, spores of decay-causing pathogens such as Penicillium spp., Mucor spp., and perhaps Botrytis cinerea, accumulate in the circulated drench solution. Skin wounds or natural openings on the fruit could be inoculated with decay-causing pathogens during drenching and packing processes and later develop decay symptoms.
• Fruit-to-fruit spread during storage. One type of fruit infection during storage is fruit-to-fruit spread of decay in fruit storage containers (bins or boxes). Nesting of gray mold due to fruit-to-fruit spread is particularly common after an extended period of storage. Botrytis cinerea produces mycelia on decayed fruit, infects the surrounding sound fruit by mycelium through fruit-to-fruit contact, and consequently forms clusters of decayed fruit. Phacidiopycnis rot also has the ability to spread among fruit during storage. Thus, for fruit destined for a long-term storage, it is very important to control decay resulting from this type of infection because one decayed fruit can potentially destroy a considerable amount of healthy fruit.