Postharvest diseases of apples account for substantial losses on an annual basis. In survey work done at the Pacific Agri-Food Research Centre (PARC) in Summerland, British Columbia, in conjunction with the Okanagan Federated Shippers Association industry research program, rot data from storage experiments were collected from 1993 to 1998. In 1998, for example, the percentage of apples that displayed fungal rot ranged from 0% to 25%, including 0% to 15% for Braeburn fruit in two experiments, 0% to 15% for McIntosh, and 0% to 10% for Galas after only four months of air or controlled atmosphere (CA) storage. Some readily identifiable trends from the survey work are evident. Postharvest rots are favored by early and particularly late picks, lengthy storage (especially in air), or at warm temperatures. Variability exists from grower to grower in the percentage of rot that occurs for a given cultivar, indicating that individual grower practices influence the storability of fruit.
Postharvest Diseases and What to Do
The two most important postharvest diseases of apples and pears identified in the survey are blue mold, caused by Penicillium sp. (approximately 50% of all rots) and gray mold caused by Botrytis cinerea (approximately 40% of all rots). In nature, each of these fungi function as important degraders of organic material. They can grow over a very wide range of temperatures and are generally not affected by environmental atmospheric conditions or pH. Thus, rot can and does occur in CA storage.
Blue mold (Penicillium sp.) is a wound pathogen; i.e., it has no means of penetrating the fruit cuticle but must rely on a puncture or tear in the fruit surface in order to establish itself. Blue mold spores are everywhere, and high levels of contamination will increase the likelihood of fruit infections. Best efforts at sanitation in the packing house will only alleviate, not eliminate, the potential for blue mold rot.
What can you as a grower do? Fortunately, the same general horticultural principles that guide production of good quality fruit also enhance the fruit's ability to resist infection by blue mold. In particular, fruit with a high calcium level and low nitrogen level will be very tolerant of the presence of blue mold. Because calcium and nitrogen levels can vary considerably on individual trees, and throughout an orchard, factors such as thinning to achieve even distribution of fruit may influence the amount of rot that occurs. The use of foliar calcium sprays is recommended to help reduce the incidence of postharvest rot by Penicillium. Data from experiments conducted from 1996 to 1998 in a Fuji and Braeburn planting indicate that if foliar sprays result in the elevation of calcium levels in the fruit, postharvest rot will be significantly reduced. The effect was more noticeable on Braeburn than on Fuji. In 1996, calcium levels in Fuji and Braeburn were significantly raised over the check by six calcium sprays before harvest (mid-August to mid-September). Postharvest rot due to blue mold was significantly reduced. In 1997 and 1998, the application of four (August only), and five (August only) calcium chloride sprays, respectively, did not significantly increase the calcium levels in the fruit and there was no difference in the amount of postharvest rot resulting in the check or calcium sprayed fruit.
Gray mold (Botrytis cinerea) is also a wound pathogen. It is present throughout the growing season in the orchard, particularly on flowers and developing fruit. During the growing season, Botrytis can cause calyx end rot of immature fruit. In storage, the fungus has the ability to "nest;" it can readily spread from one apple or pear to another in a bin, resulting in greater losses than otherwise might occur. In work done at PARC-Summerland monitoring Botrytis populations in the orchard, the fungus has been found to be present in high numbers on flowers, particularly toward the end of the bloom period, and numbers rise again toward harvest. For conventional growers, the inclusion of a fungicide effective against gray mold, as well as apple scab or powdery mildew, as part of the regular bloom spray schedule is suggested to reduce Botrytis inoculum when the calyx end of the fruit may be open and susceptible to infection. Organic growers should be vigilant in applying their protectant sprays at bloom. Experiments conducted on Jonagold, Fuji, Braeburn, and Gala plantings in 1997 and 1998 showed that a pinpoint scab spray applied one month to two weeks before harvest consistently significantly reduced the amount of postharvest rot caused by Botrytis. Cultivar differences in susceptibility to gray mold were noted. Gala was the most resistant, followed by Fuji, Braeburn, and Jonagold.
Current work being done at PARC is focused on identifying key factors that can be used potentially as a check list at the packing house to determine which lots of apples have a greater potential for postharvest rot. Orchards with a history of significant gray mold rot and orchards with a history of little gray mold rot throughout the Okanagan Valley, as identified by the survey, are being monitored for environmental conditions throughout the growing season, cultural practices, especially irrigation, and gray mold populations on fruit surfaces and ground cover. Data analysis is not yet complete, but consistently higher populations of Botrytis are present in the more susceptible orchards.
Orchard sanitation is critical for any disease management. Whatever can be done in the orchard to reduce fungal spore inoculum is beneficial for control of field diseases and postharvest diseases alike. This should be a priority for both organic and conventional growers. Removal of fruit from the orchard floor, incorporation of leaf and fruit litter into the soil for microbial breakdown, and good weed control (Botrytis grows on a number of plant species), will aid in the ability to reduce the spore load on fruit going into storage, and the amount of primary inoculum available to infect next spring.
In summary, the general principles for reducing postharvest disease problems include maintenance of good orchard sanitation; provision of adequate fruit nutrition, in particular calcium foliar sprays; timely application of appropriate fungicides, including a pinpoint scab spray preharvest; avoidance of early or late harvests; and gentle handling of fruit to limit wounding. In short, good management will ensure high-quality, storable, disease tolerant fruit.
Pacific Agri-Food Research Centre (PARC)
CANADA V0H 1Z0
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000