"One bad apple ..." - preventing postharvest decays in apples
CALS Impact Statement
Apples harvested in autumn are held for up to 10 months in refrigerated low-oxygen storages so that high-quality apples will be available in grocery stores throughout the year. The fungal pathogen Penicillium expansum causes decays in stored apples. This pathogen was controlled with benzimidazole fungicides from the early 1970s through the mid-1990s, but then fungicide-resistant strains of the pathogen resulted in mounting losses to P. expansum. Epidemiological studies in commercial orchards, storages, and apple packing houses showed that P. expansum was invading apple fruit via their stems during long-term storage and that high boron concentrations in fruit increased susceptibility to postharvest decay. Inoculum that originated in the field each year was relatively unimportant compared to much larger quantities of inoculum that cycled from year to year on wooden and plastic storage bins. Apple growers informed of these results reduced boron fertilizer regimes so as to minimize fruit susceptibility to decay and storage operators adopted sanitation measures that reduced inoculum levels on bins. One large packinghouse installed a $75,000 automated bin washer to decontaminate bins as they are emptied in the packing house. Results from this work have helped to maintain profitability of the New York apple industry.
New York produces more than 26 million boxes of apples annually. Apples are harvested in September and October. However, high-quality New York apples are available in grocery stores year round because apples can be held for up to 10 months in controlled-atmosphere cold storages before they are graded, packed, and shipped to supermarkets. During the storage period between harvest and packing, several fungal pathogens can invade apple fruit and cause postharvest decays. From the early 1970s through the mid-1990s, these postharvest pathogens were controlled by treating apples with benzimidazole fungicides. However, the most common decay pathogen, Penicillium expansum, gradually developed resistance to the benzimidazole fungicides and postharvest decays began to appear in up to 20 percent of apples that were stored for more than six months. Furthermore, although the scientific literature indicated that P. expansum invaded fruit primarily through wounds, the decayed apples that appeared in the mid-1990s were mostly devoid of wounds. By the time apples come out of long-term storage, the investment in these fruit is roughly double what it was at harvest. Furthermore, decayed apples foul packing lines, increase packing costs, and contribute inoculum that initiates more decay in packed fruit after it is shipped. Apple storage operators needed information on how and why P. expansum was invading intact fruit and new options for controlling this pathogen.
Studies were initiated in the mid-1990s to determine how P. expansum invades non-wounded fruit. Results from replicated trials using fruit from six orchards over two years showed that P. expansum invaded fruit through stems when fruit were stored for more than six months. Fruit with high concentrations of boron were more susceptible to decay than fruit with moderate boron concentrations. Contaminated field bins were identified as a primary source of inoculum and were shown to carry huge quantities of spores from one season to the next. Some empty bins carried more than one billion spores per bin. However, the cost of sanitizing bins could not be justified if that effort would be negated by spore loads on fruit surfaces at harvest or by spores carried in soil sticking to bin runners as bins are removed from the field. A selective medium was developed to allow quantification of P. expansum spores present on fruit surfaces and in orchard soils. Results from multiple orchards over two years showed that inoculum density in orchard soils and on apple fruit at harvest were so low that these sources were likely to supply less than five out of every 10,000 spores carried into apple storages, with the remainder of the inoculum coming from contaminated bins. Thus, the benefits of bin sanitation were not likely to be compromised by inoculum coming into storages on apple fruit or soil on bin runners.
Information from these studies was presented at fruit grower and storage operator meetings as results became available. As soon as growers became aware that high fruit boron concentrations increased susceptibility to decay, they decreased ground-applied and foliar boron applications so as to allow boron levels to drop back to moderate levels. One large storage operator, upon learning that inoculum recycles on harvest bins, installed a $75,000 automated bin washer to decontaminate bins as they are emptied in the packing house. Informal surveys in several large packinghouses have shown that losses to postharvest decays have decreased significantly over the past few years as packing house operators improved sanitation of bins and in packing houses. Results from this work have helped to maintain profitability of the New York apple industry.
Chris Watkins (Department of Horticulture, Cornell Univ., Ithaca, NY)
Terence Robinson (Department of Horticltural Sciences, Cornell University, Geneva, NY)
Luke Laborde (Department of Food Science, The Pennsylvania State University, State College, PA)