Postharvest Diseases and Disorders of Apples and Pears
The most common postharvest diseases and disorders of apples and pears are described below, along with current control techniques.
Alternaria Rot (Alternaria alternata). Alternaria rot may occur on apples and pears in any production stage. This fungus lives on dead and decaying plant tissue in the orchard, and its spores contaminate fruit in the orchard or during the handling process. The amount of decay depends on the condition of the fruit. Infection usually occurs through breaks in the skin or other weakened areas caused by sunburn, bruising, chemical injury or scald.
Blue Mold (Penicillium spp.). Blue mold is a common destructive rot found on fruits in storage and in the market. Several species of Penicillium can cause blue mold, the most common being P. expansum, P. solitum and P. commune. The species that causes the most extensive decay is P. expansum. Most decay caused by Penicillium is initiated at wound sites, such as cuts and stem punctures, but fruit can also become infected through lenticels on unbroken skin, particularly at bruise sites. In very mature fruit or late in the storage season when fruit has been weakened by ripening and aging, most varieties are susceptible to lenticel infection. Decay by P. solitum and P. commune is also most prevalent at this time. Infection by all species of Penicillium can start when pears or apples are handled carelessly during the packing process. Environmental conditions such as moisture, ventilation and temperatures directly influence the development of decay. Poor ventilation around storage containers leads to increased moisture around the fruit and slower cooling times, which can increase the risk of infection. Likewise, delays in cooling fruit after harvest also increase the chance of blue mold. Careful fruit handling, packinghouse sanitation, prompt fruit cooling, and proper temperature management during storage and transit to market are keys to reducing blue mold.
Bull's Eye Rot (see Anthracnose, Perennial Canker and Bull's-Eye Rot of Apple and Pear by Frank M. Dugan).
Coprinus Rot (Coprinus psychromorbidus). Coprinus rot has been found throughout the Pacific Northwest. It is often mistaken for bull's eye rot. Fruit infection occurs during the last month before harvest. This fungus appears as a white, cobwebby growth on the surface of infected fruit and will create nest or cluster rot like gray mold.
Gray Mold (Botrytis cinerea). Botrytis rot is a common decay of apples and pears. This fungus enters through punctures and wounds; therefore, minimizing fruit injury will reduce the amount of decay. The source of Botrytis spores is the orchard. The fungus grows and sporulates abundantly on dead and dying plant material found in orchard cover crops, especially during cool, moist weather. Infection can occur in the orchard or at any time in the handling process that the spores contact unprotected fruit wounds or susceptible tissue (e.g., Anjou pear stems). These initially rotted fruits spread the disease through fruit contact to produce nests or pockets of decaying fruit. The disease is often called nest or cluster rot.
Mucor Rot (Mucor piriformis). Mucor is a soilborne fungus that grows well even at cold temperatures. The fungus grows in fallen fruit on the orchard floor, so any practice that reduces rotting fruit in the orchard or movement of spore-laden soil into the packinghouse on bins will help reduce this rot. There is no effective fungicide registered for control of mucor.
Side Rot (Phialophora malorum). Side rot can affect both apples and pears but is most severe on pears, particularly the Bosc variety. Similar symptoms can be caused by the fungus Cladosporium herbarum. Spores of this fungus also enter the packinghouse on bins or fruit contaminated with orchard soil.
Bitterpit. Bitterpit is a disorder of apples related to a mineral imbalance within the fruit. It appears as depressed, dark sunken spots scattered on the surface of the fruit and is more severe on the calyx end. Peeling the affected area reveals dry, brown corky flesh. It often appears after harvest, although it can be found in fruits of certain varieties in the orchard when the problem is severe. Some orchards are affected more than others; production practices can minimize the disorder. Granny Smith, Jonagold and Golden Delicious apples are more susceptible than Red Delicious. Calcium sprays in the orchard followed by a postharvest drench of calcium have been shown to reduce the incidence of the disease. Complete control is achieved only when production practices are modified to minimize fruit stress and a complete calcium nutrition program is employed.
Cork Spot. Cork spot (Anjou pit) is a disorder of pears related to a mineral imbalance within the fruit. Affected fruit develop a bumpy, uneven appearance as they approach maturity. Frequently, the affected area will yellow and peeling reveals pockets of brown or gray corky areas. Cork is most serious in years of low relative humidity, high early season temperatures, low yield, and large fruit. Lower calcium levels have been found in affected fruit. As with bitterpit, reduction of cork spot can be achieved through modification of production practices to reduce fruit stress, including a preharvest calcium spray program.
Scald of Apples and Anjou Pears. These are physiological disorders characterized by a brown discoloration of the skin which occurs during storage. This browning is thought to be caused by the production of naturally occurring toxic compounds in the fruit peel. Fruit harvested early is at greatest risk. While the most reliable way to reduce scald is use of postharvest antioxidant drenches or sprays, researchers are looking at other methods of scald control including prediction of its occurrence and severity and use of controlled atmosphere storage.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Tree Fruit Postharvest Journal 4(1):3-4