Post-Harvest Disease Threat for Apples

Several of the fungi that cause fruit rot begin their infections at bloom or petal fall. These fungi invade fruitlets, infect sepals, or exist in a resting phase in healthy fruit, only to begin infecting and decaying them when fruit reach full size. Rots occur both in the orchard and in storage after harvest. Rotten fruit represent a significant loss to growers because much of the investment in the crop is made before the fruits show any indication of decay. When fungicide sprays are terminated too early in August there is a chance of higher incidence of summer rots that include fruit scab, black rot, white rot, and bitter rot, and sooty blotch and flyspeck (SBFS) on apple fruit during the last few weeks before harvest. These late-season outbreaks of summer diseases and fruit rots occur when inoculum is high in the orchard and when rainfall after the last fungicide application removes all fungicide residues several weeks prior to harvest. In the Penn State Fruit Research and Extension Center (FREC) research orchards, flyspeck was first observed in late July. The extended wetting periods in late July and August that accumulated 10 inches of rain favored the development SBFS, Bot rot (black  rot caused by Botryosphaeria obtusa and / or white rot caused by Botryosphaeria dothidea), and bitter rot (caused by Colletotrichum sp aka Glomerella cingulata).

Black rot and white rot can initiate infections in fruit lenticels during late summer. These infections may be invisible at harvest, or they may appear as small black spots at the lenticels. Both the latent infections and the lenticel spots can develop into severe fruit decays during storage. Under warm conditions the rot progresses rapidly. Rotted areas are soft and watery and tan to brown in color; fruit completely rot in a few days. Under cooler conditions, the rot is firmer and darker brown in color. It is difficult to separate white rot from black rot based on fruit symptoms.

Black rot fungus infects blossoms and leaves (causing frog-eye leaf spot) as well as twigs, branches, and fruits. Black rot inoculum originates from colonized dead wood within the tree or from mummified fruit and fruitlets. Fruit with black rot infections at the calyx end usually result from sepal infections that occurred early in the season (Fig. 1). These infections, which may happen as soon as the flower bud scales loosen, typically develop into blossom end rot. If black rot infections appear on the sides of fruit in summer, the source of inoculum can often be traced to one or more killed fruitlets located above the infection site within the tree canopy. Late fruit infections occur through cracks in the cuticle, wounds and lenticels. Temperatures near 70 degrees F favor black rot fruit infections with prolonged wetness. The black rot fungus can also be one of several different fungi that may be present in fruit with moldy core. Infected fruits eventually shrivel and dry down to pycnidia-covered mummies, which remain attached to the tree, serving as inoculum sources in the spring of the following year.

White rot of apple appears as a circular brown decay on the fruit surface. The fungus is ubiquitous in nature, causing diseases on a wide variety of other woody hosts such as birch, chestnut, willow, mountain ash, quince, pear, sweet gum, Rhododendron, grape, roses, stone fruit, blueberry, blackberry, currant and gooseberry.  As with black rot, the white rot fungus can also infect woody tissue and cause cankers. The white rot fungus does not infect apple leaf tissue. Latent infections may occur on immature fruit up to 7 weeks after petal fall. Fruit infections can occur throughout the growing season, but rot symptoms usually do not appear before soluble solids reach approximately 10 percent.  Fruit infection can occur in as few as 2 to 4 hours at 80 degrees F. Under warm conditions exceeding 80 degrees F the decay is soft, watery and a light tan color extending as a cylinder of decay from the surface to the core (Fig. 2). Under cooler temperatures, the decay is usually firmer and a darker tan. Most rotted fruits drop, but some may shrivel and remain attached to the tree, serving as a source of inoculum for further fruit infection.

Bitter rot appears as tan sunken lesions on the sides of fruit and the lesions will have slimy orange spores in their centers if fruit are damp. Bitter rot of apple is an economically important disease in the southern United States. This can be a devastating disease during prolonged warm, wet weather. Upon slicing the fruit in half, a V-shaped, brown, watery lesion can oftentimes be seen growing toward the apple core. Sanitation is an important strategy to managing this fungal disease. Bitter rot can spread rapidly during hot wet weather in late summer, and it sometimes causes storage decays in fruit that were exposed to high levels of inoculum shortly before harvest and that were then cooled slowly after being moved into cold storages.