By John Hartman

A good apple or pear crop can be ruined due to decay caused by parasitic fungi. Several of the fungi that cause fruit rot disease can begin their infections at bloom or shortly thereafter. The fungi may invade killed fruitlets, infect sepals, or exist in a latent phase in healthy fruit, only to begin decaying them when they reach full size. Apple and pear fruit rots can occur both in the orchard and in storage after harvest. Decayed 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.

Recent observations in the U.K. plant disease diagnostic laboratory suggest that the black rot fungus successfully invaded apple and pear flowers or young fruitlets causing black rot (Figure 8). Black rot is caused by the fungus Botryosphaeria obtusa. The fungus infects blossoms, leaves, twigs, branches, and fruits. On leaves, the fungus causes frogeye leaf spot (Figure 9). 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 (Figure 10). 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 growing 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. Black rot fruit infections are favored by temperatures about 70 degrees F 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 (Figure 11) which remain attached to the tree, serving as inoculum sources in the spring of the following year.

Growers finding black rot in their orchards now will want to review their disease management practices in order to have better results next year.

• Remove from the tree or pick up off the ground overwintering fruit mummies and destroy them.

• Prune out dead and diseased branches and remove from the orchard or chop up twig and branch prunings in late winter before the growing season begins.

• Maintain an effective spray schedule. See the U.K. Extension publication ID-92, 2010 Midwest Tree Fruit Spray Guide, available at County Extension Offices statewide.

• Fungicides containing active ingredients such as thiophanate-methyl, captan, mancozeb, azoxystrobin, or kresoxym methyl most likely have good effectiveness against black rot.