Diseased or dead tree tissue is a major source of inoculum for sphaeropsis rot and speck rot in orchards. Cankers caused by the decay organisms Sphaeropsis pyriputrescens and Phacidiopycnis washingtonensis are uncommon in commercial apple trees in Washington state, but the Manchurian crab apple is highly susceptible to both organisms.
These fungal pathogens can cause twig dieback and cankers in trees and infect the fruit. Both fungi can produce small black dots (fruiting bodies) on twigs and branches, which contain millions of spores that serve as inoculum for fruit infection.
Rain, irrigation water, or overtree cooling can spread the spores to nearby apple trees. Although fruit infection begins in the orchard, symptoms develop during storage or at the market.
Both types of decay can affect the stem end or the calyx end of the fruit. Decayed areas can turn light or dark brown, or occasionally black. Fruit with sphaeropsis rot have a distinct bandage-like odor, especially when cut. Speck rot is so named because of the brown or black specks, with lighter centers, that may appear around the lenticels.
Detailed pruning of crab apple trees to remove all infected twigs, branches, and fruit mummies will significantly reduce the risk of infection. In addition, overhead irrigation should be avoided and over-tree cooling limited to the amount necessary to prevent sunburn.
Preharvest fungicides, such as Ziram, Pristine (pyraclostrobin and boscalid), or Topsin M (thiophanate-methyl) applied near harvest can reduce both speck rot and sphaeropsis rot. However, a postharvest fungicide drench with Penbotec (pyrimethanil), Scholar (fludioxonil), or Mertect (thiabendazole) is generally more effective than a preharvest treatment.
Source: Chang-Lin Xiao, U.S. Department of Agriculture, Parlier, California, and Richard Kim, Pace International, Wapato, Washington.