Timing of Perennial Canker Development in Apple Trees Caused by Neofabraea perennans and Neofabraea kienholzii

Members of the genera Neofabraea and Phlyctema have been reported to incite canker diseases of apple trees and a postharvest decay of apple fruit referred to as “bull’s-eye rot.” Neofabraea kienholzii was recently identified as participating in the bull’s-eye rot disease complex of apple and other pome fruit. In this study, apple twigs inoculated with N. kienholzii were shown to develop symptoms of a canker disease closely resembling perennial canker of pome fruit trees caused by N. perennans. Cankers resulting from infection by either Neofabraea spp. were more likely to be induced when twig inoculations occurred in October, and to a lesser degree in April, compared with all other inoculation dates evaluated in this study. Although N. kienholzii tended to induce cankers that were smaller in size compared with N. perennans, both pathogens shared similar seasonal trends in the initiation and expansion of tree cankers. N. perennans and N. kienholzii were recovered from inoculated twigs 6 months postinoculation regardless of when inoculations were conducted, indicating that both pathogens can survive on diseased twigs year-round. In addition, acervuli were observed more often on twigs inoculated in September and April compared with those inoculated in other months. Data from this work should help further our understanding of the epidemiology of N. kienholzii. This information also highlights the importance of proper branch pruning, canker removal, and aphid control. Such management activities should be conducted in a manner that helps minimize further spread of the pathogen.

Control of Bull’s-Eye Rot of Apple Caused by Neofabraea perennans and Neofabraea kienholzii Using Pre- and Postharvest Fungicides

Bull’s-eye rot is a major postharvest disease of apple caused by several fungi belonging to the Neofabraea and Phlyctema genera. Chemical control of these fungi is a crucial component of disease management for apples that are conventionally grown. The efficacy of several preharvest and postharvest applied fungicides were evaluated to identify effective chemistries that can control bull’s-eye rot incited by Neofabraea perennans and N. kienholzii on apples. In general, the preharvest fungicide thiophanate-methyl was found to be effective at reducing disease caused by N. perennans and N. kienholzii. Two postharvest fungicides, thiabendazole and pyrimethanil, also provided disease control that was far superior to other chemical compounds evaluated in this study. The efficacy of thiabendazole and pyrimethanil was unaffected by application method (fungicide dip compared with thermofog). Despite providing satisfactory control of bull’s-eye rot, integration of these three chemicals into disease management programs should proceed judiciously with consideration of their impact on the development of fungicide resistance and influence on diversity in populations of apple postharvest pathogens.

SUPPRESSION OF BULL’S EYE ROT OF APPLES AND GROWTH OF NEOFABRAEA PERENNANS BY SCALD CONTROL AGENTS DIPHENYLAMINE AND ETHOXYQUIN

Postharvest dips of apples in diphenylamine or ethoxyquin, at concentrations that control storage scald, markedly reduced bull’s eye rot of fruit artificially inoculated with spores of Neofabraea perennans obtained from a local culture. Similar reduction in rot was obtained with the fungicide dodine, whereas the antioxidant propyl gallate was not effective.Comparative fungistatic tests indicated that dodine was a potent inhibitor of spore germination of N. perennans, and was only slightly less effective in suppressing hyphal growth. Diphenylamine inhibited hyphal growth to a greater extent than spore germination, while ethoxyquin inhibited hyphal growth only. Propyl gallate was essentially inactive in all tests. These data suggest that effectiveness of all three active materials against bull’s eye rot derives mainly from their ability to suppress hyphal growth.