3.1.1
TIMING OF INFECTION RESULTING IN STEM END GRAY MOULD OF PEAR FRUIT

CL LENNOX and RA SPOTTS

Mid-Columbia Agricultural Research and Extension Center, 3005 Experiment Station Drive, Hood River, Oregon 97031, USA

Background and objectives
An investigation conducted on the incidence of decay of d'Anjou pear fruit in storage for the 1994 through 1996 crops indicated that Botrytis cinerea is the major pathogen involved in post-harvest decay in the Hood River area. The short fleshy stems of d'Anjou pears appear to be highly susceptible to infection and decay by B. cinerea. Stem end infections cause significantly higher levels of decay than calyx end infections. The objectives of this study were to determine when stem end infections occur and to describe the progress of stem end decay in storage.

Materials and methods
Fruit from 14 commercial d'Anjou pear orchards was collected once a month from full bloom until harvest in 1996 and 1997. The fruit was triple sterilized and, using a sterile scalpel, stems were removed, leaving a small amount of pear fruit material attached to each stem. The stems were then cut in half lengthwise, and each half cut into three pieces before being plated onto an agar medium semi-selective for B. cinerea.[1]. Records were made of stem pieces infected with this fungus. At commercial harvest times, fruit was sampled from each of the 14 orchards, and placed into storage at -0.5C. This stored fruit was evaluated for decay after 3, 6 and 8 months, and 8 months plus a 1-week ripening period at 20C.

Results and conclusions
Results from the plating out of stem pieces in both 1996 and 1997 indicate that d'Anjou pear stem tissue was resistant to infection by B. cinerea until July, after which time the fungus was isolated from stems of fruit from a few of the orchards. In all cases B. cinerea was isolated from the piece of stem which included the stem abscission zone. Levels of infection were higher in stems sampled at harvest than at earlier times. From all 14 orchards sampled in 1996, little or no stem end grey mould was detected in d'Anjou pears after 3 months in storage. The incidence of stem end grey mould increased significantly between 3 and 6 months, and further increases occurred between 6 and 8 months, and 8 months followed by a ripening period of 1 week at 20C. Mean percentage stem end grey mould for the 14 orchards was 0% after 3 months; 1.8% after 6 months; 3.4% after 8 months; and 3.6% at 8 months plus the ripening period. The results from both 1996 and 1997 indicate that stem end infections occur from late summer through harvest. Contamination of the abscission zone of the stem of harvested fruit by B. cinerea spores present in the air, on fruit surfaces, and in the general fruit environment is possibly an important source of infection resulting in stem end decay in storage. The application of a contact fungicide shortly before harvest, or a pre-storage drench immediately after harvest, may reduce the inoculum levels and protect the abscission zone from infection by B. cinerea. Fruit from orchards with histories of high levels of stem end grey mould should be packed and shipped within 3-6 months in storage.

References
1. Kerssies A, 1990. Netherlands Journal of Plant Pathology 96, 247-250.