Overwintering grapevine debris as an important source of Botrytis cinerea inoculum

Production of Botrytis cinerea conidia from infected grapevine debris (trash) left on the ground and in the canopy in the season following harvest was studied in vineyards in Marlborough, New Zealand. When subsamples were incubated under high relative humidity in the laboratory, rachides had the greatest sporulation potential (P < 0·05), followed by tendrils, cane lengths and petioles. Trash remaining on the ground under the canopy had higher rates of sporulation (P < 0·05) than that in the inter‐row. The sporulation potential of rachides at different times during the growing season was assessed by placing them in vine canopies or on the inter‐row soil in three vineyards in late spring. Subsamples were removed on five occasions between flowering (capfall) and harvest, and incubated under high relative humidity in the laboratory. Mean numbers of conidia produced from the canopy rachides diminished from 3·5 × 10⁵ per rachis at capfall to 2·6 × 10⁴ at harvest, and from 3·9 × 10⁵ to 2·7 × 10³, respectively, from the ground rachides. The greater loss in sporulation capacity of ground rachides was considered to be associated with their earlier spontaneous sporulation and greater degradation in the moist inter‐row sward, where they lost 29% of their weight (P < 0·001) and 23% of their pedicels (P < 0·001), compared to the canopy rachides which lost 0% of their weight and 3% of their pedicels from capfall to harvest. This study has shown that necrotic, overwintering grapevine debris can produce B. cinerea conidia throughout the following growing season, so may contribute to the subsequent risk of bunch rot.