The influence of flower-feeding thrips on grape bunch rot

Abstract

Botrytis bunch rot, caused by the fungus Botrytis cinerea, is a serious disease of grapes. The fungus is ubiquitous in vineyards and has numerous pathways of survival, spread and infection but their contribution to the overall epidemiology of the disease is not clear. The role of flower-feeding thrips in the development of this disease has not been studied in New Zealand. The objectives of this study were to determine the species of thrips that infest grape inflorescences and their prevalence in wine-growing regions of New Zealand, and to study their effect on grape berries, both for surface damage and development of B. cinerea infection.

In spring 2001, five grape growers from each of the regions, Hawke's Bay, Marlborough and Canterbury, and three in Auckland sent 10 inflorescences of the cultivar Chardonnay, for examination of the numbers and species of thrips within them. Thrips were detected in all grape inflorescence from all regions and all were identified as New Zealand flower thrips (NZFT) (Thrips obscuratus) with the exception of one specimen of Aeolothrips fasciatus. Mean numbers of thrips per inflorescence differed significantly between regions, being 31.5, 6.9, 6.0 and 2.5 for Canterbury, Auckland, Hawke's Bay and Marlborough respectively. The highest number of thrips per bunch, 235, was recorded from Canterbury, where adjacent gorse and broom may have contributed to the higher thrips populations.

Greenhouse trials in 2001 used rooted, fruiting grapevine cuttings and NZFT collected from flowers of wild oxeye daisy (Chrysanthemum leucanthemum). The effect of NZFT numbers on grape surface damage was investigated in a threshold experiment, the different levels of thrips being enclosed in fine silk bags for 7 days around Riesling inflorescences. The thrips caused scars to develop on ripe Riesling grape berries with mean scar scores differing significantly between levels of thrips infestations. The mean scores were 0.35, 0.64, 1.47, 1.21, and 1.19 for 0, 5, 10, 20 and 40 thrips per inflorescence respectively. Signs of oviposition by thrips were observed on pedicels of inflorescences collected from the field, and from ripe berries treated with NZFT at flowering.

The effects of introducing NZFT and conidia of B. cinerea to grape inflorescences on the incidence of the fungus in green and ripe berries were investigated in two greenhouse trials. In 2001/2002, 10 thrips per inflorescence and conidia of a pathogenic marker strain of B. cinerea (883-1 Nit1) were introduced to inflorescences on rooted cuttings of Pinot Noir and Riesling grapes, in a 2x2x2 factorial experiment. Thrips were removed after one week and treatments kept free of thrips until harvest. Thrips were shown to carry conidia of B. cinerea on their bodies and to transfer them to grape inflorescences since the marker strain of B. cinerea was recovered from the ripe berries that had developed from them. This provided the first report of latent infections of B. cinerea in grapes in New Zealand. In Pinot Noir berries, the presence of thrips did not result in greater recovery of the marker B. cinerea isolate, but in Riesling berries it was recovered at 140% greater incidence in thrips infested treatments than in treatments with no thrips.

In the 2002/2003 growing season, the trial with NZFT and the marker strain of B. cinerea was repeated, using 3 year old potted Pinot Noir vines which produced larger bunches than the rooted cuttings. The marker strain of B. cinerea introduced to inflorescences was recovered from pea-size, green berries and ripe berries with incidences of 19.0 and 4.4% respectively, while recovery of total B. cinerea was 28% and 14% on green and ripe berries, respectively. However, the presence of thrips did not result in an increased incidence of marker or wild-type isolates of B. cinerea. The reduced recovery of the fungus between the two sampling periods in this study suggests that viability of latent mycelia may be influenced by host responses.

This study has demonstrated that thrips have a role in the development of botrytis bunch rot in grapes, and this needs to be further investigated in field trials.