Influence of climate on the susceptibility of grape berries to bunch rot fungi
C C Steel, L A Greer, and S Savocchia
NWGIC, School of Agricultural and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga NSW 2678
Abstract
Background and Aims - Bunch rot of grapes is typically caused by the fungus Botrytis cinerea (grey mould) however a range of other micro-organisms have been implicated in the rotting of grape berries. The aim of this work was to investigate the effect of climate on bunch rot organism predominance.
Methods and Results - Mature detached berries of Vitis vinifera (Cabernet Sauvignon) (22.4 °Brix) were inoculated with two bunch rot pathogens; B. cinerea or Greeneria uvicola (bitter rot) either singularly or in combination at one of two temperatures, either 20°C or 27°C. In co-inoculation studies B. cinerea out-competed G. uvicola at 20°C. Conversely G. uvicola out-competed B. cinerea at 27°C. In field trials conducted in the Hunter Valley in 2005 and 2006 non-Botrytis bunch rot incidence and severity was found to be greater in westerly facing canopies suggesting that the likely higher temperatures on the western-side of the row predisposed fruit to bunch rots such as G. uvicola. Bunch rot incidence and severity was further investigated over two growing seasons, 2006/7 and 2007/8 and data examined with respect to climatic records for the November to February growing period. The 2006/7 season was hotter (62 days 30 °C) and drier (176 mm rain) than the long term average. In this season B. cinerea was absent from the vineyards examined. The predominant bunch rotting organisms recorded were G. uvicola and Botryosphaeria spp. This contrasted with the 2007/8 season which was cooler (19 days 30 °C) and wetter (579.2 mm rain) than the long term average. In this season B. cinerea was recorded at the six sites examined and was the predominant bunch rot organism.
Conclusions - Bunch rot incidence and type is influenced by climatic conditions. In warm and dry years bunch rots such as bitter rot are likely to predominate in the Hunter Valley while cooler and wetter years are likely to lead to a higher occurrence of grey mould.
Significance of the study - Our findings support the hypothesis that heat stress can pre-dispose grape berries to infection by non-Botrytis bunch rots such as G. uvicola. It is hoped this work will lead to improved management practices and a greater understanding of bunch rot epidemiology and will form the basis of disease forecasting models in response to climate change.