Management of bitter rot and ripe rot of grapes in sub-tropical vineyards in Australia

Abstract: Two bunch rot diseases associated with viticulture in warm and wet conditions are bitter rot, (Greeneria uvicola) and ripe rot (Colletotrichum acutatum). Surveys of vineyards in the Hunter Valley region of New South Wales revealed that in some seasons these are the predominant fungi associated with bunch rots. Our observations also demonstrate that bitter rot is associated with growing seasons that are warmer and drier than normal while B. cinerea predominates in cooler and wetter years. Mature berries from seven Vitis vinifera cultivars (Cabernet Sauvignon, Chardonnay, Sauvignon Blanc, Semillon, Shiraz, Traminer and Riesling) were evaluated for susceptibility to C. acutatum and G. uvicola infection and all were found to be equally susceptible. Susceptibility in field grown vines is linked to date of maturity and regional climatic events (rainfall and temperature). In a series of in vitro screening trials, fungicides belonging to the strobilurin group were found to be the most effective at inhibiting mycelia growth of C. acutatum and G. uvicola. Fungicides registered in Australia and commonly used for B. cinerea management (e.g. chlorothalonil, iprodione and pyrimethanil) were not as effective. As strobilurin fungicides cannot be applied to wine grapes grown in Australia after bunch closure we examined their efficacy at preventing C. acutatum and G. uvicola flower infections. In experiments using potted and field-grown grapevines an application of Cabrio (a.i. pyraclostrobin) at flowering was found to limit C. acutatum and G. uvicola infection. A combination of canopy management that avoids over exposing fruit to heat stress and strategically applied strobilurin sprays, coupled with growing varieties that can be harvested prior to the onset of summer rainfall in sub-tropical areas appears to be the best management option for bitter rot and ripe rot of grapes in Australia.