Co‐infection by B otryosphaeriaceae and I lyonectria spp. fungi during propagation causes decline of young grafted grapevines

Decline of newly planted, grafted grapevines is a serious viticultural problem worldwide. In the R iverina ( N ew S outh W ales, A ustralia), characteristic symptoms include low fruit yields, very short shoots and severely stunted roots with black, sunken, necrotic lesions. To determine the cause, roots and wood tissue from affected plants in 20 vineyards ( V itis vinifera cv. Chardonnay grafted to V . champini cv. R amsey rootstock) were assayed for microbial pathogens. I lyonectria spp. ( I . macrodidyma or I . liriodendra, producers of phytotoxin brefeldin A , BFA , and cause of black foot disease of grapevines) and B otryosphaeriaceae spp. (predominantly D iplodia seriata ) were isolated from rootstocks of 100 and 95% of the plants, respectively. T ogninia minima and P haeomoniella chlamydospora (cause of grapevine P etri disease) were isolated from 13 and 7% of affected plants, respectively. All R amsey rootstock stems of grafted plants sampled from a supplier nursery were infected with I lyonectria spp. and D . seriata . D iplodia seriata , but not I lyonectria spp., was also isolated from 25% of canes sampled from the rootstock source block. Root inoculation of potted, disease‐free C hardonnay plants with I lyonectria isolates from diseased vineyards caused typical disease symptoms, while co‐inoculation with B otryosphaeriaceae spp. increased disease severity. This is the first study to show that a major cause of young grapevine decline can be sequential infection by B otryosphaeriaceae from rootstock cuttings and I lyonectria spp. from nursery soil. Although the P etri disease fungi were less common in young declining grafted grapevines in the R iverina, they are likely to contribute to the decline of surviving plants as they mature.