Young Vine Decline in the Riverina is caused by co-infection by Botryosphaeria and Ilyonectria at different propagation stages

During the last decade many wine grape-growers in theRiverina wine region of NSW, Australia have purchaseddiseased young grapevines. These plants typically failin the first season or are developmentally retarded, lowyielding, and die prematurely. This syndrome, YoungVine Decline (YVD), was most commonly reported ingrafted grapevines. We have shown that the cause ofYVD in the Riverina is the co-infection by Botryosphaeriaspp. and Ilyonectria spp., infecting at different stages ofthe nursery propagation process. Although both Botryosphaeriaand Ilyonectria alone cause the decline anddeath of young grapevines, co-infection leads to moresevere disease symptoms. Co-infection is initiated whencuttings are taken from source rootstock plants infectedby Botryosphaeria. Those cuttings, and others crosscontaminatedduring propagation, are then invadedby strains of Ilyonectria macrodidyma or Ilyonectria liriodendri.These Riverina strains were shown to producephytotoxic brefeldin A. Consequently the rootstock stembelow the graft union becomes internally infected withboth Ilyonectria and Botryosphaeria, while usually thescion is initially uninfected. As a result, Ilyonectria disruptsroot function and retards early plant developmentwhile Botyrosphaeria gradually invades the xylem. Botryosphaeriahyphae move basipetally to the roots and acropetallythrough the graft union to the scion, thus leadingto eventual plant death. As Botryosphaeria infections areespecially severe when the host plant has been subjectedto water stress, we speculate that water-stress inducedby brefeldin A-impaired root growth accounts for theparticularly debilitating consequences of co-infection bythese pathogens.