GRAPEVINE NECROTIC UNION, A NEWLY RECOGNIZED DISEASE OF UNKNOWN ETIOLOGY IN GRAPEVINES GRAFTED ON 110 RICHTER ROOTSTOCK IN CALIFORNIA
In Northern California, surveys of several vineyards planted to Vitis vinifera cv. Pinot noir (PN) clones 02A, 667, 777, and UCD 04 grafted onto the rootstock V berlandieri x V rupestris 110 Richter (110R) revealed 2 to 45% of vines showing solid red leaf canopies and two distinct disease stages, i.e., full canopied plants with normal grape clusters (designated as acute) or stunted shoots and straggly bunches (designated as chronic). In addition, the scion-rootstock junction of symptomatic grapevines had developed a necrotic union; hence, the name 'grapevine necrotic union' (GNU). Also, two white fruited cultivars, Chardonnay (clone 04, vineyard in Yolo county) and Pinot gris (clone 152, vineyard in Monterey county), exhibited GNU-associated symptoms, with an exception of pale yellowed leaves. A detailed survey, from 2004 to 2009, was made in a sub-block containing 664 grapevines (8 rows x 80-85 grapevines) in a PN 02A vineyard (planted in 1997) located in Sonoma county. The initial incidence of GNU was 2.1% in 2004 and rose to 21.9% by 2009 suggesting secondary spread. Molecular assays (RT-PCR and PCR) for grapevine viruses, phytoplasmas, and Xylella fastidiosa, failed to detect a putative causal agent. Likewise, bud-chip inoculations of test plants of Cabernet sauvignon scions on different rootstocks, including 110R, were inconclusive. However, several bench-grafts of collections of PN 02A, PN 91 and Chardonnay 04 onto 110R produced plants that developed GNU symptoms in a research plot. Based on the results obtained so far, GNU is being regarded as an emerging disease of unknown etiology in California vineyards.