The USDA grape rootstock improvement program, based at the Grape Genetics Research Unit, is breeding grape rootstocks resistant to aggressive root-knot nematodes. We define aggressive root-knot nematodes as those which feed on and damage the rootstocks Freedom and Harmony. In 2008 we screened 3572 candidate grape rootstock seedlings for resistance to aggressive rootknot nematodes. We select only those seedlings which completely suppress nematode reproduction and show zero nematode egg masses. Selected seedlings are propagated and then planted into the vineyard. We screened an additional 480 seedlings were screened for nematode resistance breeding and genetics studies. We tested the propagation ability of 82 selections and retested 65 selections to confirm nematode resistance in replicated trials. We identified twelve selections for a new rootstock trial to be planted at the University of California Kearney Ag Center in 2009. We pollinated 233 clusters of crosses in 31 unique combinations specifically aimed at the breeding of improved rootstocks with resistance to aggressive root-knot nematodes.
An additional 268 clusters in 32 cross combinations were pollinated specifically for rootstock genetic study.