Xiphinema Index and Grape Fanleaf Virus Disease Complex: Defining Nematode

The goal of this research is to determine how fanleaf degeneration-resistant rootstock (039-16) influences X. index and GFLV, separately and together, and their influence on grape growth and yield over an extended period. Experiments were initiated in large soil containers (4′ X 5′ X 5′), initially containing the highly susceptible Rupestris St. George rootstock to increase nematode and virus to appropriate levels within the experimental units. The grapes were planted in the spring 1996, and were maintained until the early spring of 2000. Nematode samples were collected during the previous two years on a total on 15 sampling dates in 1998 and 1999. Soil samples were collected from within each experimental unit by taking five soil cores per sampling date using an Oakfield tube to a depth of two feet in approximately one-foot vertical increments. Population densities became very low during the summer of 1999, but by fall 1999 had recovered to acceptable levels. The highest nematode densities observed in the top foot of soil in the experimental units, and densities were as high as 48 nematodes per 100 cc of soil observed. Unfortunately, due to the variability of nematode numbers within each experimental unit, quantitative demographic analyses were not possible. Root fragments were assessed for feeding damage, and at all sample periods, galled root tips indicative of nematode feed were observed. The experiment will continue in 2001, as susceptible plants will be replaced with grafted Cabernet Sauvignon on 039-16 and susceptible Cabernet Sauvignon on St. George (as a control treatment) in early summer 2000. These stocks differ in their resistance to X. index and in their ability to condition tolerance to GFLV in the scion. Research on developing a method for successfully rearing the nematode on micropropagated grape plants was conducted. Such a system will represent an improved means for preliminary screening of nematode resistance for different rootstocks. We have discovered that the salt concentrations typically found in the grape micropropagation medium is a lethal osmotic stress for the nematode. By decreasing salt concentration, we have improved nematode survival on micropropagation medium. Additionally, we have discovered that the gel medium used in micropropagation limits nematode survival. We have used alternative gelling medium, and our initial tests indicate that the nematode survives very well on the medium. This research is continuing.