Although RT-PCR assays of diseased collections were positive for RSPaV and RVFV,
the same viruses were also detected in non-diseased collections. Thus, there was no
apparent correlation between them and necrotic union disease.
Regarding the red leaf grapevines of PN777 putatively grafted on rootstock 101-14. With
the grower?s consent (because cost of analysis was anticipated), roots from both the red
leaf- and the green leaf-grapevines were submitted for DNA analysis at FPS. Test results
identified both diseased grapevines as 110R and the healthy grapevine as 101-14.
Evidently, a mixture of two rootstocks was planted in the block. Anecdotal observations
suggest further that the rootstock 101-14, unlike 110R, is likely tolerant of the putative
agent of necrotic union and that infected, asymptomatic grapevines of PN777/101-14
may have served as internal reservoirs of a causal agent. Also and perhaps a larger
question looms in that what impact might the causal agent of 110R necrotic union have in
infected asymptomatic scions on 101-14. Does it affect berry maturation or soluble solid
composition, e.g. delays sugar accumulation, pigmentation, etc?
In Trial 2005, the lack in development of necrotic union symptoms was not surprising. It
has been our experience that wood markings require, minimally, two-year incubation;
vis-à-vis stem lesion disease. We anticipate union symptoms to develop in 2007 in the
test plants inoculated in Trial 2005. Likewise, union symptoms in Trial 2006 are
anticipated in 2008.
Rapid spread had continued in the Carneros West block and less so in the south Napa
District. This suggests that a putative vector species was more active in one area, then in
the other. Secondary spread is hypothesized because necrotic union symptoms have
developed on large mature grapevines. If infected nursery stocks were planted, they
should remain small in stature with weak growth and/or succumb.
The standard cloning procedure is laborious and inefficient in cloning viruses in lowcopy
numbers. Our laboratory has struggled with this problem for quite some time.
However, we now have access to the Genome Sequencer 20tm, which can sequence 1000s
of cDNAs and should enable us to obtain sequences for the necrotic union agent (see
details in 2007-2008 proposal).