August 31, 2004:
All over the world, grape growers and scientists
are becoming increasingly aware of decline in fruit productivity
and quality with prolonged perennial production and with replanting
in un-rotated fields.
In a recent issue of Wine East: News of Grapes and Wine
in Eastern North America, Dr. Elwin Smith and Nancy Wenner
of Pennsylvania State University detail some of the pathological
agents associated with grapes in decline. These include: fungal
root diseases, crown gall bacterium, tomato ringspot virus
(TRSV) and dagger nematodes. (Dagger nematodes are both parasitic
on grape roots and transmit TRSV to roots and vines.)
The decline of grape vineyards depends on the age of the
grape vines and on the variety planted. Vine decline appears
to affect all grape varieties, but American grapes are much
more tolerant of the condition than French American or French
varietal types. Within each class of grapes there is much
variation in susceptibility as well. In terms of TRSV, for
example, 37 percent of French American Hybrids, 23 percent
of American and 18 percent of French varieties showed susceptibility.
Vineyards over 10 years old are particularly affected by
decline. For organic and conventional vineyards, the selection
of grape varieties should include their ability to withstand
over time and not just short term quality considerations.
The type of grapes and variety selection within the grape
class are very important in this regard.
In another study focusing on grape decline after re-planting,
master's student Fritz Westover and Dr. Jim Travis, also of
PSU, investigated biological and non-biological causes of
this malady. Results presented at a recent field day sponsored
by Pennsylvania Association of Wine Growers showed that by
pasteurizing soil and comparing growth to non-pasteurized
soil, Westover and Travis found that 4 out of 6 farmer re-plant
problems could be significantly helped by heat treatments.
Soil amended with compost effectively increased vine growth
in 2 out of 3 test situations. Mixing soil with compost reduced
root discoloration and increased grape root mass.
Although soil from around the vines was associated with poor
growth and showed improvement after pasteurization, taking
soil from adjacent grassy areas showed no poor growth, and
pasteurization decreased grape performance. This indicates
that grass strips have a grape-positive microflora that is
inhibited by pasteurization.
Besides the buildup of pathogens in soil, Westover and Travis
found that herbicide injury to grapevines is common. Bioassays
with lettuce and cucumber seedlings showed that simazine,
a triazine herbicide related to atrazine, can persist in soils
and cause subsequent replanting problems.
Paul Reed Hepperly, Ph.D., is research and training manager
at The Rodale Institute.