The Crush - May 2019 - 2

[ FEATURE STORY ]
VIRUS BASICS AND MANAGEMENT
Cornell University professor and virus expert Dr. Marc Fuchs said
viruses are not mechanically transmitted in vineyards by the use
of tools such as pruning shears or machine operations. Viruses
are graft transmissible and can be transmitted between scions and
rootstocks by top working/grafting with virus-infected scions.

and the VMB are prevalent
here. We had to find a way to
manage this disease in order to
maintain our wine industry."

Use these practices:

He added, "There is probably
not a single vineyard in South
Africa where the grower is not
actively controlling leafroll
by the time the vineyard is
eight years old. Growers are
replanting their vineyards
about every 20 to 25 years."

* Scout for virus symptoms, such as red leaves around harvest
time, and test (don't guess).
* Rogue individual vines that test virus-positive when they make
up 25 percent or less of the total vineyard.
* Remove the entire vineyard if more than 25 percent of the vines
are infected.
* Carefully select and screen vine material when planting new
vines and replacing rogued vines. Work with nurseries to obtain
clean and certified plant material.

Pietersen said early leafroll infections are sometimes the result of
infected planting material, and may be seen as individual infected
plants randomly distributed in a vine block with specific scion/
rootstock combinations. Leafroll spread is most commonly seen in
adjacent vines within a vine row, and can then move to the next
nearest rows. Leafroll virus is spread by VMB motility, by ants
carrying VMB eggs and instars, and by workers and tools moving
VMB. Wind and birds can also move VMB, causing random
spread within a vineyard block.

"Leafroll and red blotch viruses only affect grapes. Once a virus
infects a grapevine there is no cure, and no chemical can be used
to treat the virus," Fuchs said. "Focus all your disease efforts on
eliminating the virus inoculum in your vineyards."

Fuchs said, "Zero virus
tolerance is a lofty goal, but
one we should work toward.
Each of you in the room are the
drivers of the goal. If you don't
ask for virus-tested vines, you
won't get them."
Fuchs said insecticides can
be used when leafroll viruses
and mealybugs are present to
reduce mealybug populations.
Dr. Marc Fuchs
Photo: Ted Rieger
However, he does not
recommend using insecticides
if only red blotch virus is present, because its known vector, the
three-cornered alfalfa hopper (3CAH) (Spissistilus festinus), is
not abundant in vineyards and does not reproduce on grapes.
Lab studies indicate the 3CAH can reproduce or feed on some
cover crop and weed species found in vineyards, but Fuchs said
currently, scientists do not recommend making changes regarding
cover crop choices or weed control.
VIRUS MANAGEMENT IN SOUTH AFRICA
Professor Gerhard Pietersen, of the University of Stellenbosch in
South Africa, discussed ongoing virus and mealybug standard
management practices in South African vineyards, where
grapevine leafroll associated virus-3 (GLRaV-3) is the most
important virus. Pietersen said, "We had no choice. Leafroll virus

2 / MAY 2019

Professor Gerhard Pietersen
Photo: Ted Rieger

Pietersen said, "Rogueing to remove infected plant material is
incredibly effective. Get into your vineyard as soon as possible
and rogue. It's the least painful course of action, and you will
remove the least number of plants over the long run." He added,
"It's important to totally remove the vine and as much of the root
system as possible."
When planning vineyard replant, after the last season's harvest
apply a systemic insecticide to help kill VMBs present. After
vineyard removal, Pietersen recommends a fallow period to
remove volunteer vines that may sprout from remaining roots.
He has no specific recommendation for a fallow time period, it
can depend on the soil type and how well vine root systems were
removed. Keep removing volunteer plants from fallow ground
and in newly planted vineyards.
Have a system and a location to wash mealybugs from tools
and equipment before entering new vineyards. Pietersen said
releasing parasitoid insects will help with mealybug control.
Meeting attendee Aaron Lange of LangeTwins Family Winery
and Vineyards said, "I think the most important message to come
out of this is we can't adequately control or eradicate the VMB,
so the best way to control virus spread is by rogueing infected
vines." He added, "I don't think enough people are walking their
vineyards to identify virus problems. This is a statewide problem,
and all grape growers need to address this issue."
The Crush - May 2019

Table of Contents for the Digital Edition of The Crush - May 2019
