Virus Season Part 2: Leafroll Virus, the Scurge of California

There are over 55 diseases in grapevines, so I won’t be talking about all of them in the coming weeks.  Merely highlighting the main culprits of low yield, low quality, and ruined vineyards.  In this entry, the focus is on Leafroll Virus.

Leafroll Virus in a red cultivar
Leafroll Virus in a red cultivar

Leafroll, or Grape Leafroll Virus, is reaching epidemic levels in grapevines in California.  It is easier to detect in red varietals as a reddish coloring to the leaves and rolling.  It white varietals, you merely observe the rolling of the leaves.

Comparison between Leafroll virus in red (A) and white (B) cultivars
Comparison between Leafroll virus in red (A) and white (B) cultivars

In regards to economic affect, according to Deborah Golino, plant pathologist and director of the University of California, Davis’ Foundation Plant Materials Service,

What you get (with leafroll virus) is delayed fruit maturity, poor color and reduced yields.

Of course, to complicate matters, not all viral strains produce the same symptoms.  In fact, it is technically incorrect to refer to Leafroll as one single virus.  Leafroll is an effect produced by a family of viruses.  The best comparison would be that of the flu in humans.  A general set of symptoms, not everyone gets the exact same flu symptoms, and the flu is caused by a different strain of influenza each season.

Transmission of Leafroll occurs through mealybugs, tiny little insects that feed on the phloem, or sugar water, of the plant.  They are very good at hiding under the bark of the plant and attracting other insects, including ants, to feast on the vulnerable vine.  There are actually a slew of different kinds of mealybugs, each with a particular strain of the virus they can transmit.  Suffice to say, the mealybug is quite devastating despite its size.

Despite the spike in cases and spread, Leafroll has been around for a while.  It is not known why the sudden spike has occurred.  Some suggest it may be due to the use of different and new rootstocks that are not as tolerable as the older varieties.  Others think it may be due to evolving strains of leafroll.  The plants have not developed immunity to the new strains and thus are more susceptible.  Nonetheless, vigilance in pest management and procuring plant material from reputable sources is a must.

Mealybugs overtaking a grapvine
Mealybugs overtaking a grapevine

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