Red Blotch: An Old Vector and New Inoculum Confirmed

By Marc Fuchs, Cornell University

Red blotch disease was described almost 15 years ago in California as a new threat to the grape and wine industries. Caused by grapevine red blotch virus (GRBV), the disease lowers vine vigor and yield, reduces fruit quality and degrades wine composition, resulting in reported losses of up to $170,000 per acre over the lifespan of a vineyard. Substantial progress has been made on understanding how red blotch causes these issues. But how it’s spread—including confirmed vectors and a full picture of the sources of inoculum—has remained unclear. Until now.

Spread of GRBV in the Vineyard
In diseased vineyards, planting stocks have been observed as a primary source of virus inoculum. From infected vines, it may be spread via insect vectors, resulting in distinct distribution patterns and disease progress. For example, the annual increase of newly diseased vines varies from 0% in New York (no spread) to 0.1 to 8% in California, and 0.5 to 29% in southern Oregon. The virus is most commonly spread within-vineyard vs. vineyard-to-vineyard. And although the spread typically starts slowly and at random, it can quickly accelerate once disease incidence is high and infected vines are aggregated at vineyard edges near riparian areas. (Keep reading for a twist.)

The three-cornered alfalfa hopper (TCAH) was identified early on as a vector candidate in diseased California vineyards where spread was documented. But its capacity to acquire GRBV from infected vines and transmit it to healthy vines was only recently documented in a vineyard (vs. greenhouse) setting by graduate student in the Fuchs lab, Madison Flasco, confirming TCAH as a vector of GRBV. Other insect vector candidates have been identified, but their ability to transmit GRBV is so far unknown.

Transmission of GRBV
Why did it take so long to confirm TCAH as a vector? In part because transmission of GRBV by TCAH is unique. The virus must move all the way through the body of the insect and back to its salivary glands to be transmitted. No other grapevine virus is transmitted in this circulative way. Add to that the fact the virus lasts a lifetime in TCAH’s body. A nymphal TCAH can acquire GRBV and transmit it through adulthood without ever visiting another infected vine.

Also, we now know that GRBV is present in wild Vitis species in northern California (V. californica and its hybrids) and in southern Oregon (V. riparia), but not in New York. In addition, the TCAH can transmit GRBV from and to V. californica hybrid vines. Research by another graduate student in the Fuchs lab, Victoria Hoyle, recently documented transmission of GRBV by the TCAH from V. californica hybrids to Cabernet franc vines and vice versa. This means that wild vines can be a source of GRBV inoculum transmissible by the TCAH not only to other wild vines but to winegrape cultivars. This finding illustrates the interconnectivity of the two virus hosts in riparian areas and commercial vineyards, respectively, for virus spread. Furthermore, GRBV transmission by the TCAH was higher to wild vines (77%) than to Cabernet Franc (16%) vines, suggesting wild vines are preferred feeding hosts.

Ecology of the TCAH
We also have found that TCAHs’ behavior on grapevine is distinctive. TCAH populations are usually low in vineyards and often more abundant along parcel edges near water sources. Limited information is available on the behavior of the TCAH in a vineyard ecosystem. Legumes, including some used in cover crop mixes in vineyard middle-rows, have been described as feeding and reproductive hosts of the TCAH. But what else might it eat?
The Fuchs lab recently analyzed the dietary preferences of the TCAH to gain insights into the diversity of its feeding hosts and landscape movement. This collaborative research with Drs. Monica Cooper (UC ANR), and Rodney Cooper (USDA-ARS) was achieved by characterizing the plant material present in the gut of specimens caught on yellow sticky cards in vineyard sites in Napa County, CA. Preliminary results revealed a prevalence of plant species in the family Asteraceae (sowthistle, prickly lettuce, etc.), Zygophyllacea (puncture vine), Fabacae (clover, medic, etc.) and Vitaceae (winegrapes and wild vines), among others. This work showed that, in a vineyard ecosystem, the TCAH feeds on many diverse plant species, not only legumes. This research also revealed flying distances of nearly one mile for an adult TCAH.

Management Guidance
Counterintuitively, although the TCAH has now been confirmed as a red blotch vector, the new research cited here indicates that no action should be taken to control the pest. That’s because TCAH (1) inefficiently transmits GRBV, (2) uses grapevine as an opportunistic host, (3) only transiently visits vineyards, (4) has low populations in vineyards, (5) can fly relatively long distances, and (6) has numerous feeding hosts in a vineyard ecosystem, none of which carry GRBV.

Managing feeding hosts could potentially control TCAH populations; however, it would be virtually impossible, given how widespread these plants are in vineyard ecosystems. Additionally, the fact that the TCAH can travel a mile or more may likely render any direct effort to reduce its populations in the vineyard practically ineffective, especially since it does not spend extended time in the grapevine canopy.

Instead, red blotch disease management should focus on reducing virus inoculum sources by (1) eliminating infected vines and replacing them with planting stock derived from GRBV-free scions and GRBV-free rootstocks, especially when disease incidence is less than 30%, and (2) removing wild vines near vineyards where GRBV spread is a problem, pending an environmental permit if/as needed in riparian corridors.

This article is excerpted from the original, titled “Updates on Red Blotch Disease Ecology,” published in the January 2023 edition of Practical Winery & Vineyard Journal, inside Wine Business Monthly. Click to the digital edition at the link above and scroll to pages 152-155 for the full story.