The Daane lab joins Houston Wilson and other researchers on a nationwide search to confirm the three cornered alfalfa treehopper as a red blotch vector and/or identify other insects that are spreading the disease. Part of that work includes sampling for suspect insects using the D-vac and yellow sticky traps in vineyards where red blotch spread is evident. The trap count information is used to understand the life cycles of these potential vectors. Wilson, who was one of several speakers covering red blotch research progress during my 2020 Foothill Grape Day, recently published a newsletter on his work of the seasonal ecology of the treehopper in North Coast vineyards. Our foothill monitoring will help us understand if the insect's life cycle is similar here as compared to what is known to occur in the cooler N. Coast.
And no, those vineyard fall colors are not a good thing! This is the time of the year when GRBaV symptoms become evident in red varieties in the vineyard-the characteristic red "blotches" on the leaves for which the disease is named begin to show in summer and become more visible as harvest nears and the season progresses. (Note: the disease affects both white and red varieties. In whites, the "blotches" are yellow and more difficult to see). While diseased vineyards turn red, healthy vines remain green, then gold, in fall.
Thanks to the hard work of those like McCalla and Flores, even during this time of the pandemic (and wearing masks while working outdoors in summer heat is not fun, let me tell you), we will get closer to solving the red blotch mystery.
Hello Fodder Followers! My "Bahder Discovers Red Blotch Vector!" post has received over 1600 hits to date-a testament to how critical this disease and research finding is to the grape industry.
First, just in case I need to (do I?), let me say this: no one in science works in a vacuum. Research is ALWAYS a collaborative effort, and every person involved, from grower-collaborator to first author, plays a role. And sometimes, Extension works! and plays a small but critical role in scientific inquiry.
So it started as it often does for us UC Cooperative Extension Farm Advisors: a bag arrives with a grower name, a phone number, and a short message "Can you help me identify this problem?"
In this case the route was circuitous: the grower attended a meeting where I made a Red Blotch presentation. I have worked with Sudhi Sudarshana (USDA virologist) since 2013, collecting photos of the disease on many grape varieties grown in the foothills. As most of you know, over 50 grape varieties are grown here, in a range of soils and
My presentation made this grower think about some red leaf symptoms in his own vineyard. He had been told that the red leaf was due to potassium (K) deficiency and so he had made applications of K for the past couple of years. Yet, the red leaf continued and appeared to be spreading. So off to the UCCE office he went to drop off a bag of red leaves with a note.
Certainly Brian would likely have made his findings without this site: the team was already hot on the trail before I came along. I did not know about Brian's transmission test results before the webinar, though I knew he had narrowed the vector candidates. So when I watched his presentation I felt pride and excitement for him and for the team.
Diagnosing problems is a critical and sometimes undervalued role for us UCCE Farm Advisors. We are the "boots on the ground" for our colleagues on campus and most of us love this aspect of our work. But few realize the number of hours that can go into a single farm call. Most often the reward is a grower's gratitude (which is enough!), and sometimes, it's a little bit more.
Badher and Zalom have been working as a team with Mysore "Sudhi" Sudarshana (USDA virologist) and several farm advisors and UC researchers, including Rhonda Smith (UCCE Sonoma), Mike Anderson (Oakville station) and myself, to monitor and map vineyards where patterns of red blotch spread are evident. Bahder narrowed the candidates of suspect vectors to those insects he found in common present in vineyards with pattern of red blotch spread from locations across the state. He then conducted arduous greenhouse tests consisting of rearing suspect vectors in completely virus free cages; placing them on RBaV infected vines, and moving them, a single insect per cage, onto virus-free vines to allow them to feed and possibly infect. He then used a highly sensitive PCR test, which allows detection of very small amounts of virus, to look for virus periodically in the vines after insect feeding. He found the virus in his greenhouse controlled vines that the three cornered alfalfa leafhopper had fed on 4 months after initial feeding (and transmission) took place.
Very little is known about the feeding habits and biology of the three-cornered alfalfa treehopper, but you can bet all that is about to change. What we do know is that the insect prefers other plants, such as alfalfa, grasses and legumes, to feed on; and spends very little time feeding on grapevines. The treehopper has been known to cause minor damage to grapevine leaf petioles in the foothills and N. Coast, girdling the petioles and then causing just one leaf to turn red.
I plan on doing some sweep net surveys of groundcovers to determine if there are any treehoppers overwintering as adults (we know they overwinter as eggs).
Bahder and Zalom noted that the treehopper may not be the ONLY species that is capable of transmitting RBaV; and they plan on continuing their studies of other insect vector candidates.
The Red Blotch webinar was a huge success, with over 300 participants online and a wealth of expert information on the disease. For those of you who missed it, I will let you know when the webinar is posted online.