How a Bag of Red Leaves Led to the Red Blotch Scoop.

The Red Blotch webinar is available for viewing now on YOUtube.  

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

microclimates. I've found Red Blotch infection can show slightly different symptoms on different varieties-though this is difficult to precisely say without a complete panel of virus testing done-something I haven't the funds for.  Mixed virus infections (such as leafroll virus), growing conditions (dry farming), other disease status (i.e. Esca or canker) and nutritional status (P, B and/or K deficiency) can and do affect "red leaf" symptom severity.  This is why laboratory polymerase chain reaction (PCR) testing- a test that identifies virus genetic material (DNA or RNA), is required to confirm the presence of the virus.

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. 

When I made my farm call, I could barely contain my excitement.  Because, you see, in most vineyards that I've seen the pattern of Red Blotch is throughout the entire block-a sign that it likely came in with the planting material. But here was a vineyard with an evident pattern of spread: a cluster of symptomatic vines towards the middle with more symptomatic vines scattered here and there and on the edges. Furthermore, the wood had never seen a nursery-it was on its own root, and had remained "green" until a few years ago. The grower, being an astute farmer, had even tagged vines that he saw as "new" infections from the previous years. Here, I thought, is a site with a vector at work. And then, with the grower's permission, I contacted Sudhi, Frank (Zalom) and Brian (Bahder).

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.  

Zinfandel RBaV(-): Test, don't guess!

K deficiency in Zinfandel. Photo credit: P. Christensen

A vineyard with spreading pattern of Red Blotch.