Texas seeing an increase in disease that damages grape crops

Pierce’s Disease “is a very controllable disease as long as vineyard managers take timely action,” says Jacy Lewis of the Texas A&M AgriLife Extension Service.

By Rhonda Fanning & Alexandra HartSeptember 6, 2024 12:54 pm,

According to an economic impact study from the Texas Wine and Grape Growers Association, the state’s wine industry generates $20 billion in economic activity for the state.

But grape growers in Texas have noticed an uptick in something else, too: A pathogen called Pierce’s Disease that could hurt harvests and, if it’s left unaddressed, potentially cause large scale damage to vineyards.

Jacy Lewis, manager of the Texas A&M AgriLife Extension Service Viticulture and Fruit Lab in Fredericksburg, joined the Standard with more about the disease and how to prevent it.

This transcript has been edited lightly for clarity:

Texas Standard: Tell us a little bit about this Pierce’s Disease. I understand that this uptick was noticed this year. Are we talking about a parasite or some kind of bacterial infection? What is it?

Jacy Lewis: It is a disease, and it is caused by a bacterial infection in grapes. The pathogen is actually called Xylella fastidiosa. It does affect other crops, but in grapes, the disease that it causes is called Pierce’s Disease.

What does the disease do to the grapes?

It’s a fairly complicated process, but the short version is it occludes xylem so that the plant is no longer able to transport xylem fluid through its tissues. And this results in foliar damage and then eventually death.

Foliar damage – so the leaves start to wither and drop off? You don’t end up with a good grape crop?

Absolutely. So that’s one of the first signs of the disease. And obviously a plant is not able to survive and photosynthesize without its leaves. Now, again, there’s other damage that happens, but that’s the most apparent damage from the outside of the plant.

How does this compare with previous years, and what do viticulturists think is driving this increase?

I wouldn’t want to say that this is the worst year we’ve ever had; in the past, we’ve definitely had years like this.

The difference is that we have come off of a handful of years where disease pressure was relatively low, and now all of a sudden this year, we’re seeing a fairly dramatic increase in the number of calls that we’re getting with regard to concerns that people’s plants may have Pierce’s Disease and the number of diagnoses of the disease that we’re seeing.

And the reason that we believe that’s probably happening is primarily that over the past several years, where we’ve had some really cold spells in the winter and some really dry summer seasons and dry winters for that matter, the vectors for the pathogen – this pathogen is vectored by insects – we’re not really, really back.

These insects like warmer weather. They like wet weather. So, the number of insects out transmitting the disease had decreased relatively sharply.

This past winter was mild. We had a very wet spring. And so we’re seeing a bounce back in the population of the insects that vector the pathogen. And that’s likely the reason that we’re seeing an increase in disease diagnoses this year.

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What sort of bugs carry it, to your knowledge?

A glassy-winged sharpshooter.

This is a xylem limited pathogen. That means it lives only in the xylem of the plants. And so it’s only transmitted by insects that feed on the xylem sap of plants. And those are a suite of insects that we refer to generally as sharpshooters.

So a sharpshooter, one of the most commonly known is glassy-winged sharpshooter, though there are several others. And essentially these are insects that have the ability, when they land on a plant, to stick their proboscis down into the plant, into the xylem of the plant – the fluid – and feed on that fluid.

Let’s talk economic impact here. How much is the Texas wine industry set to take a hit from all this?

Well, I wouldn’t want to put a number on it. And I think that the industry is not going to take a large hit as long as people go ahead and start doing what needs to be done to control the pathogen. This is a very controllable disease as long as vineyard managers take timely action.

What is timely action in this context? What do they need to do?

The first thing that we do is we always recommend prevention. There is really no cure for this pathogen once it’s in a plant. So first of all is a solid [integrated pest management] program director.

We recommend a vector control program that’s targeted at the insect pests feeding on the protected plant – in this case grapevines – as opposed to some sort of a broad spectrum spray that has a greater potential to harm off target insects. So that’s the first step.

The next step is, once infection has been observed in an individual vine, that vine needs to be removed from the vineyard as quickly as possible. In that way, we remove the pathogen from the vineyard, and that decreases the amount of opportunity for that pathogen that can be moved to other vines in the vineyard.

How confident are you that this is controllable at this stage?

I’m very confident, yeah. It’s just a matter of people making sure that they’re getting their control programs in order and making sure that they’re observant, making sure they recognize and get correct diagnoses when they see potential symptoms for Pierce’s disease. And roguing those bonds out immediately.

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