Walnut blight research yields positive findings

The incidence of walnut blight has been trending downward, according to Steve Lindow, professor at the department of plant and microbial biology at the University of California, Berkeley.

"To understand how we manage the disease, we have to understand where the pathogen is overwintering in the tree and really where it causes infection," Lindow said.

The highest numbers of the pathogen are typically on the buds, with the lowest numbers actually on the nuts, but with the right conditions it will move from the buds into the nuts, Lindow said.

Pathogen populations vary greatly between orchards and between buds within a given orchard, Lindow said.

"Some years the numbers are much higher, others lower, and we're not exactly sure why it varies, but it's probably driven a lot by the weather," Lindow said. "When it gets into these buds, it's almost immortal. It will stay within those buds. And if we look at buds at a given time, we'll come back a week or two or a month later, and we'll see that very similar populations are found in a bud."

The buds are very small, about the size of an adult's pinkie fingernail, and yet they will have as many bacteria as within the large shoot that will emerge from that bud, he said.

Lindow said it is his observation that the disease is not nearly as prominent as it was 10 years ago when he first started the research. He attributed this in part to the fact that when growers apply chemicals properly, they are getting better control of the disease. Before better control, orchards were getting wiped out with blight, he said.

"They'd have 50 percent or more disease. Now they're much less than that," Lindow said, adding, "It's still a very serious disease, and one growers have to be aggressive in controlling, but it hasn't been the disaster that it was 10 years ago."

The addition of chemicals like Mancozeb with copper has made a big difference in controlling the blight, Lindow said.

"Copper alone, especially up north, will not control the disease at all well," Lindow said, adding this is because the strains of the pathogen are resistant to copper.

"Our earliest work had shown that the copper-resistant strains are common and are growing in abundance. But they can be controlled relatively well by adding this other material to the copper, which kind of magically improves the toxicity of the copper to the bacteria without causing any damage to the tree," Lindow said.

"So that's kind of a gimme that you have to use a copper/Mancozeb mixture," Lindow said, adding that there are three or four other materials that are basically the same as Mancozeb, but with different formulations and different names.

Weather is certainly a major factor in blight, Lindow said. The pathogen survives in the buds, and wet conditions allow the pathogen to move out of the buds, onto the nuts, and cause the disease. But even if there is a lot of pathogen in the orchard, that doesn't necessarily mean there will be blight. It all hinges on rainfall in April, Lindow said.

"If you get a period of dry weather in that early springtime, in April, typically when the buds are emerging to make shoots, the disease is much less likely to occur than if it had been wetter," Lindow said, "but if there is pathogen in the orchard, combined with rain as the buds emerge, it's likely there will be a lot of blight."

Managing the disease comes down to knowing whether your orchards have a lot of disease pathogens present, he said.

There are ways to forecast the risk of blight to a particular orchard based on the abundance of the pathogen in the buds in the wintertime, Lindow said.

"In February, we can access whether a given orchard has a lot of the pathogen around, or whether it's much lower," he said.

"Growers can send their buds in for testing to a commercial lab," Lindow said. "They will be given a report, which basically gives them the risk level."

Testing is reasonably inexpensive, about $100 per orchard, he continued.

Lindow recommended chemical applications at least once regardless of the risk level in the orchard, and then if testing shows low risk for the disease, a much less aggressive program.

The earliest application should be at the 30 to 40 percent prayer stage—just when the buds are starting to open, Lindow said.

"It's a very critical time to make sure they have their first treatment on at that point," Lindow said, adding that putting that it on later could be a big risk because the pathogen may have already moved out onto the shoots.

Even though this is a drought year, there could still be problems with blight, Lindow said. It's all about the timing of the rains, he added.

"It might be dry, dry, dry, and then if you get a couple of rains in springtime, you're still going to get a lot of blight," Lindow said, adding a few rains at the wrong time is all that's needed to spread the disease.

Jeff Cripe, a walnut grower in Chico, said 2013 was definitely a light year for walnut blight.

Cripe agreed the current dry pattern isn't favorable to blight.

"If we get a shift in the weather, that'll increase the likelihood. However, we've had two or three dry springs, in a row, so even if we get a wet spring, as long as a guy has been diligent in his blight applications, fungicides, the inoculant just hasn't really built up in the orchards," Cripe said.

Lindow advised growers to make their applications early and provide good coverage. Then, based on the pathogen abundance in the orchard, they can determine how aggressively to make applications.

(Kathy Coatney is a reporter in Corning. She may be contacted at kacoatney@gmail.com.)