When LSU AgCenter plant pathologist Jonathan Richards began investigating narrow brown leaf spot a few years ago, he ended up going down a bit of a scientific rabbit hole in his quest for ways to manage the rice disease.

Narrow brown leaf spot is caused by a fungal pathogen called Cercospora janseana — and so are two other diseases affecting rice. But, as Richards has found, these diseases are surprisingly distinct in their behavior. What keeps one disease at bay doesn’t work for the others.

Richards has made it his goal to understand why, and these findings are helping him identify rice varieties that are resistant to the trio of diseases.

Narrow brown leaf spot infects rice leaves, causing discolored lesions. The Cercospora pathogen also is responsible for Cercospora net blotch, which affects the sheath, and Cercospora panicle blight.

Richards was starting at square one when he began studying this set of diseases.

“Historically, rice diseases caused by Cercospora janseana have been really understudied,” he said. “The diseases have been present in Louisiana for over 100 years and were studied a decent amount in the 1930s and 1940s. But then there was a really long gap in the research that was done.”

Interest in this research has grown in the past couple of decades as incidence of the diseases has ticked upward. Like many diseases, those caused by Cercospora flourish in Louisiana’s warm, humid climate.

The diseases pose late-season threats to mature rice, often showing up after heavy rains that are common in late July and early August.

With existing fungicides unable to provide effective control, Richards and his colleagues looked to the concept of host resistance as a tool to fight these diseases.

He worked with AgCenter rice breeder Adam Famoso to pinpoint genes that impart resistance to narrow brown leaf spot. They were able to identify a few varieties that fit the bill.

The researchers made a significant, if discouraging, discovery in the past year: Those resistance genes only guard against narrow brown leaf spot.

“It does not work for the other diseases caused by Cercospora,” Richards said. “It’s the same fungal pathogen, but the resistance is tissue specific. It works in the leaves but it doesn’t work in the sheath or the panicles.”

So, Richards has turned his attention to learning more about Cercospora net blotch, figuring out how to reproduce it in the lab for studies and developing a method to screen for resistance.

“With that method, we’re now able to rapidly and reliably screen different rice lines for the resistance, which will help breeding programs identify the lines they can use when they’re making crosses to increase genetic resistance to Cercospora net blotch in their elite varieties,” he said.

Several resistant lines have been identified in the lab, and they appear to hold up against the disease in the field, too.

Richards plans to focus on Cercospora panicle blight in the next year.

“That’s another challenging one,” he said. “Similar to Cercospora net blotch, the methods for doing these experiments and evaluations don’t exist yet.”

In studying the diseases caused by Cercospora, Richards has determined that there are different races of the pathogen.

“A race is defined by the host genotype that it can affect,” he said. “If a rice line has a certain resistance gene, that gene may only be effective against a certain number of races, and other races have overcome that gene.”

He’s working to characterize those races to aid efforts to identify resistant varieties.

Richards is proud to be working toward solutions that could help rice producers conquer yield-robbing diseases. As a scientist, he also is enjoying the process of uncovering new information about an understudied area of plant pathology.

“Every discovery that we make opens a lot of new doors of questions to ask,” Richards said.