The fungal pathogen Cercospora janseana is responsible for two diseases that have become economically important to rice growers — narrow brown leaf spot and Cercospora net blotch.

Jonathan Richards, a scientist in the LSU AgCenter Department of Plant Pathology and Crop Physiology, is studying the pathogen and the diseases, which he said had been sporadic in rice until the past 15 to 20 years.

There hadn't been a ton of research conducted on these diseases really since the 1930s or 1940s,” Richards said. “So, when we started working with these diseases, we kind of had to start at square one.”

The researcher said more farmers have noticed the diseases in their fields. He said Louisiana’s hot, humid conditions are conducive to the diseases’ development.

Over the past few years, Richards’ research has been addressing the knowledge gap of these diseases so that the AgCenter can help develop effective management strategies.

“We've come to realize that even though this is one fungal pathogen, it causes two distinct diseases that likely need to be managed differently, and because we just don't have enough research done on either disease, it's going to be critically important for us to understand and develop management strategies that are going to be effective against both,” Richards said.

The two diseases also affect different areas of the plant. Narrow brown leaf spot causes brown lesions most commonly found on the leaf blade, while Cercospora net blotch affects the sheath and stem of the plant. The diseases occur late in the season so they also can affect the ratoon rice crop.

Richards conducted field evaluations of the rice breeding germplasm panel for resistance to narrow brown leaf spot and Cercospora net blotch.

He observed one variety that is highly resistant to Cercospora net blotch, and Richards said he is working to understand and characterize the gene that can control resistance and ultimately identify a linked molecular marker that can be used in the breeding program. He said a priority in the research, which he is conducting with rice breeders Adam Famoso and Brijesh Angira, is a genetic study to map the resistance gene.

“Basically, it will let us find the approximate location where that gene is in the rice genome, and then the breeder can then select for that gene in their program,” he said.

For narrow brown leaf spot, the CRSP2.1 gene has been effective in controlling it, but recently some varieties are showing waning resistance

“The CRSP2.1 gene has been deployed in rice varieties like PVL03, which has a high acreage in Louisiana, and it was very effective, very high level of resistance in leaf tissue. However, we've noticed in the last couple of years that that resistance seems to potentially have been defeated,” Richards said.

Richards theorized that a new race could have emerged that has overcome the CSP2.1 resistance. Lab tests confirmed his theory, but Richards said there are still some lines that may be resistant to the new race.

In addition to host resistance, Richards said they are looking at how to effectively treat or manage this disease with chemical applications. He said research has been conducted on fungicides to control narrow brown leaf spot, but there haven’t been studies on fungicide control of Cercospora net blotch.

“So, we don't know if the same recommendations for narrow brown leaf spot chemical applications would be effective toward Cercospora net blotch,” he said.

With his research, Richards is trying to build the foundation of tools that can help farmers manage these two diseases. His plans for the coming year include continued efforts to characterize pathogen races for both narrow brown leaf spot and Cercospora net blotch through controlled inoculation experiments and screening of the rice breeding germplasm for Cercospora net blotch resistance.

Typical symptoms for narrow brown leaf spot, at top, and Cercospora net blotch, below, observed in the field. Photos provided by Jonathan Richards