Study focuses on narrow brown leaf spot

LSU AgCenter plant pathologist Jonathan Richards is studying ways to control narrow brown leaf spot, a disease that has become more widespread in Louisiana rice in recent years.

“Many popular commercial varieties are susceptible, and yield losses can range from 10% to 40%,” Richards said.

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The disease, which causes linear brown lesions on rice plants’ leaves and a netlike pattern on the sheaths, tends to show up late in the growing season.

“It can have a substantial impact on ratoon crops,” Richards said.

He has been studying several facets of the disease and control methods. First, he is working to determine which varieties have resistance traits, and he wants to identify different races of the pathogen that are present in Louisiana and Texas.

Genetic analyses play a large role in this part of the project. With help from AgCenter rice breeder Adam Famoso, Richards is using some high-tech tools to look for genetic markers that indicate resistance to narrow brown leaf spot and that could influence future variety development.

So far, they’ve found a major resistance gene plus two quantitative resistance loci. When combined, these loci — specific points on a chromosome where genes or genetic markers are located — may offer durable resistance.

Technology also can help provide a clearer picture of the genetic diversity of pathogen populations. Richards used whole-genome sequencing to identify evidence of sexual reproduction within pathogen populations as well as gene flow between Louisiana and Texas rice fields. That means new pathogen races may develop quickly and migrate to threaten existing resistance genes, making monitoring efforts all the more important.

Another aspect of Richards’ work is evaluation of how well fungicides, particularly propiconazole and azoxystrobin, control the disease and the possibility for resistance.

This year, Richards has examined genome sequencing data for numerous isolates of Cercospora janseana, the fungal pathogen that causes narrow brown leaf spot. He found a mutation in 75% of those samples that appears to lead to azoxystrobin resistance, meaning it’s not a viable option for disease control.

Propiconazole still seems to be a good choice for leaf spot management. However, Richards added, “fungicide resistance can develop quickly in fungal pathogen populations, and constant monitoring is required to ensure maintenance of efficacy.”

12/15/2021 6:36:00 PM
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