For LSU AgCenter pathologists Felipe Dalla Lana and Jonathan Richards, the unprecedented heat and drought conditions the state has experienced has made disease research a bit more challenging this year, but their quest to control Cercospora janseana, which causes narrow brown leaf spot, continues.
Narrow brown leaf spot is typically considered a late-season disease or a concern for the second crop, with severe outbreaks occurring intermittently over the years. Dalla Lana says the name is inspired by the short and elongated lesions along the leaf veins.
“The disease can infect and colonize various plant tissues. Although its classical manifestation primarily occurs on the leaves, it can also affect the sheath and, in some instances, the panicle,” he said.
Unlike this year, during the late season of 2022 (from the end of July to September), the researchers encountered extended periods of cloudy and rainy weather, creating favorable conditions for disease development. Toward the end of the season, they received numerous reports from commercial fields with severe Cercospora symptoms on the sheath and panicle and important yield losses.
Therefore, said Dalla Lana, one of the objectives for this season's studies is to validate the efficacy of the current fungicide timing and assess the varieties' resistance to sheath and panicle symptoms.
According to Dalla Lana, the current recommended approach of combatting the disease involves using varieties with some degree of resistance, avoiding late planting and applying a demethylation inhibitors (DMI) fungicide, such as propiconazole, at the 2 to 4 inches panicle stage up to the heading growth stage. However, all field research on disease management and phenotyping focuses exclusively on leaf symptoms.
“This study encompasses two application timings: the recommended 2 to 4 inches panicle stage and a later application in the season, and it involves eight different varieties,” he said. “The study is conducted across six trials, including three at the H. Rouse Caffey Rice Research Station, with plantings on March 15, April 15 and May 15, one at the Lake Arthur breeding program field, another at the Macon Ridge Research Station in Winnsboro with AgCenter pathologist Trey Price, and one at the University of Arkansas in Stuttgart with pathologist Camila Nicolli.”
Dalla Lana said another study is exploring the impact of planting dates on disease incidence and its subsequent effect on yield. Using the same set of eight varieties, the researchers aim to quantify how delayed planting dates and disease onset affect disease intensity. Additionally, their laboratory continues its efforts to phenotype commercially available varieties and lines within the breeding pipeline.
“While we did observe symptoms of Cercospora on the sheath during the season and in the leaves in late September, the disease did not reach significant levels to cause substantial yield reductions,” he said. “Looking ahead to the next year, our program plans to continue these ongoing studies and explore new methods for fast and reliable quantification of Cercospora.”
Dalla Lana’s colleague Jonathan Richards said that great progress has been made in characterizing the major narrow brown leaf spot resistance gene CRSP2.1, which provides a high level of resistance in rice leaves. However, lines that harbor this gene, such as PVL03, remain susceptible in the sheath.
“We shifted our research focus onto identifying narrow brown leaf spot sheath resistance in 2023 and developed a sheath phenotyping protocol, identifying three lines with high levels of sheath resistance,” Richards said. “Following our initial screening, we conducted a genetic study and discovered that sheath resistance is potentially governed by a single gene and is independent from leaf resistance.”
Richards looks forward to further dissecting narrow brown leaf spot sheath resistance next year and ultimately developing a diagnostic marker that can be used for selection in breeding programs.
“Over the past few years, we have evaluated several different rice populations in the field and greenhouse to identify additional resistance genes. Besides the major CRSP2.1 locus, we have identified several smaller effect loci, which may be useful to complement the major effect gene CRSP2.1,” he said. “The deployment of several resistance genes can increase durability and make it more difficult for the pathogen to overcome.”
The research team has also focused on the diversity of Cercospora janseana. Previously, they discovered that pathogen populations have high levels of genetic diversity and appear to migrate easily between Louisiana and Texas.
In addition, according to Richards, the pathogen apparently undergoes sexual reproduction which could lead to the development of novel races. Cercospora janseana isolates collected from the sheath are genetically similar to those collected from the leaf and the researchers are currently expanding their efforts to characterize similarities and differences between these groups of isolates.
“Our greenhouse trials have identified at least six pathogen races with one race being predominant in the isolates we have characterized so far,” Richards said. “Our future work will include screening more isolates collected from Louisiana to gain a better picture on variation in pathogen virulence and understand which varieties provide the highest level of protection.”