The Geographic Distribution, Genetic Structure and Cultural Management of Waitea circinata var. circinata, the Causal Agent of Brown Ring Patch
- Author(s): CHEN, CHIMIN
- Advisor(s): Douhan, Greg W.
- et al.
Waitea circinata var. circinata (Wcc), the causal agent of brown ring patch, is an emergent disease of turfgrass in the U. S. It causes serious problems on golf course putting greens by disrupting the uniformity and aesthetic value. Based on morphological and molecular data, we identified that Wcc has a broad host range and is present in multiple states across the U.S. Besides annual bluegrass (Poa annua L.), Wcc was also found as a pathogen on rough bluegrass (Poa trivialis L.) and creeping bentgrass (Agrostis palustris). A new variety, W. circinata var. prodigus, was also reported in 2011 causing basal leaf blight of kikuyugrass in southern California. To demonstrate the distribution of Wcc in the U.S., ribosomal DNA internal transcribed spacer regions and 5.8S region (ITS) of 42 Wcc isolates collected from annual bluegrass obtained from nine States were sequenced. A total of 17 ITS haplotypes were observed and there was no obvious relationship between ITS haplotype and the geographic distribution of the isolates. This preliminary data showed that Wcc in the U.S. may be genetic diverse. To further demonstrate the genetic variation of Wcc and examine how Wcc exchange genes, a 3 year study was conducted at a golf course in San Diego, California. High genotypic diversity and few clonal genotypes were found among the 116 isolates collected from two putting greens. Results suggest that the pathogen may be sexually reproducing in the field and that inbreeding could account for the gametic disequilibrium detected. Before the above studies had been conducted, very little information about the epidemiology of brown ring patch was known. The disease was confused with another turf disease called yellow patch that produced similar yellow rings on greens. Attempts to manage brown ring patch similar to yellow patch makes the disease more severe. There was also concern that using trinexapac-ethyl (TE), a commonly used plant growth regulator to control turfgrass height, might be increasing disease severity. Our study showed that three nitrogen sources (nitrate, ammonium and urea) decreased brown ring patch severity. No difference in disease severity was found between treatments containing TE and the control plots that received no treatment. Azoxystrobin plus nitrogen or azoxystrobin plus nitrogen plus TE showed significant effect in alleviating brown ring patch severity in this study. To accurately evaluate disease severity of brown ring patch, digital image analysis (DIA) was compared to visual rating. Linear regression analysis showed that a bias existed between different raters regardless of the rater experience. Overall DIA and visual rating showed similar disease trends and cameras from digital camera to digital single-lens reflex cameras yielded similar results. This study showed that Wcc is a pathogen with high genetic diversity and the sexual stage may play an important role in epidemiology. Brown ring patch could be detected without using expensive equipment and this technique could help to develop a model for predicting this important turfgrass disease. Although similar to other turf grass diseases, this disease needs to be managed differently.