Seashore paspalum (P. vaginatum) is a warm-season, perennial turfgrass (Morton, 1973) that is used primarily as a fine-bladed turfgrass in recreational areas. This littoral, C4 species is typically found in tropical to warm temperate regions (Chapman and Peat, 1992). Several biotic stresses affect seashore paspalum including nematodes, insects, and pathogens. In particular, plant diseases can be a major problem for turfgrass practitioners managing seashore paspalum. Dollar spot, caused by S. homoeocarpa (Bennett, 1937), is one of the most important fungal diseases that affects turfgrass, and can cause extensive damage to highly maintained turfgrasses such as those found on golf course fairways and putting greens. Dollar spot can reduce turf quality and playability of fine-textured paspalum. This disease is a widespread problem throughout the world, and nearly all cultivated turfgrass species can be a host for this pathogen (Allen et al., 2005).
Turfgrass cultural practices, such as proper nitrogen fertility and irrigation, are commonly used to reduce dollar spot symptoms in a turfgrass sward. The removal of guttation fluids with dew whips or the use of lightweight rollers can also reduce disease severity (Giordano et al., 2012). Chemical and biological fungicides are also tools used to manage dollar spot (Walsh et al., 1999). However, facultative saprophytes like S. homoeocarpa can develop acquired pesticide resistance to site-specific fungicides. Benzimidazoles, demethylation inhibitors, dicarboximides, and nitriles are common classes of fungicides that have been effective for management of dollar spot. Boscalid (Emerald®; BASF, Research Triangle Park, NC), a carboximide, can also be used to control S. homoeocarpa, and is one of the few fungicide classes to which S. homoeocarpa has not developed resistance (Allen et al., 2005).
Genetically controlled host plant resistance is the most promising of the management strategies for control of dollar spot (Bonos et al., 2003), and some resistant or partially resistant cultivars are available for many common turfgrass species. Screening by Raymer et al. (2008) showed substantial differences in S. homoeocarpa resistance among eight standard seashore paspalum varieties. Paspalum vaginatum germplasm with disease resistance can be used as parents in a breeding program to develop cultivars with improved dollar spot resistance. Development of seashore paspalum cultivars that exhibit resistance to S. homoeocarpa would greatly reduce the cost and environmental impact of fungicide applications and other management practices.
Much of the research on genetic host resistance has been conducted on the cool-season turfgrass, creeping bentgrass (Agrostis stolonifera L.). Cultivars of creeping bentgrass differ in their response to S. homoeocarpa; however, no cultivar is completely resistant to the pathogen (Morris, 2005; Vincelli et al., 1997). Bonos et al. (2003) found that a population of creeping bentgrass clones from a dollar spot resistant by dollar spot susceptible cross formed a continuous distribution of dollar spot severity phenotypes, indicating that resistance is likely quantitative with two to five effective factors associated with resistance to the pathogen. High, narrow-sense heritability was also observed for this trait (Bonos, 2006). Further research indicated that a few genes were likely interacting in an additive manner to confer dollar spot resistance in creeping bentgrass (Bonos, 2011). Quantitative trait loci (QTLs) associated with dollar spot resistance have also been identified for creeping bentgrass, and could help facilitate marker-assisted selection in breeding programs (Chakraborty et al., 2006a; Honig et al., 2014). Chromosomal regions associated with dollar spot resistance have been identified in colonial bentgrass (Agrostis capillaris L.) as well (Rotter et al., 2011). Molecular markers linked to dollar spot resistance gene(s) have not been developed in seashore paspalum, but if available, they could be used to transfer the resistance into elite cultivars using marker-assisted selection.
Artificially inoculating turfgrasses with the S. homoeocarpa pathogen can be an effective way to screen for resistance or susceptibility. Preparation of inoculum typically involves autoclaving flasks containing seed and then placing small pieces of S. homoeocarpa colonized potato dextrose agar (PDA) media inside the flask to infect sterile seeds with the pathogen. This procedure has been accomplished using seeds of kentucky bluegrass (Poa pratensis L.) (Bonos et al., 2003; Chakraborty et al., 2006a; Oliveira et al., 2010); wheat (Flor et al., 2013); and equal mixtures of wheat, barley, and oats (Goodman and Burpee, 1991). These inoculum sources have been created using both single and combinations of S. homoeocarpa isolates.
It is unclear if screening with multiple isolates is necessary to identify seashore paspalum genotypes with broad-spectrum dollar spot resistance. The objectives of this study were to 1) evaluate the disease expression of seashore paspalum genotypes inoculated with S. homoeocarpa isolates collected from cool- and warm-season turfgrass species, 2) identify which of these isolates are most effective to use for resistance screening, and 3) determine if significant genotype by isolate interactions occurred which would indicate a need for screening with multiple S. homoeocarpa isolates. This experiment is necessary to determine how to best screen for resistance to this important disease.
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