Detection of Pythium spp. in Golf Course Irrigation Systems.

Many Pythium spp. are causal agents of diseases of turfgrasses. Pythium spp. disseminate through irrigation systems in agricultural settings, and this study provides evidence that Pythium spp. also disseminate through golf course irrigation. Water samples were collected from irrigation heads and water sources at 10 golf courses in Missouri and Kansas, U.S.A. Samples were collected from 2018 to 2019 in April, July, and October. Phosphorus, nitrogen, and chloride concentrations were measured from irrigation head samples to determine if these parameters influence frequency of Pythium spp. detected. Pythium spp. were detected in samples through baiting and membrane filtration. Cultures were isolated on PARP media, and DNA was extracted from putative Pythium isolates. The internal transcribed spacer region was PCR-amplified and sequenced. Phylogenetic trees were constructed using representative sample sequences, sequences from seven morphologically identified reference isolates of Pythium, and similar GenBank accessions. Detected oomycete species include Lagenidium giganteum, Pythium biforme, Pythium insidiosum, Pythium marsipium, Pythium plurisporium, and Saprolegnia hypogyna. Twenty-one clades lacked species-level resolution, and 14 of these clades were associated with Pythium species. Clades A, C, D, E, I, and M contain Pythium species that cause root and crown rot on creeping bentgrass. Detected Pythium communities were dependent on the detection method used and sampling source. Pythium frequency and diversity were highest in April 2019. Sample temperature, sampling site, and chloride and nutrient concentrations did not influence Pythium frequency in samples. Irrigation systems using surface water sources contained at least three Pythium spp. over the course of 2 years.

Sensitivity of Sclerotinia homoeocarpa Isolates to Propiconazole and Impact on Control of Dollar Spot.

In response to reports of reduced efficacy of propiconazole for control of dollar spot, isolates of Sclerotinia homoeocarpa were collected from several locations in Georgia and tested for sensitivity to propiconazole and other demethylation-inhibiting (DMI) fungicides. Two discriminatory concentrations of propiconazole (0.02 and 0.2 µg ml-1) were used to detect lower in vitro sensitivity in two populations that had been exposed repeatedly to propiconazole than in four nonexposed populations. Mean 50% effective concentration (EC50) values for a nonexposed population (baseline) and a DMI-exposed population were 0.0049 and 0.0283 µg ml-1, respectively. Positive correlations were significant among log10 EC50 values for propiconazole, fenarimol, and myclobutanil but not between triadimefon and any of the other three fungicides, indicating cross-resistance relationships in this pathogen may not be universal among the DMIs. In greenhouse experiments, propiconazole-treated bentgrass was inoculated with seven isolates of S. homoeocarpa differing in sensitivity to propiconazole. Incubation period decreased and relative area under the disease progress curve and disease severity 28 days after inoculation increased linearly with increasing log10 EC50 value of the isolate. Results of this study confirm a significant relationship between in vitro sensitivity of S. homoeocarpa and in planta control efficacy of propiconazole and provide evidence of field resistance to propiconazole in S. homoeocarpa in Georgia.