Turfgrass Diseases Research Articles

The role of oxalic acid in Clarireedia jacksonii virulence and development on creeping bentgrass.

Dollar spot is a destructive foliar disease of amenity turfgrass caused by the fungus Clarireedia spp., and mainly Clarireedia jacksonii on the northern US region's cool-season grass. Oxalic acid (OA) is an important pathogenicity factor in related fungal plant pathogens such as Sclerotinia sclerotiorum, however, the role of OA in the pathogenic development of C. jacksonii remains unclear due to its recalcitrance to genetic manipulation. To overcome these challenges, a CRISPR/Cas9-mediated homologous recombination approach was developed. Using this novel approach, the oxaloacetate acetylhydrolase (Oah) gene that is required for the biosynthesis of OA was deleted from C. jacksonii wild-type stain. Two independent knockout mutants, ΔCjoah-1 and ΔCjoah-2, were generated and inoculated on potted creeping bentgrass along with a wild-type isolate (WT) and a genome sequenced isolate LWC-10. After 12 days, bentgrass inoculated with the mutants ΔCjoah-1 and ΔCjoah-2 exhibited 59.41% lower dollar spot severity compared to the WT and LWC-10 isolates. Oxalic acid production and environmental acidification were significantly reduced in both mutants when compared to the WT and LWC-10. Surprisingly, stromal formation was also severely undermined in the mutants in vitro, suggesting a critical developmental role of OA independent of plant infection. These results demonstrate that OA plays a significant role in C. jacksonii virulence and provide novel directions for future management of dollar spot.

Turf-Type Tall Fescue Brown Patch Resistance as Influenced by Morphological Characteristics

Brown patch (caused by multiple species of Rhizoctonia and Rhizoctonia-like fungi) is one of the major turfgrass diseases impacting turf-type tall fescue [ Schedonorus arundinaceus (Schreb.) Dumort., nom. cons.] lawns during the summer season. Selection of tall fescue cultivars with brown patch resistance is an important cultural management practice to reduce disease severity and limit reliance on fungicides. A greenhouse experiment was conducted to quantify differences in brown patch resistance among 15 tall fescue cultivars and correlate disease resistance with plant morphological characteristics. Prior to R. solani inoculation, leaf blade width, sheath width and length, endophyte stem infection, and relative growth rate were measured. Disease resistance was determined as a percentage of the canopy blighted as measured using digital image techniques. Brown patch severity after 21 days in Experiments 1 and 2 ranged from 36.7 to 72.7% and 20.8 to 41.8%, respectively, among all cultivars. There were significant differences among morphological characteristics for the cultivars. Increased brown patch was associated with cultivars with a faster growth rate, wider sheath width, and decreased sheath length. This study demonstrates the importance of cultivar selection for reducing brown patch severity in tall fescue lawns. Additionally, tall fescue cultivars with higher shoot density and narrow leaves should not be avoided due to concerns of increased brown patch susceptibility.

Draft Genome Sequences of 11 Isolates of Ophiosphaerella spp., the Causal Agents of Several Turfgrass Diseases Including Spring Dead Spot of Bermudagrass and Necrotic Ring Spot of Kentucky Bluegrass

Ophiosphaerella herpotricha, O. korrae, and O. narmari are the causal agents of spring dead spot of warm-season grasses worldwide, and O. korrae is the causal agent of necrotic ring spot of cool-season grasses that primarily occurs in North and South America. All three species are considered important pathogens of turfgrasses, and there is limited genomic information about these ectotrophic, root-infecting fungi. Herein are reported the draft genome sequences of three isolates of O. herpotricha, five O. korrae, and three O. narmari isolates, furthering the knowledge about these three important turfgrass disease-causing species. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Iron Sulfate and Phosphorous Acid Affect Turfgrass Surface pH and Microdochium Patch Severity on Annual Bluegrass.

Microdochium patch is a turfgrass disease caused by the fungal pathogen Microdochium nivale. Iron sulfate heptahydrate (FeSO4•7H2O) and phosphorous acid (H3PO3) applications have previously been shown to suppress Microdochium patch on annual bluegrass putting greens when applied alone, although either disease suppression was inadequate or turfgrass quality was reduced from the applications. A field experiment was conducted in Corvallis, Oregon, U.S.A., to evaluate the combined effects of FeSO4•7H2O and H3PO3 on Microdochium patch suppression and annual bluegrass quality. The results of this work suggest that the addition of 3.7 kg H3PO3 ha-1 with 24 or 49 kg FeSO4•7H2O ha-1 applied every 2 weeks improved the suppression of Microdochium patch without substantially compromising turf quality, which occurred when 98 kg FeSO4•7H2O ha-1 was applied with or without H3PO3. Spray suspensions reduced the pH of the water carrier, therefore two additional growth chamber experiments were conducted to better understand the effects of these treatments on leaf surface pH and Microdochium patch suppression. On the application date in the first growth chamber experiment, at least a 19% leaf surface pH reduction was observed compared with the well water control when FeSO4•7H2O was applied alone. When 3.7 kg H3PO3 ha-1 was combined with FeSO4•7H2O, regardless of the rate, the leaf surface pH was reduced by at least 34%. The second growth chamber experiment determined that sulfuric acid (H2SO4) at a 0.5% spray solution rate was always in the group that produced the lowest annual bluegrass leaf surface pH, but did not suppress Microdochium patch. Together, these results suggest that while treatments decrease leaf surface pH, this decrease in pH is not responsible for the suppression of Microdochium patch.

Environmental and Edaphic Factors that Influence Spring Dead Spot Epidemics.

Spring dead spot (SDS) (Ophiosphaerella spp.) is a soilborne disease of warm-season turfgrasses grown where winter dormancy occurs. The edaphic factors that influence where SDS epidemics occur are not well defined. A study was conducted during the spring of 2020 and repeated in the spring of 2021 on four 'TifSport' hybrid bermudagrass (Cynodon dactylon × transvaalensis) golf course fairways expressing SDS symptoms in Cape Charles, VA, U.S.A. SDS within each fairway was mapped from aerial imagery collected in the spring of 2019 with a 20 MP CMOS 4k true color sensor mounted on a DJI Phantom 4 Pro drone. Three disease intensity zones were designated from the maps (low, moderate, high) based on the density of SDS patches in an area. Disease incidence and severity, soil samples, surface firmness, thatch depth, and organic matter measurements were taken from 10 plots within each disease intensity zone from each of the four fairways (n = 120). Multivariate pairwise correlation analyses (P < 0.1) and best subset stepwise regression analyses were conducted to determine which edaphic factors most influenced the SDS epidemic within each fairway and each year. Edaphic factors that correlated with an increase in SDS or were selected for the best fitting model varied across holes and years. However, in certain cases, soil pH and thatch depth were predictors for an increase in SDS. No factors were consistently associated with SDS occurrence, but results from this foundational study of SDS epidemics can guide future research to relate edaphic factors to SDS disease development.

Spo0A-dependent antifungal activity of a plant growth promoting rhizobacteria Bacillus subtilis strain UD1022 against the dollar spot pathogen (Clarireedia jacksonii)

Dollar spot is an economically important foliar disease of turfgrass caused by Clarireedia spp. Increased use of chemical fungicides for disease management has possible negative effects on the environment and human health. The quest for eco-friendly alternatives to fungicides is driving the development of new biological strategies for managing dollar spot. In the present study, an analysis of antifungal activity of the plant growth promoting rhizobacteria (PGPR) Bacillus subtilis strain UD1022 (hereafter UD1022) and plant health products (PHPs) (Acibenzolar-S-methyl, kelp extract, fosetyl-Al, and trinexapac-ethyl) against Clarireedia jacksonii for control of the turfgrass fungal disease dollar spot was conducted. In vitro assays revealed that UD1022 has significant inhibitory effects against the mycelial growth of C. jacksonii. Of all the treatments, wild type UD1022 and a non-ribosomal peptide mutant (sfp¯) showed potential antagonism against C. jacksonii with inhibition rate of 17.2 % and 18.4%, respectively. Interestingly a Bacillus-derived broad antifungal compound (surfactin) and PHPs showed no direct antagonistic activity against dollar spot pathogen and were statistically like the untreated control. All the mutants of UD1022 except spo0A¯ showed varying but significant inhibition of the fungal pathogen suggesting that spo0A gene may be directly involved in the antagonism against C. jacksonii. Bacterial volatile compounds (BVCs) of UD1022 may not be involved in antagonism as no fungal inhibition was observed in the indirect assay using split petri plates. Filter sterilized cell free lysate (CFL) as well as heat treated CFL of UD1022 bacterial culture did not show any fungal inhibition indicating a primary role of live bacterial cells and not bacterial-derived or secreted compounds in fungal growth inhibition. The results indicate UD1022 to possess the potential to be extended for the biocontrol of dollar spot of turfgrass.

Sensitivity of Clarireedia spp. to benzimidazoles and dimethyl inhibitors fungicides and efficacy of biofungicides on dollar spot of warm season turfgrass.

Dollar spot caused by Clarireedia spp. (formerly Sclerotinia homoeocarpa) is an economically destructive fungal disease of turfgrass that can significantly compromise turf quality, playability, and aesthetic value. Fungicides are frequently used to manage the disease but are costly and potentially unfavorable to the environment. Repeated use of some active ingredients has resulted in reduced efficacy on C. jacksonii causing dollar spot in cool-season turfgrasses in the US. Experiments were conducted to study fungicide sensitivity of Clarireedia spp. as well as to develop alternatives to fungicides against dollar spot on warm-season turfgrass in Georgia. First, 79 isolates of Clarireedia spp. collected across the state were tested on fungicide-amended agar plates for their sensitivity to thiophanate-methyl (benzimidazole) and propiconazole (dimethyl inhibitor). Seventy-seven isolates (97.5%) were sensitive (0.001 to 0.654 μg/mL) and two isolates (2.5%) were found resistant (>1000 μg/mL) to thiophanate-methyl. However, in the case of propiconazole, 27 isolates (34.2%) were sensitive (0.005 to 0.098 μg/mL) while 52 isolates (65.8%) were resistant (0.101 to 3.820 μg/mL). Next, the efficacy of three bio- and six synthetic fungicides and ten different combinations were tested in vitro against C. monteithiana. Seven bio- and synthetic fungicide spray programs comprising Bacillus subtilis QST713 and propiconazole were further tested, either alone or in a tank mix in a reduced rate, on dollar spot infected bermudagrass 'TifTuf' in growth chamber and field environments. These fungicides were selected as they were found to significantly reduce pathogen growth up to 100% on in vitro assays. The most effective spray program in growth chamber assays was 100% B. subtilis QST713 in rotation with 75% B. subtilis QST713 + 25% propiconazole tank mix applied every 14 days. However, the stand-alone application of the biofungicide B. subtilis QST713 every seven days was an effective alternative and equally efficacious as propiconazole, suppressing dollar spot severity and AUDPC up to 75%, while resulting in acceptable turf quality (>7.0) in field experiments. Our study suggests that increased resistance of Clarireedia spp. to benzimidazoles and dimethyl inhibitors warrants continuous surveillance and that biofungicides hold promise to complement synthetic fungicides in an efficacious and environmentally friendly disease management program.

Biocontrol of the causal brown patch pathogen Rhizoctonia solani by Bacillus velezensis GH1-13 and development of a bacterial strain specific detection method.

Brown patch caused by the basidiomycete fungus Rhizoctonia solani is an economically important disease of cool-season turfgrasses. In order to manage the disease, different types of fungicides have been applied, but the negative impact of fungicides on the environment continues to rise. In this study, the beneficial bacteria Bacillus velezensis GH1-13 was characterized as a potential biocontrol agent to manage brown patch disease. The strain GH1-13 strongly inhibited the mycelial growth of turf pathogens including different anastomosis groups of R. solani causing brown patch and large patch. R. solani AG2-2(IIIB) hyphae were morphologically changed, and fungal cell death resulted from exposure to the strain GH1-13. In addition, the compatibility of fungicides with the bacterial strain, and the combined application of fungicide azoxystrobin and the strain in brown patch control on creeping bentgrass indicated that the strain could serve as a biocontrol agent. To develop strain-specific detection method, two unique genes from chromosome and plasmid of GH1-13 were found using pan-genome analysis of 364 Bacillus strains. The unique gene from chromosome was successfully detected using both SYBR Green and TaqMan qPCR methods in bacterial DNA or soil DNA samples. This study suggests that application of GH1-13 offers an environmentally friendly approach via reducing fungicide application rates. Furthermore, the developed pipeline of strain-specific detection method could be a useful tool for detecting and studying the dynamics of specific biocontrol agents.

First Report of Pyricularia oryzae Causing Gray Leaf Spot of Perennial Ryegrass (Lolium perenne) in Oregon

Gray leaf spot of perennial ryegrass ( Lolium perenne), caused by Pyricularia oryzae, is a devastating turfgrass disease in many regions of the United States. In 2021, P. oryzae was detected for the first time in Oregon, causing disease on a perennial ryegrass athletic field. This report describes the occurrence of gray leaf spot and pathogenicity testing on the original infected cultivar mixture. The confirmation of this pathogen in Oregon could have management implications for turfgrass managers of the state.

First Report of In Vitro Insensitivity to Fluazinam of Clarireedia jacksonii Causing Dollar Spot of Turfgrass on New England Golf Courses

Dollar spot, caused by the newly renamed ascomycete fungus Clarireedia spp., contributes to the aesthetic and functional damage and ultimate death of turfgrass. Fluazinam, a protective fungicide belonging to the 2,6-dinitroaniline chemical group, has proven to be one of the most efficacious fungicide classes of managing a variety of turfgrass diseases, including dollar spot, due to its low risk for fungicide resistance as a broad-spectrum fungicide. Dollar spot isolates collected from six New England golf courses with suspected resistance to multiple fungicide classes had shown reduced sensitivity to fluazinam using a mycelial growth inhibition assay. This is the first report of reduced sensitivity to fluazinam in dollar spot field isolates in the United States. Briefly, in vitro sensitivity assays of 22 isolates collected from 12 golf courses and UMass turf research facilities showed that reduced fluazinam sensitivity of isolates was detected on six golf courses since 2017, demonstrating that fungi with higher exposure to fluazinam tend to develop reduced sensitivity over time. To date, there is no reported field resistance of fluazinam in C. jacksonii. However, the findings highlight the need for ongoing monitoring of in vitro and field sensitivity to fluazinam in C. jacksonii, as well as emphasizing judicious use of fluazinam in managing dollar spot disease to delay the development of fungicide resistance.

A one-step multiplex PCR assay for the detection and differentiation of four species of Clarireedia causing dollar spot on turfgrass.

Dollar spot (DS) is one of the most destructive and economically important diseases of cool- and warm-season turfgrasses worldwide. A total of six species causing DS disease in the genus Clarireedia have been described, and four of them have been reported to be distributed countrywide in China. Identification of different species of Clarireedia is a prerequisite for the effective management of DS disease. Here we report a novel polymerase chain reaction (PCR)-based method for the detection and differentiation of the four species of Clarireedia associated with DS on turfgrass in China: C.jacksonii, C.paspali, C.monteithiana and C.hainanense. Species-specific genes were identified for each species by comparative genomics analysis. Four primer pairs were designed and mixed to amplify species-specific PCR fragments with differential sizes for the four species of Clarireedia in a single multiplex PCR assay. No PCR products were generated from the DNA templates of other common fungal pathogens associated with multiple turfgrass diseases. The multiplex PCR method developed can be used for the rapid and accurate detection and differentiation of the four species of Clarireedia from pure cultures as well as from infected turfgrass blades with DS symptoms. The study developed a one-step multiplex PCR assay for the detection and differentiation of four species of Clarireedia causing DS on turfgrass in China, which will have important implications for DS management in China and worldwide. © 2022 Society of Chemical Industry.

In vitro fungicide sensitivity of Clarireedia, Fusarium, and Microdochium isolates from grasses

AbstractClarireedia spp., Fusarium culmorum, and Microdochium nivale are destructive and widespread fungal pathogens causing turfgrass disease. Chemical control is a key tool for managing these diseases on golf greens but are most effective when used in a manner that reduces overall inputs, maximizes fungicide efficacy, and minimizes the risk of fungicide resistance. In this study, sensitivity to eight commonly used fungicides was tested in 13 isolates of Clarireedia spp., F. culmorum, and M. nivale via in vitro toxicity assays. Fungicide sensitivity varied significantly among the three species, with isolates of F. culmorum showing the least sensitivity. The sensitivity of M. nivale to all tested fungicides was high (with the exception of tebuconazole), but only four fungicides (Banner Maxx®, Instrata® Elite, Medallion TL, and Switch® 62,5 WG) suppressed the growth of M. nivale completely at a concentration of 1% of the recommended dose. All three fludioxonil‐containing fungicides either alone (Medallion TL) or in combination with difeconazole (Instrata® Elite) or cyprodinil (Switch® 62,5 WG) had the same high efficacy against isolates of both M. nivale and Clarireedia spp. On average, the Clarireedia isolates tested in this study showed high sensitivity to the tested fungicides, except for Heritage (azoxystrobin). The observed variation in sensitivity among isolates within the same fungal species to different fungicides needs further investigation, as an analysis of the differences in fungal growth within each fungal group revealed a significant isolate × fungicide interaction (p &lt; .001).

Assessment of Temperature and Time Following Application as Predictors of Propiconazole Translocation in Agrostis stolonifera

Propiconazole is a xylem-mobile fungicide used to suppress psychrophilic fungi on amenity turfgrass in temperate climates. The ability of turfgrass plants to translocate propiconazole during cold temperatures is unclear, with important implications for adequate fungal suppression and potential environmental contamination. We assessed the impact of temperature and time following application on the amount of propiconazole translocated into individual turfgrass plants and created a linear mixed-effects (LME) model to predict propiconazole uptake. Propiconazole was applied near the base of individual creeping bentgrass plants grown in controlled environment chambers at four different temperatures (22, 14, 10, and 1 °C). Propiconazole quantification was performed on individual 1 cm leaf segments using liquid chromatography-tandem mass spectrometry and assessed 24, 48, and 72 h following application. Propiconazole concentrations were consistently highest in the lowest leaf segment (0–1 cm above the soil surface) regardless of temperature or time following application. Peak propiconazole concentrations were observed between 24 and 48 h after application. Using the LME model, leaf segment correlated most strongly with higher propiconazole concentration, followed by time after application and temperature. Our results indicate that propiconazole translocation in amenity turfgrass is primarily limited to the lowest segment of the plant regardless of temperature or time following the application. These findings provide important information for the effective use of xylem-mobile fungicides to treat low-temperature turfgrass diseases.

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.

Magnaporthiopsis species associated with patch diseases of turfgrasses in Australia

Magnaporthiopsis species associated with patch diseases of turfgrasses in Australia

The Known Antimammalian and Insecticidal Alkaloids Are Not Responsible for the Antifungal Activity of Epichloë Endophytes.

Asexual Epichloë sp. endophytes in association with pasture grasses produce agronomically important alkaloids (e.g., lolitrem B, epoxy-janthitrems, ergovaline, peramine, and lolines) that exhibit toxicity to grazing mammals and/or insect pests. Novel strains are primarily characterised for the presence of these compounds to ensure they are beneficial in an agronomical setting. Previous work identified endophyte strains that exhibit enhanced antifungal activity, which have the potential to improve pasture and turf quality as well as animal welfare through phytopathogen disease control. The contribution of endophyte-derived alkaloids to improving pasture and turf grass disease resistance has not been closely examined. To assess antifungal bioactivity, nine Epichloë related compounds, namely peramine hemisulfate, n-formylloline-d3, n-acetylloline hydrochloride, lolitrem B, janthitrem A, paxilline, terpendole E, terpendole C, and ergovaline, and four Claviceps purpurea ergot alkaloids, namely ergotamine, ergocornine, ergocryptine, and ergotaminine, were tested at concentrations higher than observed in planta in glasshouse and field settings using in vitro agar well diffusion assays against three common pasture and turf phytopathogens, namely Ceratobasidium sp., Drechslera sp., and Fusarium sp. Visual characterisation of bioactivity using pathogen growth area, mycelial density, and direction of growth indicated no inhibition of pathogen growth. This was confirmed by statistical analysis. The compounds responsible for antifungal bioactivity of Epichloë endophytes hence remain unknown and require further investigation.

In vitro screening of turfgrass species and cultivars for resistance to dollar spot

AbstractDollar spot, caused by at least five Clarireedia species (formerly Sclerotinia homoeocarpa F. T. Benn.), is one of the economically most important turfgrass diseases worldwide. The disease was detected for the first time in Scandinavia in 2013. There is no available information from Scandinavian variety trials on resistance to dollar spot in turfgrass species and cultivars (http://www.scanturf.org/). Our in vitro screening (in glass vials) of nine turfgrass species comprising a total of 20 cultivars showed that on average for ten Clarireedia isolates of different origin, the ranking for dollar spot resistance in turfgrass species commonly found on Scandinavian golf courses was as follows: perennial ryegrass = slender creeping red fescue &gt; strong creeping red fescue &gt; Kentucky bluegrass = velvet bentgrass &gt; colonial bentgrass = Chewings fescue ≥ creeping bentgrass = annual bluegrass. Significant differences in aggressiveness among Clarireedia isolates of different origin were found in all turfgrass species except annual bluegrass (cv. Two Putt). The U.S. C. jacksonii isolate MB‐01 and Canadian isolate SH44 were more aggressive than C. jacksonii isolates from Denmark and Sweden (14.10.DK, 14.15.SE, and 14.16.SE) in velvet bentgrass and creeping bentgrass. The Swedish isolate 14.112.SE was generally more aggressive than 14.12.NO despite the fact that they most likely belong to the same Clarireedia sp. The U.S. C. monteithiana isolate RB‐19 had similar aggressiveness as the Scandinavian C. jacksonii isolates, but was less aggressive than two U.S. C. jacksonii isolates MB‐01 and SH44. Thus, aggressiveness of Clarireedia isolates was more impacted by their geographic origin and less by species of the isolate and/or the host turfgrass species.

Fungicide sensitivity of Clarireedia spp. isolates from golf courses in China

Dollar spot, caused by Clarireedia spp. (formerly Sclerotinia homoeocarpa), is a devastating turfgrass disease worldwide. Strong selection pressure imposed by continuous fungicide applications has caused the emergence of fungicide resistance in fungal populations. In this context, a total of 358 isolates of Clarireedia spp., collected from 25 locations in the north, east and south regions of China, were assayed for in vitro sensitivities to thiophanate-methyl, iprodione, propiconazole, and boscalid using discriminatory concentrations of 10 μg/mL, 1 μg/mL, 0.1 μg/mL and 10 μg/mL, respectively. Relative mycelial growth (RMG) was used to determine sensitivity or resistance based on comparison with baseline populations. A total of 18, 11 and 14 of the 25 locations exhibited significantly reduced sensitivity (P < 0.05) to propiconazole, boscalid and iprodione, respectively. Thirteen locations had isolates resistant to thiophanate-methyl while 81%, 67%, 63% and 62% of the isolates had reduced sensitivity to propiconazole, boscalid, iprodione and thiophanate-methy1, respectively. Of the 328 isolates with the history of fungicide use screened, 94% had reduced sensitivity to at least two fungicides. Isolates resistant to thiophanate-methyl had significantly higher EC50 values than sensitive isolates for propiconazole but not for boscalid. Low to highly significant correlations were observed between fungicide sensitivities for isolates from the 25 locations, the three regions, and for all isolates in total. Our findings suggest wide-spread cross-resistance to commonly employed fungicides to manage Clarireedia spp. throughout China and underscore the challenges for managing dollar spot.

Field Assessment of Six Point-Mutations in SDH Subunit Genes Conferring Varying Resistance Levels to SDHIs in Clarireedia spp.

Dollar spot, caused by Clarireedia spp. (formerly Sclerotinia homoeocarpa F.T. Bennett), is the most economically important turfgrass disease causing considerable damage on golf courses. While cultural practices are available for reducing dollar spot infection, chemical fungicide use is often necessary for maintaining optimal turf quality. Since the release of boscalid in 2003, the succinate dehydrogenase inhibitor (SDHI) class has become an invaluable tool for managing dollar spot. However, resistance to this class has recently been reported in Clarireedia spp. and many other plant pathogenic fungi. After SDHI field failure on four golf courses and one university research plot, a total of six unique SDH mutations conferring differential in vitro sensitivities to SDHIs have been identified in Clarireedia spp. In 2018 and 2019, turf research plots were inoculated with sensitive, non-mutated isolates of Clarireedia spp., as well as resistant isolates harboring each unique identified mutation. Fungicide efficacy trials were conducted on inoculated plots to assess differential sensitivity to five SDHI active ingredients (boscalid, fluxapyroxad, isofetamid, fluopyram, and pydiflumetofen) across mutations under field conditions. Results indicate unique mutations are associated with distinct SDHI field efficacy profiles as shown in in-vitro sensitivity assays. Isolate populations with B subunit mutations (H267Y/R) were more sensitive to fluopyram, whereas isolate populations with C subunit mutations (C-G91R, C-G150R) showed resistance to all SDHIs tested. Mutation-associated differential sensitivity observed under field conditions indicates a need for a nation-wide survey and frequent monitoring of SDHI sensitivity of dollar spot populations on golf courses in the USA. Further, the information gained from this study will be useful in providing sustainable management recommendations for controlling site-specific resistant populations of Clarireedia spp.

Biological control of turfgrass diseases with organic composts enriched with Trichoderma atroviride

The increasing demand of natural products for the control of plant diseases has led to the search for biological control agents, namely fungi, often isolated from composts of organic residues. By composting two different mixes of agro-industrial residues, P1 and P2 composts were obtained, from where a known antagonist, Trichoderma atroviride, was isolated. Later, a second composting process was performed and when the composting mix reached room temperature, the previously isolated T. atroviride was inoculated in both composts to enrich its population (E) and named as P1E and P2E. The suppressive capacity of these two composts against Sclerotium rolfsii, Clarireedia spp. and Rhizoctonia solani was tested two weeks after T. atroviride inoculation, in 100 mL pots with turfgrass seeded with Agrostis stolonifera. The tested treatments were: composts P1 and P2 without any treatment; thermally treated P1 and P2 (P1t, P2t); P1 and P2 enriched without and with previous thermic treatment (P1E, P2E, P1tE, P2tE), and a commercial peat-based substrate, natural (P) and thermally treated (Pt), enriched (PE) and enriched after thermal treatment (PtE). Enriched composts achieved the highest disease control. P1E was more effective in controlling Clarireedia spp.; pots with infected plants grown on P1E, showed 53.5% less affected area compared to P1t. P2E was more effective against R. solani; results showed 69.3% less affected area compared to P2t and both were effective in S. rolfsii control, with a reduction of 38.5% in P1E and 43.5% in P2E. The larger population of Trichoderma spp. observed in the enriched substrates associated to the greater enzyme activity, namely and N-acetyl-β-glucosaminidase, standing out in P2E, will have contributed strongly to the biocontrol of the studied diseases. An increase in some macro and micronutrients was observed in the enriched heat-treated substrates.