The effects of N source, fall fertilization, and preventive fungicides were evaluated in bermudagrass [Cynodon dactylon (L.) Pers.] artificially inoculated with Ophiosphaerella korrae and O. herpotricha, the most common spring dead spot (SDS) pathogens in the United States. The source of N applied to bermudagrass from May to August 2006–2008 had a significant effect on SDS symptoms appearing in the spring of 2007–2009. Ammonium sulfate [(NH4)2SO4] provided excellent suppression of O. herpotricha but had no effect on O. korrae, which was suppressed instead by calcium nitrate (CaNO3). More research is needed to determine the underlying mechanisms responsible for suppression of SDS with N sources and the potential role of Mn and Ca in development of the disease. Fall applications of K, S, gypsum, or dolomitic lime had no influence on SDS development. Preventive applications of propiconazole, propiconazole + azoxystrobin, tebuconazole, or fenarimol provided effective control of O. herpotricha but failed to provide significant suppression of O. korrae. The differential response of SDS pathogens to fertilization practices and preventive fungicide applications highlight the importance of pathogen identification in development of integrated management programs.
The efficacy of five application methods and four fungicides were evaluated for control of spring dead spot (SDS) of bermudagrass from 2002 to 2004. Fenarimol and propiconazole were most effective in reducing SDS, providing from 66% to 89% and 51% to 52% control, respectively. Application water volume (2.5, 5, or 10 gal/1000 ft2), post-application irrigation, and high-pressure injection did not affect SDS control. Further research with fenarimol was conducted from 2003 into 2005 to optimize application rate and timing. In both years, all rates (6, 4 + 4, and 6 + 6 fl oz/1000 ft2, with split applications 2 weeks apart) provided equivalent control of SDS when averaged across all application timings. No significant differences were detected among application timings ranging from August 1 to October 1 in 2003 and from August 23 to November 5 in 2004. Accepted for publication 22 May 2006. Published 1 September 2006.
Since 2002, symptoms of an unknown disease have been observed in ‘El Toro’ zoysiagrass (Zoysia japonica Steud.) in several locations across North Carolina. Symptoms become evident in the spring as the zoysiagrass comes out of winter dormancy. Circular or irregularly shaped patches, 10 to 30 cm in diameter, remain dormant as the surrounding turf resumes growth. These patches eventually collapse and die, leaving sunken depressions in the turf stand. After the initial appearance of symptoms, the zoysiagrass slowly recolonizes the patches by spreading inward from the perimeter. Microscopic observation revealed necrotic stolon and root tissue that was colonized by ectotrophic fungal hyphae, whereas leaf and sheath tissue was colonized by species of Curvularia, Colletotrichum, Alternaria, Ascochyta, Drechslera, or Fusarium. Sections of necrotic root and stolon tissue were washed under flowing tap water for 10 min, submersed in 0.6% NaOCl for 5 min, rinsed with sterile dH2O, blotted dry, and placed on ¼ strength potato dextrose agar amended with 100 μg/ml each of streptomycin sulfate and chloramphenicol. A total of 50 isolates were obtained from four locations during 2002 and 2003. A fungus resembling Ophiosphaerella spp. was consistently isolated and was confirmed to be Ophiosphaerella korrae by species-specific PCR assays (3) and rDNA internal transcribed spacer (ITS) sequencing. Pathogenicity tests were conducted in the field on ‘El Toro’ zoysiagrass at the Lake Wheeler Turfgrass Field Laboratory in Raleigh, NC. Autoclaved rye grain (Secale cereale L.; 200 g of grain, 5.75 g of CaCO3, and 220 ml of H2O) was infested with one of eight O. korrae isolates. Plots (1 × 1 m) were inoculated on 13 October 2004 by removing an 11-cm-diameter core from the center of each plot to a 5-cm depth, placing 10 cm3 of infested rye grain in the bottom of the hole, and replacing the core. Noninoculated and uninfested rye grain treatments served as controls, and each treatment was replicated eight times in a randomized complete block. No symptoms were observed in the experimental area during 2005. In April 2006, five isolates (Zrr20, Zrr36, Zrr57, Zrr58, and Zrr59) incited spring dead spot symptoms in at least four of eight inoculated plots. The average diameter of patches induced by these isolates ranged from 7.9 to 11.4 cm. In April 2007, three isolates (Zrr20, Zrr36, and Zrr57) incited symptoms in at least four plots, with average patch diameters ranging from 14.5 to 16.0 cm. These inoculation success rates and patch diameters were similar to those resulting from O. korrae inoculation of bermudagrass conducted on the same date (L. P. Tredway, unpublished data). No symptoms were observed in noninoculated plots or those amended with uninfested rye grain. O. korrae was consistently reisolated from symptomatic stolons and roots in May 2007 to complete Koch's postulates. To the best of our knowledge, this is the first report of spring dead spot of zoysiagrass caused by O. korrae in the United States. Previously, O. herpotricha was shown to induce spring dead spot symptoms on zoysiagrass in Kansas (1), and O. korrae was reported as a zoysiagrass pathogen in Japan (2). To date, we have only observed spring dead spot on the Zoysia japonica ‘El Toro’. References: (1) D. E. Green et al. Plant Dis. 77:1040, 1993. (2) T. Tani. Color Atlas of Turfgrass Diseases. Ann Arbor Press, Chelsea, MI, 1997. (3) N. A. Tisserat et al. Phytopathology 84:478, 1994.
Twenty isolates of a fungus resembling Rhizoctonia zeae were obtained from three golf courses with bermudagrass [Cynodon dactylon (L.) Pers.] greens that were exhibiting leaf and sheath spot symptoms in 2013 and 2014. Morphological and molecular identification showed that 17 isolates were identified as R. zeae and three were identified as Waitea circinata var. zeae. Pathogenicity of the 20 isolates was determined by inoculating hybrid bermudagrass (‘Champion’) plants in a growth chamber. After inoculation, plants were incubated in a growth chamber at 34/30°C (day/night) and sealed in a closed container to ensure 100% humidity. After 7 d, disease was visually estimated using a modified Horsfall‐Barrett scale (0–10). All isolates were pathogenic to the hybrid bermudagrass cultivar Champion and aggressiveness among isolates varied. Sensitivity of seven isolates collected in 2013 to 16 fungicides (flutolanil, pyraclostrobin, fluoxastrobin, azoxystrobin, propiconazole, triticonazole, triadimefon, penthiopyrad, fluxapyroxad, azoxystrobin + difenoconazole, tebuconazole, difenoconazole, iprodione, polyoxin‐D zinc salt, fluxapyroxad + pyraclostrobin, and chlorothalonil) was tested. The sensitivity of 13 isolates collected in 2014 to five fungicides (chlorothalonil, azoxystrobin, pyraclostrobin, difenoconazole, and flutolanil) was tested. In general, all the isolates were extremely sensitive to demtheylation and succinate dehydrogenase inhibitor fungicides (50% effective concentration [EC50] < 1 mg a.i. L−1) and all isolates showed insensitivity to quinone outside inhibitor fungicides (EC50 > 10 mg a.i. L−1). These results demonstrated that R. zeae is a pathogen of ultradwarf bermudagrass under the imposed environmental conditions. Rhizoctonia zeae is sensitive to many fungicides, but based on in vitro sensitivity data presented here, some may not perform well under field conditions.
Tall fescue lawns can be fertilized during the summer months at modest N rates without affecting brown patch severity.• When an appropriate fungicide such as Heritage was used for brown patch management, the method of fungicide delivery did not affect brown patch suppression.• Simulated rainfall up to 0.5 inches within 15 min of fungicide application did not compromise fungicide efficacy. AbstractIn North Carolina, tall fescue (Festuca arundinacea Schreb.) is widely grown throughout the Mountain and Piedmont regions. North Carolina is in the transition zone, which is subject to hot, humid summers that predispose tall fescue to brown patch (Rhizoctonia solani Kühn). Field trials were conducted over a 2-year period (2015)(2016) to evaluate the effects of nitrogen rate and timing, application method of a fungicide, and rainfall following fungicide application on brown patch severity on lawn height tall fescue. Seven rates of urea providing 0 to 6 lb N 1000 ft -2 year -1 were initiated each year in March with repeat applications monthly at 1 lb N 1000 ft -2 . In a separate study, various timings of urea were conducted throughout the year for a total of 3 lb N 1000 ft -2 yr -1 . In 2015, no significant differences in disease severity or turfgrass quality were observed among the seven N rates. Only the application of 6 lb N 1000 ft -2 yr -1 resulted in significantly higher brown patch compared with the non-treated control in 2016. No significant differences in disease severity or turfgrass quality were observed in the timing study in both years. Azoxystrobin was applied with a ride-on spreader/sprayer (11 gal water-carrier acre -1 ), a commercial applicator gun (130 gal water-carrier acre -1 ), and a research spray boom (88 gal water-carrier acre -1 ). No differences were detected among application methods. A rainfall event of 0.5 inches was simulated with overhead irrigation 30 min after application of fungicides. No differences were detected among the fungicide treatments, and all provided excellent control of brown patch.
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