Occurrence and Distribution of Triticum mosaic virus in the Central Great Plains.

Wheat curl mite (WCM)-transmitted viruses-namely, Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and the High Plains virus (HPV)-are three of the wheat-infecting viruses in the central Great Plains of the United States. TriMV is newly discovered and its prevalence and incidence are largely unknown. Field surveys were carried out in Colorado, Kansas, Nebraska, and South Dakota in spring and fall 2010 and 2011 to determine TriMV prevalence and incidence and the frequency of TriMV co-infection with WSMV or HPV in winter wheat. WSMV was the most prevalent and was detected in 83% of 185 season-counties (= s-counties), 73% of 420 season-fields (= s-fields), and 35% of 12,973 samples. TriMV was detected in 32, 6, and 6% of s-counties, s-fields, and samples, respectively. HPV was detected in 34, 15, and 4% of s-counties, s-fields, and samples, respectively. TriMV was detected in all four states. In all, 91% of TriMV-positive samples were co-infected with WSMV, whereas WSMV and HPV were mainly detected as single infections. The results from this study indicate that TriMV occurs in winter wheat predominantly as a double infection with WSMV, which will complicate breeding for resistance to WCM-transmitted viruses.

Genomic analysis of Xanthomonas oryzae isolates from rice grown in the United States reveals substantial divergence from known X. oryzae pathovars.

The species Xanthomonas oryzae is comprised of two designated pathovars, both of which cause economically significant diseases of rice in Asia and Africa. Although X. oryzae is not considered endemic in the United States, an X. oryzae-like bacterium was isolated from U.S. rice and southern cutgrass in the late 1980s. The U.S. strains were weakly pathogenic and genetically distinct from characterized X. oryzae pathovars. In the current study, a draft genome sequence from two U.S. Xanthomonas strains revealed that the U.S. strains form a novel clade within the X. oryzae species, distinct from all strains known to cause significant yield loss. Comparative genome analysis revealed several putative gene clusters specific to the U.S. strains and supported previous reports that the U.S. strains lack transcriptional activator-like (TAL) effectors. In addition to phylogenetic and comparative analyses, the genome sequence was used for designing robust U.S. strain-specific primers, demonstrating the usefulness of a draft genome sequence in the rapid development of diagnostic tools.

Quantifying the Effects of Lance Nematode Parasitism in Creeping Bentgrass.

We compared photosynthesis and multispectral radiometry (MSR) measurements with visual quality ratings for assessment of feeding injury to creeping bentgrass caused by the lance nematode (Hoplolaimus galeatus) using artificially infested microplots and a naturally infested putting green. Nematode feeding resulted in negative visual and MSR effects on creeping bentgrass in microplots. Visual quality ratings were correlated more consistently with nematode densities than either individual MSR variables or factor models of MSR variables. Threshold estimates for H. galeatus population densities associated with unacceptable bentgrass quality in microplots varied widely by month and year. Similarly, the relationship between H. galeatus population density and turf health indicators (including MSR measurements, visual ratings, and net photosynthetic rate) varied with cultivar and management practice (irrigation frequency and mowing height) in the naturally infested putting green. Notably, negative effects of nematode feeding were not consistently associated with more stressful management practices, suggesting that stress avoidance is not a reliable deterrent to H. galeatus damage in creeping bentgrass. Damage thresholds for this nematode-host association are dynamic and should be used with caution.

Population Dynamics of the Lance Nematode (Hoplolaimus galeatus) in Creeping Bentgrass.

The effects of management practices and nematode population density on the seasonal fluctuationsin lance nematode (Hoplolaimus galeatus) populations in creeping bentgrass were studiedin a naturally infested experimental putting green and in artificially infested microplots. In general, H. galeatus populations increased from late spring through midsummer, declined in August, and increased again in the fall. Population increase in microplots was strongly density dependent, with final population densities inversely proportional to inoculum levels. Ectoparasitic populationsof H. galeatus in both studies were composed of adults and juveniles, whereas endoparasiticpopulations were almost exclusively juveniles. H. galeatus populations in the naturallyinfested site were aggregated spatially, but the aggregation was not temporally stable. Nematodepopulations were not affected by bentgrass cultivar selection or irrigation frequency.

Development of Brown Patch and Pythium Blight in Tall Fescue as Affected by Irrigation Frequency, Clipping Removal, and Fungicide Application.

We studied the effects of irrigation frequency, clipping removal, and fungicide application on the development of Rhizoctonia brown patch (Rhizoctonia solani) and Pythium blight (Pythium aphanidermatum) in tall fescue. Brown patch severity was not significantly different between plots irrigated daily and those irrigated on alternate days. Similarly, no differences in brown patch were observed in plots where grass clippings were returned to the sward with a mulching mower compared with plots where clippings were removed. Preventive applications of azox-ystrobin at 35-day intervals or postinfection applications of chlorothalonil reduced brown patch severity, but only the azoxystrobin treatment provided aesthetically acceptable (<10%) levels of brown patch control. However, azoxystrobin applications also increased Pythium blight compared with untreated or chlorothalonil-treated tall fescue, especially in plots that received daily irrigation.

First Report of Gaeumannomyces graminis var. graminis on Kikuyugrass (Pennisetum clandestinum) in the United States.

Kikuyugrass (Pennisetum clandestinum) is a warm-season grass and invasive weed in the landscape, but can be used for golf course fairways in southern California. In 1999, a decline of kikuyugrass was observed on golf courses in southern California beginning in late summer or early autumn. Symptoms included sunken, bleached patches of turf with individual plants having chlorotic foliage and reduced vigor. Roots and stolons were often covered with dark, ectotrophic fungi, and lobed hyphopodia were visible on the stolons. On colonized roots, the cortex was rotted, and the stele showed evidence of colonization by the fungus. In March 2002, a sample of kikuyugrass exhibiting decline symptoms was obtained from a golf course fairway in Los Angeles, CA. Sections of roots and stolons were surface sterilized for 60 s in a 0.3% sodium hypochlorite solution and placed on acidified water agar. Emerging colonies were transferred to potato dextrose agar (PDA). Isolates were characteristic of Gaeumannomyces spp. (2) with dark hyphae and curled colony edges. The rDNA internal transcribed spacer (ITS) regions of two isolates (HCC-5 and -6) were amplified by polymerase chain reaction (PCR) using universal fungal rDNA primers ITS 4 (5'-TCCTCCGCTTATTGATATGC-3') and ITS 5 (5'-GGAAGTAAAAGTCG TAACAAGG-3') (3). PCR products were sequenced and exhibited 99% sequence identity to G. graminis var. graminis (GenBank Accession No. 87685). These isolates were grown separately on autoclaved sand and cornmeal media (1) for 21 days at 25°C. Styrofoam cups were partially filled with autoclaved medium-coarse sand, and 10 g of inoculum was spread evenly in a layer on top. This layer was covered by an additional centimeter of autoclaved sand and 5 g of kikuyugrass seed (cv. 'AZ-1'). Both isolates were tested separately using six replicate cups per isolate. Controls were prepared using only a 10 g layer of autoclaved sand and cornmeal. Cups were misted at 1 h intervals on a greenhouse bench maintained at 25°C. Seeds germinated and emerged after ≈10 days. In cups inoculated with isolate HCC-5 or -6, dark mycelia were evident on the coleoptiles of the emerging plants. Plants were removed and washed 21 days after planting. Inoculated plants were chlorotic and had reduced root and foliar growth compared to the controls. Coleoptiles, hypocotyls, and roots were covered with dark, ectotrophic fungi with lobed hyphopodia present on the hypocotyls. In colonized roots, cortical tissue was rotted with extensive colonization of the epidermis and penetration of the fungus into the root cortex. Sections of infected root tissue were surface disinfested, placed on acidified water agar, and the resulting colonies transferred to PDA. Isolates exhibited the same colony morphology and characteristics as those previously identified as G. graminis var. graminis. To our knowledge, this is the first report of this fungus as a pathogen of kikuyugrass. References: (1) M. J. C. Asher. Ann. Appl. Biol. 70:215, 1972. (2) P. C. Cunningham. Isolation and culture. Pages 103-123 in: Biology and Control of Take All. M. J. C. Asher and P. J. Shipton, eds. Academic Press, London, 1981. (3) T. J. White et al. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al. eds. Academic Press, San Diego, CA, 1990.

Bermudagrass Dead Spot: A New Disease of Bermudagrass Caused by Ophiosphaerella agrostis.

Hybrid bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy) is widely used on golf course putting greens in the southern United States. In March and April 1998, circular patches of dead grass 2 to 10 cm in diameter were observed on a bermudagrass putting green in College Station, TX, that had been overseeded with rough bluegrass (Poa trivialis L.) the previous October. Rapid death and deterioration of the rough bluegrass within the spot revealed extensive foliar and crown necrosis and root decay of the remaining bermudagrass. Diseased bermudagrass leaves in the patch were reddish brown to tan. Dark ectotrophic hyphae were not found on the roots or stolons, but dark hyphae were observed within the affected root tissues. Numerous pseudothecia were embedded in necrotic leaf and stolon tissues. The characteristics of the pseudothecia and ascospores coincide with the description of Ophiosphaerella agrostis Dernoeden, Camara, O'Neil, van Berkum, and Palm (1,2). This fungus was consistently isolated from stolons and roots, and single-ascospore isolates were obtained from pseudothecia. Inoculum was prepared by transferring fungal mycelium from a single-spore isolate grown in potato dextrose agar (PDA) to a moistened, autoclaved mixture of rice hulls (Oryza sativa L.) and milled rice (2:1, vol/vol) for 28 days at 24°C. 'FloraDwarf' bermudagrass was grown from stolons in 15-cm-diameter pots containing a mixture of sand, peat moss, and perlite (8:3:1, vol/vol). The bermudagrass was maintained at a height of 1 to 1.5 cm for ≈ 1 month. Plants were inoculated by forming a hole that was 0.8 cm in diameter and 7 cm deep in the center of the pot, using a rod and filling the hole with inoculum. Control pots received the same treatment, except uninoculated rice hull-milled rice mixture was used. The treatments were replicated three times, and the experiment was performed twice. The pots were maintained in a greenhouse for 6 weeks. In all inoculated pots, patches of dead bermudagrass 6 to 10 cm in diameter developed. Roots, stolons, and leaves were necrotic, and pseudothecia were abundant in stolon and leaf sheath tissues. O. agrostis was consistently reisolated from infected root and stolon tissues. All isolates produced colonies identical in appearance to the culture used for inoculation. To our knowledge, this is the first report that O. agrostis is pathogenic to hybrid bermudagrass. References: (1) M. P. S. Camara et al. Mycologia 92:317, 2000. (2) P. H. Dernoeden et al. Plant Dis. 83:397, 1999.

Genetic Diversity and Aggressiveness of Ophiosphaerella korrae, a Cause of Spring Dead Spot of Bermudagrass.

The distribution of three Ophiosphaerella spp. that cause spring dead spot (SDS) of bermudagrass was studied by sampling at 24 locations in the southeastern United States. O. korrae was isolated from 73% of the 204 bermudagrass cores collected and was the only SDS pathogen recovered at most sites. O. herpotricha was isolated at three locations in Kentucky and one in North Carolina, and O. narmari was found at two locations in North Carolina. Most O. korrae isolates collected from Alabama, Kentucky, Mississippi, Tennessee, and Virginia clustered in an amplified fragment length polymorphism group (AFLP group II) that was distinct from Kentucky bluegrass isolates collected throughout North America and similar to bermudagrass isolates from Kansas and Oklahoma (AFLP group I). A third AFLP group (III) consisting of bermudagrass isolates from Mississippi and Virginia was identified. Isolates representing AFLP groups II and III grew more rapidly on potato dextrose agar at 25 and 30°C than those in group I. O. korrae isolates differed in their aggressiveness to two bermudagrass cultivars in greenhouse studies, but these differences were not associated with AFLP group, location, or host from which the isolate was collected.

Bermudagrass Resistance to Spring Dead Spot Caused by Ophiosphaerella herpotricha.

Field and greenhouse studies were conducted to evaluate the resistance of seed- and vegetatively propagated bermudagrass entries (Cynodon spp.) to spring dead spot caused by Ophiosphaerella herpotricha. In Kansas greenhouse studies, O. herpotricha caused root discoloration and root weight reductions in all entries tested. However, in Kansas field plots, root weight reductions were not different among entries and were not correlated with disease severity ratings. In an inoculated field study in Oklahoma, diseased areas ranged from 47 cm2 for the entry Jackpot to 262 cm2 for Poco Verde in 1995, and from 121 to 1,810 cm2 for the entries Guymon and Common in 1996. African bermudagrass (Cynodon transvaalensis) exhibited the greatest number of live shoots per diseased area in both years, due in part to its greater shoot density, but also indicating greater potential to recover from the disease. African bermudagrass, Guymon, Sundevil, Midlawn, Midfield, Ft. Reno, Mirage, and several experimental seed-propagated entries were most resistant to spring dead spot, having the lowest diseased area and greatest number of live shoots within diseased areas. In Oklahoma, severity of spring dead spot among bermudagrass entries was correlated with feeeze injury that occurred during the first winter after planting.

Condition of Green Ash, Incidence of Ash Yellows Phytoplasmas, and Their Association in the Great Plains and Rocky Mountain Regions of North America.

About 50% of 1,057 green ash (Fraxinus pennsylvanica) systematically sampled in the Great Plains and Rocky Mountain regions had substantial dieback (>10% of crown branches with dieback), and the average growth ring width during the last 20 years was 2.9 mm. The overall condition of the population was rated fair. Ash yellows phytoplasmas were identified at 102 of 106 sites throughout six U.S. states (North Dakota, South Dakota, Wyoming, Nebraska, Colorado, Kansas) and three Canadian provinces (Alberta, Saskatchewan, Manitoba). These phytoplasmas had not previously been known in Alberta, Saskatchewan, Manitoba, Wyoming, Colorado, or Kansas. Incidence of phytoplasmal detection ranged from 16% in Wyoming to 71% in South Dakota. Incidence varied in the range 41 to 67% across site types and crown dieback classes. Incidence was highest in rural plantings, in trees with the most crown dieback, and in larger diameter trees. No significant relationships were detected between presence of ash yellows phytoplasmas and radial growth rates of trees.

The effect of temperature on survival of Pityophthorus juglandis (Coleoptera: Curculionidae).

The walnut twig beetle (Pityophthorus juglandis Blackman) vectors Geosmithia morbida, the causal agent of thousand cankers disease in Juglans, and is particularly damaging to Juglans nigra L. (black walnut). Native hosts of P. juglandis are distributed in the southwestern United States where winter temperatures tend to be higher than those found within the native range of black walnut. To better understand temperature effects on survival of P. juglandis, we initiated studies to determine: 1) seasonal variations in cold tolerance, as measured by the supercooling point (SCP), and 2) upper and lower lethal temperatures (LT). In the lower LT study, Xyleborinus saxeseni (Ratzeberg) was tested for comparison. Insects were either exposed to increasing or decreasing temperatures and then checked for survival. Upper and lower LTs were estimated using a logistic model. For the SCP study, data were grouped into seasons. Seasonal mean SCPs were highest in summer (-15.4°C) and lowest in fall (-18.1°C). The upper lethal limit estimations required to kill 99% of the population (LT99) for adults and larvae were 52.7 and 48.1°C, respectively, and lower limit LT99 estimations for adults and larvae were -18.1 and -18.7°C, respectively. The lower median LT (LT50) of X. saxeseni was -24.7°C. These studies, as well as beetle survival in infested Colorado trees where temperatures reached -29°C in February 2011, suggest P. juglandis could survive the winter in much of the native range of black walnut, but may be limited in trees where temperatures regularly exceed the lower LT.