Disease updates

Reports of previously unknown diseases, sightings in areas where diseases have not been seen previously and other news of turfgrass diseases

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Editor’s note: Each year, GCM publishes reports of previously unknown diseases, sightings in areas where diseases have not been seen previously and other news of turfgrass diseases. The following reports were previously published in the journal Plant Disease.

First report of Curvularia inaequalis and Bipolaris spicifera causing leaf blight of buffalograss in Nebraska

Disease updates: photo 1

Patches of thinning buffalograss are the result of leaf blight caused by Curvularia and Bipolaris species in mid-summer in Lincoln, Neb.
Photos by B.S. Amaradasa

Buffalograss (Buchloe dactyloides [Nutt.] Engelm.) is a warm-season turfgrass native to the mid-plains of North America having exceptional heat, cold and drought tolerance. In the past few decades, many turf-type buffalograss cultivars have been commercially released. During the summer of 2011, foliar blight was observed on buffalograss lawns in Lincoln and Waverly, Neb. Disease symptoms were common when buffalograss was growing above 86 F (30 C) and in drought conditions. Disease symptoms began as dark brown, oblong leaf spots, followed by leaf tip dieback and eventual blighting of entire tillers. Leaf infections would progress into patches of thinning turf.

Diseased leaf pieces were cultured and observed under a microscope to identify the causal organisms. Two fungal species having conidial morphology of Curvularia and Bipolaris were isolated. Colonies of Curvularia isolates grown on potato dextrose agar at 77 F (25 C) appeared velvety and dark greenish to grayish black after one week, while Bipolaris cultures were brownish gray with olive-green margins. The two species were identified as Curvularia inaequalis (Shear) Boedijn and Bipolaris spicifera (Bainier) Subram.

Disease updates: photo 2

Leaf spots and leaf tip dieback are initial symptoms
of buffalograss leaf blight.

Conidia of C. inaequalis were mostly straight to slightly curved, 17.4 to 37.1 × 7.2 to 12.6 micrometers, pale brown to brown, and three to four septate. Conidia of B. spicifera were 18.5 to 30.3 × 7 to 11.4 micrometers, ellipsoidal or oblong, light brown and three-septate. DNA testing was used to confirm the identity of the two pathogens. Pathogenicity of the two species was tested on the buffalograss cultivar Prestige. Stolons of Prestige were established in 4-inch (10-centimeter) square pots filled with potting medium. The pots of buffalograss were kept in an 86 F greenhouse with a 12-hour photoperiod for 12 weeks. One isolate of each species representing each collection site (two isolates per each species) was cultured on potato dextrose agar plates, and conidial suspensions of 1.5 × 106 spores/milliliter in sterile water were prepared. Each isolate was inoculated to three pots of Prestige by spraying 15 milliliters of spore suspension per pot. Control pots of Prestige were sprayed with water. Pots were sealed in transparent plastic bags, and every other day, the bags were opened for a few hours and the plants were sprayed with water to encourage infection. Isolates of C. inaequalis were more virulent, with initial symptoms of foliar spots appearing seven days after inoculation, followed by leaf tip dieback and necrosis of infected tillers. Bipolaris spicifera isolates induced similar symptoms 14 days after inoculation. Control pots were asymptomatic. Curvularia inaequalis and B. spicifera were successfully re-isolated from symptomatic tissue. To our knowledge, this is the first report of identification of foliar blight causal pathogens on buffalograss in Nebraska.

Source: Plant Disease, February 2014, 98(2):279. http://dx.doi.org/10.1094/PDIS-05-13-0487-PDN

B.S. Amaradasa, Ph.D., is a post-doctoral research associate and K. Amundsen, Ph.D., is an assistant professor in the department of agronomy and horticulture, University of Nebraska, Lincoln, Neb.

First report of Ustilago cynodontis causing smut of bermudagrass in Washington state

Bermudagrass (Cynodon dactylon) is an important perennial turf and forage grass that is typically grown in warm, tropical and subtropical climates. Smutted inflorescences of bermudagrass were observed and collected in Benton County, Wash., in October 2012 in an unmanaged, naturalized area located near the banks of the Columbia River and adjacent to large expanses of managed turf containing bermudagrass. The climate in this area is favorable to bermudagrass because of the relatively mild winters and hot, dry summers that usually occur in this region.

Disease update: photo 3

Healthy (left) and diseased (right) bermudagrass inflorescences collected from a naturalized riparian area in Washington state. The diseased inflorescences on the right were found to be infected with bermudagrass smut, which is caused by the fungus Ustilago cynodontis.
Photos courtesy of Jeremiah Dung

The infected plants occurred in patches alongside healthy plants, and several disease foci were observed along a 328-foot (100- meter) transect of non-contiguous bermudagrass. The disease was severe wherever it occurred. Diseased inflorescences were distorted, frequently failed to fully emerge and develop, and were covered with black-brown teliospores, which serve as resting spores of the fungus. Teliospores (n = 80) were irregularly globose to subglobose, 5.3 to 7.0 × 4.5 to 6.2 micrometers (mean 6.4 × 5.9 micrometers) and 6.2 to 8.8 × 5.3 to 7.0 micrometers (mean 7.0 × 6.5 micrometers), with a smooth wall approximately 1 micrometer thick, and were consistent with previous descriptions of Ustilago cynodontis teliospores. Teliospores germinated within 24 hours when plated on 0.2% malt agar at 61 F (16 C) and produced four-celled basidia in an arrangement also consistent with U. cynodontis. Basidia gave rise to lateral and terminal, ovoid-tolong ellipsoidal basidiospores. Basidiospores budded or germinated by hyphae and produced lateral or terminal aerial sporidia. Collectively, the morphology of the teliospores, basidia and sporidia were similar to previous descriptions of U. cynodontis.

DNA was extracted from sporidia of three single-spored isolates grown in malt extract broth. Genetic testing of the three isolates showed that they exhibited 99% to 100% identity with U. cynodontis strains previously deposited in GenBank. Representative specimens were deposited in the WSU Mycological Herbarium as WSP 72345 to WSP 72348.

This is the first report of U. cynodontis causing smut on bermudagrass in Washington state and represents the northernmost record of this fungus in North America. The occurrence of U. cynodontis in Washington suggests that the pathogen may exist in other hot and dry areas of northwestern North America where bermudagrass can be associated with recreational, landscape or natural settings.

Source: Plant Disease, February 2014, 98(2):280. http://dx.doi.org/10.1094/PDIS-05-13-0560-PDN  

J.K.S. Dung, Ph.D., is an assistant professor in the department of botany and plant pathology, Central Oregon Agricultural Research Center, Oregon State University, Madras, Ore.; L.M. Carris, Ph.D., is an associate professor in the department of plant pathology, Washington State University, Pullman, Wash.; and P.B. Hamm is station director and professor emeritus in the department of botany and plant pathology, Hermiston Agricultural Research and Extension Center, Oregon State University, Hermiston, Ore.

Dollar spot disease on the oceanside sedge Trichophorum cespitosum

Sclerotinia homoeocarpa is a fungal pathogen that causes dollar spot disease on more than 40 plant species, mostly in the family Poaceae, and is considered the most widespread pathogen of golf course turfgrasses in the St. Lawrence River region.

Disease updates: photo 4

Dollar spot lesions on tufted bulrush (Trichophorum cespitosum) on the seashore near Peggys Cove, Nova Scotia, Canada.
Photos by T. Hsiang

In June 2011, lesions were observed on tufted bulrush, Trichophorum cespitosum (Poales, Cyperaceae), on the seashore near Peggys Cove, Nova Scotia, Canada. Single bunches had up to 40% of the leaves affected. The foliar symptoms were large hourglassshaped lesions, up to 2 inches (5 centimeters) long, with a straw-colored portion capped at two ends by dark zone lines on surrounding green foliar tissue. These lesions were similar to dollar spot lesions found on turfgrasses such as Kentucky bluegrass (Poa pratensis).

A fungus was isolated from symptomatic leaf segments and, after three days of growth on nutrient agar at room temperature, white fluffy mycelia covered the entire petri dish. Brown columnar structures began to form in the colony centers after seven days with abundant aerial growth, and cultures became cinnamon-colored after 14 days. Dark brown or black substratal stroma were formed on or in the agar, and cultures appeared dark brown from the bottom.

DNA was extracted and amplified using ribosomal DNA primers ITS1 and ITS4, and the DNA fragment sequenced (GenBank Accession No. KF447776). The sequence showed a top match of 522/524 bp identity with the ITS sequence of an isolate of S. homoeocarpa, with the next 40 top matches also identified as S. homoeocarpa. This was an unexpected finding, so attempts were made to test the ability of this isolate to cause disease on turfgrasses.

Disease updates: photo 5

Dollar spot lesions on Kentucky bluegrass
(Poa pratensis) in mid-summer.

Two-week-old seedlings of Penncross creeping bentgrass (Agrostis stolonifera), Touchdown Kentucky bluegrass (Poa pratensis) and Express perennial ryegrass (Lolium perenne) were inoculated by placing 0.2- inch (5-millimeter) diameter mycelial plugs from five-day-old cultures onto the leaves of plants grown in small containers, and incubating under enclosed humid conditions throughout the test. White aerial hyphae on the leaves and straw-colored leaf lesions were observed by seven days after inoculation on Kentucky bluegrass and perennial ryegrass, but no lesions or hyphal growth were observed on creeping bentgrass. No signs or symptoms were observed on leaves where sterile agar plugs were used as inoculum. These tests were repeated three times with the same results, and a positive control was included by using an S. homoeocarpa isolate known to be pathogenic to creeping bentgrass under the same test conditions. Disease was observed on creeping bentgrass with the control isolate but never with the isolate from T. cespitosum. Sclerotinia homoeocarpa was reisolated from the lesions on Kentucky bluegrass and perennial ryegrass to satisfy Koch’s postulates. To the best of our knowledge, this is the first report of S. homoeocarpa on T. cespitosum worldwide, involving an isolate that was found to cause disease on Kentucky bluegrass and perennial ryegrass, but was not pathogenic to creeping bentgrass in vitro. The original host was not used in pathogenicity tests because it is considered an endangered species in many locations.

These findings extend the known host range of S. homoeocarpa and may indicate another source of inoculum of this fungus, especially for oceanside golf courses. We are continuing research to figure out why this isolate could cause disease on tufted bulrush, perennial ryegrass and Kentucky bluegrass, yet not on creeping bentgrass. The answers may lead to a better understanding of the  genes involved in pathogenicity and eventually help to improve disease management.

Source: Plant Disease, January 2014, 98(1):161. http://dx.doi.org/10.1094/PDIS-07-13-0703-PDN

T. Hsiang, Ph.D., is a professor and F. Shi is a research associate in the School of Environmental Sciences, University of Guelph, Ontario, Canada. This work was supported by the Natural Sciences and Engineering Research Council of Canada.

First report of Xanthomonas translucens causing etiolation on creeping bentgrass in Illinois, Kentucky and North Carolina

Symptoms of etiolation, which is an abnormal elongation and yellowing of tillers, have been observed on creeping bentgrass (Agrostis stolonifera L.) putting greens for decades; however, symptoms are typically transient and not problematic. Reports of etiolation have become more frequent recently, and research supports the involvement of bacteria.

Disease updates: photo 6

Comparison of inoculated (left) to non-inoculated (right) Penn A-1 creeping bentgrass maintained in the greenhouse at 86 F. Etiolation symptoms were continually observed in turf inoculated with Xanthomonas translucens as shown here at four weeks after inoculation.
Photo by J.A. Roberts

During stressful summer periods in 2011 and 2012, 62 creeping bentgrass putting green samples were submitted to the North Carolina State University Turf Clinic exhibiting symptoms of etiolation, chlorosis and/ or general decline. Microscopic examination of stem and leaf tissue often showed bacterial streaming from the xylem tissue. Symptomatic tissue was surface disinfested in sodium hypochlorite (10% Clorox) for five minutes, blotted dry and rinsed in sterile distilled water. Disinfested tissue was placed in a small drop of sterile distilled water on a glass microscope slide and cut to allow bacteria to stream into the water for two minutes.

The resulting bacterial suspension was streaked onto three nutrient agar plates and incubated at 86 F (30 C) overnight. Bacterial colonies varied in morphology, and those present in the greatest number based on morphology were re-streaked to isolate individual colonies. Bacterial isolates were tentatively identified to species using rDNA sequencing. Sequencing results showed isolates obtained from six locations (Illinois, Kentucky and North Carolina) having a positive match to Xanthomonas translucens. Additional research is needed to confirm pathovar designation as X. translucens isolates were similar to both poae and graminis pathovars.

A representative isolate was also examined for carbon source utilization resulting in a positive identification of X. translucens. This isolate was used to inoculate six-week-old seeded Penn A-1 creeping bentgrass plants maintained at a height of 0.4 inch (1 centimeter) in 1.38-inch (3.5-centimeter) diameter conetainers. Scissors were dipped in a cell suspension and used to cut healthy creeping bentgrass plants at a height of 0.4 inch, and the remaining suspension was applied to the foliage until runoff using an atomizer bottle. Non-inoculated plants were cut and misted using sterile water. After inoculation, plants were placed in a sealed clear plastic container for 48 hours and then transferred to the growth chamber bench (86 F) receiving irrigation twice daily with distilled water. Etiolation was rated within each of the four replicates by counting the number of etiolated leaves that were easily observed as significantly higher than the rest of the turf canopy.

Plants inoculated with X. translucens exhibited etiolation of the youngest leaf within 48 hours, whereas the non-inoculated plants did not. Symptoms were similar to observations in the field, as etiolated leaves were chlorotic and easily extracted from the turf surface. Microscopic examination showed bacterial streaming and identification of bacteria (using the previously described methods) was positive for X. translucens. Etiolation symptoms persisted over multiple weeks, but a decline in turf quality was not observed.

Etiolation has been previously suggested as a precursor to bacterial wilt, caused by X. translucens pv. poae, on annual bluegrass (Poa  annua L.), and Acidovorax avenae has also been shown to produce etiolation on creeping bentgrass. To our knowledge, this is the first confirmation of X. translucens as a cause of etiolation in creeping bentgrass.

Source: Plant Disease, 2014, in press. http://dx.doi.org/10.1094/PDIS-05-13-0565-PDN  

Joseph Roberts, M.S., is a graduate research assistant; Lane Tredway, Ph.D., is an associate professor; and David F. Ritchie, Ph.D., is a professor and Extension specialist in the department of plant pathology, North Carolina State University, Raleigh, N.C.

First report of stubby root caused by Trichodorus obtusus on zoysia and bermuda in South Carolina

Disease updates: photo 7

Trichodorus obtusus female from South Carolina
(length = 1,235 micrometers).
Photos by Brad Shaver

In September 2011, diagnostic samples were taken from Tifway bermudagrass (Cynodon dactylon × C. transvaalensis) tees and from Emerald zoysiagrass (Zoysia japonica) roughs of a golf course in Charleston, S.C. Additional samples were taken from a sod farm located near Charleston from a field of Empire zoysiagrass.

The soil was sandy loam, and the samples were taken at a depth of 4-6 inches (10-15 centimeters) from symptomatic turf. Symptoms on bermudagrass and zoysiagrass included stubby roots and lightly to severely chlorotic or dead patches of irregular sizes and shapes. Nematodes were extracted by sugar centrifugal- flotation and counted. The predominant nematode species recovered was a stubby root nematode, Trichodorus obtusus Cobb. Nematode densities were 30 to 170/6.1 cubic inches (100 cubic centimeters) of soil (average 94, n = 5) at the sod farm, and 30 to 230 (average 107, n = 7) at the golf course.

Trichodorus obtusus has been reported as a pathogen of bermudagrass in Florida, where it is more damaging than Paratrichodorus minor, the other stubby root nematode commonly associated with turfgrass. In Florida, a density of 120 T. obtusus/6.1 cubic inches is considered high risk. We have encountered several additional samples from across South Carolina with similar or higher densities since our first diagnosis.

Infested soil (94 individuals/6.1 cubic inches) collected from the sod farm was put into columns and planted with Empire sod and maintained in the greenhouse. After 140 days, the population density increased to an average of 230 individuals/6.1 cubic inches of soil. Plants were prone to wilting, and new root growth showed symptoms similar to those observed in the field.

Disease updates: photo 8

Root symptoms of the stubby root
nematode, T. obtusus, on zoysiagrass.

Morphologic and morphometric identification of T. obtusus was made by examining male and female specimens in temporary water mounts. Males had ventrally curved spicules with three ventral precloacal papillae, with the posterior papilla just anterior to the head of the retracted spicules, one ventromedian cervical papilla anterior to the excretory pore, and tail with non-thickened terminal cuticle. Females had a deep, barrel- shaped, pore-like vulva, and one or two postadvulvar lateral body pores on each side. Males and females had distinctly offset esophagus. Females (n = 10) were 1,100 to 1,440 (1,250) micrometers long, body width 40 to 53 (45) micrometers, onchiostyle 63 to 75 (67) micrometers, and V 583 to 770 (673) micrometers. Males (n = 10) were 1,076 to 1,353 (1,222) micrometers long, body width 33 to 45 (39) micrometers, onchiostyle 62 to 69 (65) micrometers, and spicule 55 to 63 (59) micrometers.

A section of the rDNA region was sequenced from individuals representing the two locations. A search revealed no similar sequences to those of our two populations. As such, it appears that these are the first sequences of this portion of the rDNA for T. obtusus, although a different, non-overlapping portion was found under the synonym T. proximus. To our knowledge, this is the first report of T. obtusus on zoysiagrass and the first report of the species on bermudagrass in South Carolina.

Source: Plant Disease, June 2013, 97(6):852. http://dx.doi.org/10.1094/PDIS-10-12-0932-PDN  

J.B. Shaver is a graduate student in plant and environmental sciences; P. Agudelo, Ph.D., is an associate professor and nematologist; and S.B. Martin, Ph.D., is a professor in turfgrass pathology in the School of Agricultural, Forest and Environmental Sciences, Clemson University, Clemson, S.C.