G. Ruhl

First Report of Gray Leaf Spot on Perennial Ryegrass in Indiana

Pyricularia grisea, the causal agent of gray leaf spot on turfgrass, was isolated from symptomatic perennial ryegrass (Lolium perenne) leaves collected from a golf course in north-central Indiana in August 1999. Gray leaf spot is an emerging threat to stands of perennial ryegrass in the mid-Atlantic and Midwestern United States (1). Symptoms were first evident in taller (6 cm) mown, rough areas surrounding golf course fairways. Field symptoms included diffuse patches (1 to 4 m in diameter) of thin, yellow-tan turf. Within larger affected areas, some of the turf was dead and matted. Close inspection revealed the presence of typical tan-gray lesions with brown margins and fish hook-like distortion of infected leaf blade tips. Incubation of affected turf in a saturated environment at 23°C for 16 h resulted in production of numerous three-celled, pear-shaped conidia characteristic of those produced by P. grisea. A pure culture of the isolate was grown on V8-juice agar in darkness at 29°C. After 10 days, the culture was exposed to continuous light for 4 days at 23°C to induce sporulation. Conidia were washed from the colony surface with sterile distilled water. Two-week-old perennial ryegrass plants in 8-cm-diameter pots were inoculated with the conidial suspension. Typical gray leaf spot symptoms resulted after incubation of inoculated plants at 27°C for 72 h in a saturated atmosphere. Uninoculated control plants exposed to the same environmental conditions remained healthy. This is the first report of gray leaf spot on perennial ryegrass in Indiana. Reference: (1) P. J. Landschoot and B. F. Hoyland. Plant Dis. 76:1280, 1992.

First Report of Goss's Bacterial Wilt and Leaf Blight on Corn Caused by Clavibacter michiganensis subsp. nebraskensis in Indiana

In August of 2008, leaves of hybrid corn (Zea mays L.) and popcorn from Pulaski and Jasper counties in northwest Indiana were submitted to the Purdue Plant and Pest Diagnostic Lab with symptoms characteristic of a bacterial disease. Symptomatic leaves had large, tan-to-gray necrotic lesions with dark freckling present within the lesions. Shiny bacterial exudate was present on the surface of many of the lesions. Microscopic observation revealed no fungal structures within the lesions, and bacterial streaming was observed from the cut edge of symptomatic tissue under ×100 magnification with phase contrast. A commercially available ELISA test (Agdia Inc., Elkhart, IN) determined that samples were negative for Pantoea stewartii, the causal agent of Stewarts bacterial leaf blight and wilt. A bacterial suspension was prepared from symptomatic tissue and streaked onto Kings B medium and subcultured on semiselective CNS medium (1,2). Axenic, peach-colored colonies present on the CNS medium tested gram positive with a KOH test. Analysis of fatty acid methyl esters (MIDI Inc, Newark, DE) indicated that the strain was very similar (0.611) to Clavibacter michiganensis. Amplification of the 500-bp 16S rRNA region of the bacterial gene and subsequent BLAST alignments of the resulting sequence indicated a 99% match for C. michiganensis subsp. nebraskensis (GenBank Accession Nos. AM410697 and U09763; D16S2 gene bacterial library, version 2.10; MIDI Inc,). Kochs postulates were used to confirm pathogenicity of the isolated bacteria on corn inbred B73. Eighteen plants were mechanically inoculated at growth stage V1 to V2 with a bacterial suspension of approximately 1 × 108 CFU/ml prepared from cultures grown on CNS for 10 days at 28°C (2). Inoculum was rubbed onto leaves dusted with Carborundum and 0.1 ml of the bacterial suspension was injected into stems with a hypodermic needle. Nine control plants were inoculated with sterile water. Plants were kept at greenhouse conditions (24°C) with supplemental 400W high-pressure sodium light. Within 5 to 8 days, leaves and stems of all 18 inoculated plants developed water-soaked, necrotic lesions. No symptoms were observed in control plants. Bacteria were reisolated from symptomatic plants on CNS medium as described above, and gram-positive colonies were obtained. Reisolated strains were identical to C. michiganensis subsp. nebraskensis by D16S2 DNA sequence analysis, confirming the causal agent of the disease. Disease incidence in affected fields ranged from 20 to 60% and significant yield loss was reported. This confirmation is of regulatory importance because of potential export restrictions of Indiana-grown seed corn and popcorn to select countries. To our knowledge, this is the first report of Gosss bacterial wilt and leaf blight on corn in Indiana. References: (1) D. C. Gross and A. K. Vidaver. Phytopathology 69:82, 1979. (2) L. M. Shepherd. M.S. thesis. Iowa State University. Ames, 1999.

A Survey of Phytophthora spp. in Midwest Nurseries, Greenhouses, and Landscapes

A survey of nurseries, greenhouses, and landscapes was conducted from 2006 to 2008 in order to determine the prevalence and diversity of Phytophthora spp. From sites in Iowa, Michigan, Ohio, and, predominantly, Indiana, 121 Phytophthora isolates were obtained from 1,657 host samples spanning 32 host genera. Based on sequence of the internal transcribed spacer (ITS) region of the ribosomal DNA, 11 Phytophthora spp. and two hybrid species were identified. A majority of the isolates were P. citricola (35.9%) or P. citrophthora (27.4%). Six isolates were confirmed as hybrids (four of P. cactorum × hedraiandra and two of P. nicotianae × cactorum) by cloning and sequencing the ITS region. Three P. cactorum × hedraiandra isolates were obtained from the same site, from three Rhododendron spp., which are known hosts to the parental species. The fourth isolate, however, was recovered out of a different location in a Dicentra sp., which is not a known host to either parental species, suggesting an expansion of host range of the hybrid isolate as compared with either parental species.

What's A Picture Worth? The Use of Modern Telecommunications in Diagnosing Plant Diseases

Plant disease diagnosis is an art as well as a science (8). The diagnostic process (i.e., the recognition of symptoms and signs) is inherently visual and requires intuitive judgment as well as the use of scientific methods. Photographic images of plant disease symptoms and signs are used extensively to enhance descriptions of plant disease and are invaluable in research, teaching, extension, and diagnostics. Modern telecommunications systems permit individuals to share high-resolution digital images among multiple locations within seconds. Plant pathologists can thus incorporate these digital imaging and digital image transfer tools into the process of plant disease diagnosis. As with the introduction of most new tools, techniques, or changes in protocol, there is controversy about how this tool should be used (5,12,13,16–18,20,21). The objective of this paper is to discuss the benefits, risks, and limitations associated with digitally assisted diagnosis (DAD) of plant diseases as well as the current status of this practice in the United States. Case scenarios included in this paper provide evidence that the transfer of high-quality digital images has been a successful tool in assisting long-distance consultation and the accurate diagnosis of plant problems. Examples of inappropriate uses of image transfer are also discussed.

Potential Damage to Sensitive Landscape Plants from Wood Chips of Aminocyclopyrachlor Damaged Trees

Abstract Applications of aminocyclopyrachlor in 2011 to turf resulted in brown and twisted shoots, leaves, and needles; shoot dieback; and in some cases, death of trees and ornamental plants adjacent to treated turf areas. Our research objective was to determine if a sensitive plant could be injured from wood chips (mulch) obtained from aminocyclopyrachlor-damaged trees, and to quantify movement of aminocyclopyrachlor from contaminated wood chips into soil and its subsequent uptake by roots into landscape plant tissues. Tomatoes were grown under greenhouse conditions and mulched with chipped tree branches collected from honey locust and Norway spruce damaged 12 mo previously by aminocyclopyrachlor. Analysis of tomato tissue for aminocyclopyrachlor residues 32 d after mulching found aminocyclopyrachlor in all mulched tomato plants, which was consistent with observations of epinasty on tomato leaflets. Aminocyclopyrachlor residues ranged from 0.5 to 8.0 ppb in tomato plants while chipped tree branches contained 1.7 to 14.7 ppb. Aminocyclopyrachlor residues in the potting soil below the mulch ranged from below the quantifiable limit to 0.63 ppb, indicating that aminocyclopyrachlor can leach from wood chips into soil, causing plant injury. These results indicate that trees damaged by aminocyclopyrachlor should not be chipped and used for mulch or as an ingredient in compost. Nomenclature: Aminocyclopyrachlor; honey locust; Gleditsia triacanthos L.; Norway spruce; Picea abies (L.) Karst.; tomato; Solanum lycopersicum L. Resumen En 2011, aplicaciones de aminocyclopyrachlor en céspedes resultó en tejido aéreo y hojas café y enrolladas, muerte del tejido aéreo, y en algunos casos, la muerte de árboles y plantas ornamentales adyacentes a las áreas tratadas en el césped. El objetivo de nuestra investigación fue determinar si una planta sensible podría ser dañada por una cobertura de chips de madera (mulch) que se obtuvo a partir de árboles dañados con aminocyclopyrachlor, y cuantificar el movimiento de aminocyclopyrachlor desde chips de madera hacia el suelo y su subsiguiente absorción por las raíces de plantas presentes en el paisaje. Plantas de tomate fueron crecidas en invernadero y con cobertura de chips hecha a partir de ramas colectadas de árboles de Gleditsia triacanthos y Picea abies dañados 12 meses antes con aminocyclopyrachlor. El análisis de aminocyclopyrachlor en el tejido de tomate 32 d después de poner la cobertura encontró aminocyclopyrachlor en todas las plantas de tomate con cobertura, lo cual fue consistente con observaciones de epinastia en las hojas de tomate. Los residuos de aminocyclopyrachlor variaron entre 0.5 y 8.0 ppb en plantas de tomate mientras que en las ramas de los árboles fue de 1.7 a 14.7 ppb. Los residuos de aminocyclopyrachlor en la mezcla de suelo de las macetas debajo de la cobertura varió desde niveles por debajo del límite de cuantificación a 0.63 ppb, indicando que aminocyclopyrachlor puede lixiviarse desde los chips de madera al suelo, causando daño en las plantas. Estos resultados indican que árboles dañados con aminocyclopyrachlor no deberían ser usados para producir coberturas o como ingrediente en compost.