Interpretive Summaries

Survival of Tilletia indica Teliospores in Different Soils. M. R. Bonde, D. K. Berner, S. E. Nester, and G. L. Peterson, USDA-ARS, Foreign Disease-Weed Science Research Unit, Fort Detrick, MD 21702-5023; M. W. Olsen, Department of Plant Pathology, University of Arizona, P.O. Box 210036, Tucson 85721-0036; B. M. Cunfer, University of Georgia, Griffin 30223-1797; and T. Sim, Kansas Department of Agriculture, Topeka 66619-0282. Plant Dis. D-2004-0202-02R, 2004 (online). Accepted for publication 24 November 2003.

Karnal bunt of wheat, caused by a fungal pathogen discovered for the first time in the United States in 1996, is considered by many people to be a minor disease, generally causing, at most, only small reductions in grain quality and yield. Because many wheat-importing countries do not accept grain with even a trace of the disease, Karnal bunt is of concern to the United States. In order to gain information on the ability of the pathogen to spread and become established in new areas, a study was undertaken to compare its survival in different soils. Propagules of the pathogen in soil were enclosed in polyester-mesh bags and placed in soil within PVC pipes. The pipes were open at the ends to allow contact with the outside environment, but covered with a series of screens to prevent any possibility of pathogen escape. The pipes were buried in field plots in four states. At each location, temperature was monitored in the pipes. Identical sets of bags of infested soil were kept in the laboratory. Over 3 years, the pathogen was tested to determine numbers of propagules still alive. Comparison of results for the various soils showed that the fungus did not survive well in some soils. The determination of the reason may make it possible to eliminate Karnal bunt from infested wheat fields.

Mycelial Compatibility Grouping and Aggressiveness of Sclerotinia sclerotiorum. L. S. Kull and W. L. Pedersen, Department of Crop Sciences, University of Illinois, Urbana 61801; D. Palmquist, United States Department of Agriculture–Agricultural Research Service (USDA-ARS), MWA Biometrician, Peoria, IL; and G. L. Hartman, USDA-ARS and Department of Crop Sciences, National Soybean Research Center, University of Illinois, Urbana. Plant Dis. D-2004-0120-02R, 2004 (online). Accepted for publication 25 October 2003.

On soybean, Sclerotinia stem rot is an important yield-reducing disease in the United States. Isolates of the fungus that cause this disease vary in several ways, including how fast they can colonize soybean tissue and what asexual group they belong to. In this study, isolates from two fields in Illinois and from other locations were compared. Among 299 isolates tested, 42 asexual groups were identified. Experiments testing isolate aggressiveness indicated a large variation among isolates. A soybean cultivar–isolate interaction was not detected, but resistant and susceptible cultivars performed similarly when inoculated with either less-aggressive or highly aggressive isolates. This study is important in documenting how different isolates from various geographic regions group together based on several attributes, and may be useful to other soybean pathologists, mycologists, and soybean breeders who have an interest in soybean fungal pathogens. Pathogen population structure and variability in isolate aggressiveness may be important considerations in disease-management systems.

Identification of Fungi and Fungal Pathogens Associated with Hypolixus haerens and Decayed and Cankered Stems of Amaranthus hybridus. J. T. Blodgett, former Post-Doctoral Fellow, and W. J. Swart, Professor, Department of Plant Science, and S. vdM. Louw, Professor, Department of Zoology & Entomology, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa. Current address of J. T. Blodgett: USDA-Forest Service, Forest Health Management, 1730 Samco Rd., Rapid City, SD 57702. Plant Dis. D-2004-0127-02R, 2004 (online). Accepted for publication 29 October 2003.

Smooth amaranthus (Amaranthus hybridus) is a nutritious leafy vegetable well suited for cultivation in semiarid regions throughout the world. Discoloration, cankers, and decay in branches, stems, and root collars of smooth amaranthus were observed in Bloemfontein, South Africa. Examination of symptomatic stems revealed larval galleries of the pigweed weevil (Hypolixus haerens). Therefore, we identified the most common fungal species associated with this disease and the associated pigweed weevils. The objectives of our study were to test if the associated fungi can cause the stem canker disease observed in the field, and to determine if the adult pigweed weevil might transport the disease-causing fungi. Three of the seven most common fungal species produced cankers following inoculation, with Fusarium sambucinum and F. oxysporum being the most aggressive. Results suggest that pigweed weevils might be vectors of the Fusarium disease. There is significant potential for yield loss associated with this disease. The identification of this insect–fungal association and the pathogens involved in disease sets the stage for further research on the management of this important insect–fungal relationship.

Detection and Confirmation of Potato mop-top virus in Potatoes Produced in the United States and Canada. H. Xu, T.-L. DeHaan, and S. H. De Boer, Canadian Food Inspection Agency, Centre for Animal and Plant Health, 93 Mount Edward Road, Charlottetown, PEI, Canada, C1A 5T1. Plant Dis. D-2004-0120-01R, 2004 (online). Accepted for publication 21 November 2003.

Potato mop-top virus (PMTV, a pomovirus) is a highly destructive and harmful pathogen to potato because it can cause significant tuber quality damage by causing tuber necrosis. This virus was detected in commercial potatoes grown in the United States and Canada by a reverse transcription–polymerase chain reaction (RT-PCR) targeting the coat protein (CP) gene in RNA3. Out of 3,221 lots of seed and ware potatoes that were tested in a survey conducted in 2001 and 2002, 4.3% were positive for PMTV. The reliability of the survey results was confirmed by re-extraction of selected samples and additional RT-PCR tests using two primer sets targeting gene segments in RNA2 and RNA3. PCR products generated from RNA2 and RNA3 were identified by the analysis of fragment length polymorphisms. PMTV was further identified by enzyme-linked immunosorbent assay, bioassay on Nicotiana debneyi, and transmission electron microscopy. Sequence of a portion of the coat protein gene revealed near 100% identity among isolates from the United States and Canada and >97% homology of the North American isolates with European isolates.

Isolation and Partial Nucleic Acid Characterization of a New Isolate of Potato virus V with Distinct Biological and Serological Properties. P. J. Shiel, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83844; L. Miller and S. A. Slack, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; and P. H. Berger, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83844. Plant Dis. D-2004-0116-01R, 2004 (online). Accepted for publication 12 November 2003.

Breeding of potatoes for better quality and disease resistance is greatly facilitated by incorporation of exotic potato species and land races into the breeding programs. During screening for potato pathogens in an exotic potato line ‘Papa Amarillo’ (Yellow potato), we found plants that showed viruslike symptoms even though none of the known potato viruses were detected using serological assays. Several lines of analysis were used to determine that a new strain of Potato virus V (PVV) was present, and this new strain can escape detection with routine assays. This isolate has been characterized, and it is now possible to develop new antibody and nucleic acid probes to detect it in the future. Our work shows that viruses not previously described, often with distinct biological and serological characteristics, can still be found and can potentially escape detection.

Sensitivity of Monilinia fructicola from Stone Fruit to Thiophanate-Methyl, Iprodione, and Tebuconazole. Michael A. Yoshimura, Biological Sciences Department, California Polytechnic State University, San Luis Obispo 93407; and Yong Luo, Zhonghua Ma, and Themis J. Michailides, Department of Plant Pathology, University of California-Davis, Kearney Agricultural Center, Parlier 93648. Plant Dis. D-2004-0127-01R, 2004 (online). Accepted for publication 2 December 2003.

Brown rot, caused by Monilinia fructicola (G. Wint.) Honey, is the most destructive fungal disease causing significant losses in stone fruit production in California. Application of fungicides during the growing season is a major control strategy in stone fruit orchards. Frequent use of specific fungicides can lead to the development of resistance by fungi and the loss of efficacy of specific chemicals. Resistance of M. fructicola to the benzimidazole fungicide benomyl was first reported in California in 1977 but levels of resistance to benomyl have been relatively low in California. Iprodione has been used in the United States for about two decades, but resistance to iprodione in M. fructicola has not yet been reported in the field. The demethylation-inhibiting fungicides also have been used widely in the last several years, and no resistance to them in M. fructicola has been found. Sensitivity in Monilinia fructicola to these three fungicides was determined by measuring mycelial growth in fungicide-amended media. Frequency of resistance to the benzimidazole thiophanate-methyl was found to be 75% in isolates collected from 1992 to 1998 (historic population) and 22% in isolates collected in 2002 (current population). Three groups having distinct ranges of values for 50% effective concentration (EC(50)) to thiophanate-methyl were identified. Benzimidazole-sensitive (benS) isolates had EC(50) less than 2.0, low-resistant (benL) isolates between 2.0 and 30.0, and high-resistant (benH) isolates greater than 30.0 µg/ml. One (2%) isolate from the historic and three (3%) isolates from the current populations were benH. Inoculation experiments showed that the benS, benL, and benH isolates were equally pathogenic and competitive. The use of thiophanate-methyl at 300 µg a.i./ml (half dosage) and 600 µg a.i./ml (full dosage) effectively reduced the percentage of blighted blossoms (PBB) caused by the benS group but not that caused by the benL and benH groups. The benH isolates caused significantly greater PBB than the benL isolates at either dosage levels. None of the tested isolates of M. fructicola were resistant to either iprodione or tebuconazole.

Simulations of Fungicide Runoff Following Applications for Turfgrass Disease Control. P. Vincelli. Department of Plant Pathology, University of Kentucky, Lexington 40546-0091. Plant Dis. D-2004-0202-01R, 2004 (online). Accepted for publication 5 December 2003.

There has been great interest in recent years in the environmental fate and ecological impact of pesticides applied to turfgrasses. This study was conducted to address the question, “Do fungicide programs commonly used on turfgrasses in Kentucky pose a significant risk to water quality in surface runoff?” Computer simulations were conducted to evaluate potential contamination of runoff from fungicide-treated golf course fairways and lawns. For each simulation, the fungicide concentration predicted in runoff was compared to values of 50% lethal concentration (LC(50) values) known to be hazardous to water flea (Daphnia magna) and rainbow trout, which serve as indicator species for primary and secondary consumers in aquatic ecosystems. Predicted concentrations of four fungicides—metalaxyl, propiconazole, thiophanate methyl, and triadimefon—were well below LC(50) values of the indicator species. However, for all applications of chlorothalonil, predicted concentrations greatly exceeded LC(50) values of both indicator species. Some applications of azoxystrobin, iprodione, and pentachloronitrobenzene also resulted in predicted concentrations that exceeded LC(50) values of at least one indicator species. Although actual amounts of fungicide predicted to move into surface runoff were relatively low, these simulations suggest that turfgrass applications of fungicides that are highly toxic to the indicator species could pose a risk to the health of aquatic ecosystems. These results indicate a need for educational programs directed at turfgrass managers about practices for reducing these risks.

Detection, Isolation, and Pathogenicity of Colletotrichum spp. from Strawberry Petioles. J. C. Mertely and D. E. Legard, University of Florida, Gulf Coast Research and Education Center, Dover 33527. Plant Dis. D-2004-0206-02R, 2004 (online). Accepted for publication 18 November 2003.

Fungi in the genus Colletotrichum cause several strawberry diseases of worldwide importance. Colletotrichum acutatum is particularly destructive, causing root necrosis disease immediately after transplant and anthracnose fruit rot in production fields. This fungus often spreads to new plantings on infected transplants. Unfortunately, colonized or infected plants may appear healthy and often are difficult to detect. Paraquat has been used in the past to kill plant tissues and accelerate the growth of hidden colonists such as C. acutatum. However, paraquat is a toxic herbicide which must be handled carefully. In our study, freezing was investigated as an alternative to paraquat for the detection of Colletotrichum spp. on strawberry petioles. Petioles were frozen for 1 to 2 h in a conventional freezer, thawed, soaked for 1 to 5 min in dilute bleach (to eliminate surface contaminants), rinsed in sterile water (or not rinsed in one treatment), and incubated. After 5 to 7 days, Colletotrichum spp. that grew on the petiole tissue were identified microscopically. The freeze method was as sensitive as the standard paraquat method for the detection of Colletotrichum spp. on symptomless petioles from field-grown plants and for the detection of C. acutatum on inoculated petioles of greenhouse-grown plants. In addition, the freeze method successfully detected C. acutatum and C. dermatium on healthy-appearing transplants from Canadian nurseries. This technique can eliminate a hazardous pesticide from the laboratory, and may have wider use for the detection of other fungi that colonize plants.