Interpretive Summaries

Distribution of Potebniamyces pyri in the U.S. Pacific Northwest and Its Association with a Canker and Twig Dieback Disease of Pear Trees. C. L. Xiao and R. J. Boal, Department of Plant Pathology, Washington State University, Tree Fruit Research and Extension Center, 1100 North Western Avenue, Wenatchee 98801. Plant Dis. DOI: 10.1094/PD-89-0920. Accepted for publication 5 May 2005.

The fungus Potebniamyces pyri is the pathogen responsible for Phacidiopycnis rot, a recently reported postharvest fruit rot disease on pears in the United States. We observed that P. pyri was associated with cankers, dead bark, and twig dieback of pear trees in the orchards. However, little is known about whether P. pyri is a canker pathogen on pear trees. In a region-wide survey, we found that P. pyri was present in all pear-producing areas in Oregon and Washington; trees infected by P. pyri based on the presence of viable fruiting bodies of the fungus in pear orchards ranged from 0 to 100%; and pear trees in the Wenatchee area, WA, were heavily infected by the fungus. In a research orchard, 2-year-old twigs of d’Anjou pear trees were wounded using a sterile cork borer with or without spraying with a commercial aerosol tissue-freezing product at the wound sites. Wounds were then inoculated with P. pyri. Canker development was monitored approximately monthly for up to 6 months after inoculation. We found that P. pyri in general did not cause cankers on non-cold-injured, wound-inoculated twigs, but apparently became established on cold-injured, wound-inoculated twigs and caused small cankers. Our results indicate that P. pyri appears to be a weak canker-causing pathogen on pear trees. The information on the geographic distribution of the pathogen should help the pear industry know where control measures may be needed.

The Relationship Between Aphis glycines and Soybean mosaic virus Incidence in Different Pest Management Systems. M. E. Lee Burrows, USDA-ARS Plant, Soil and Nutrition Laboratory, Ithaca, NY 14853; and C. M. Boerboom and J. M. Gaska, Department of Agronomy, and C. R. Grau, Department of Plant Pathology, University of Wisconsin–Madison, Madison 53706. Plant Dis. DOI: 10.1094/PD-89-0926. Accepted for publication 28 April 2005.

The soybean aphid, Aphis glycines, was discovered in North America for the first time in 2000. This is the only aphid that can colonize soybeans (Glycine max) in North America, and the aphid can transmit several important viruses. We investigated whether insecticide and herbicide would alter the transmission of Soybean mosaic virus (SMV) in soybean. This virus is common in the United States and is primarily moved by winged (alate) aphids. We observed very different aphid populations in 2001 and 2002. In 2001, there were five times as many soybean aphids as in 2002. The incidence of SMV increased from 2 to 80% in 18 days in 2001. In 2002, the incidence of SMV increased from 1 to 44% within 21 days. The increase in the quantity of SMV within the field was associated with the arrival of A. glycines alates in pan traps in both years. In 2001, insecticide had no effect on SMV, whereas in 2002 insecticide application was associated with lower frequency of SMV. The herbicides glyphosate and imazamox (Roundup and Raptor) did not alter the frequency of SMV-infected soybean plants. Insecticide and herbicide applications did not significantly alter seed quality. Insecticide did reduce colonizing aphid populations, but in 2001, a year of high aphid abundance and SMV, insecticide did not produce a yield benefit in our study. A yield benefit associated with insecticide application was observed in 2002. Herbicides did not alter colonizing or winged aphid populations in the soybean canopy and did not significantly alter the incidence of SMV.

Virulence of Oat Crown Rust in Mexico. K. J. Leonard, U.S. Department of Agriculture – Agricultural Research Service, Cereal Disease Laboratory, University of Minnesota, St. Paul 55108; J. Huerta-Espino, Campo Experimental Valle de Mexico – INIFAP, Apdo. Postal 10, 56230, Chapingo, Edo. de Mexico; and J. J. Salmeron, Campo Experimental Sierra de Chihuahua, Apado Postal 554, Cd. Cuauhtemoc, C.P. 31500, Chihuahua, Mexico. Plant Dis. DOI: 10.1094/PD-89-0941. Accepted for publication 27 April 2005.

Crown rust is the most damaging disease of oat in North America. Oat production in the southwestern United States may be vulnerable to spread of crown rust from adjacent areas of Mexico where oat is grown primarily as a forage crop. Likewise, new virulent races of crown rust that appear on formerly resistant oat varieties in Texas may spread by windborne spores into Mexico and endanger oat crops there. We surveyed crown rust races in collections of infected oat plants from four regions of Mexico, looking for possible evidence of spread of rust races between the United States and Mexico and between regions within Mexico. We found that races in each of the four regions of Mexico were largely distinct, indicating relatively little spread of rust from west to east across Mexico. Many rust races found in Texas were also found in the adjacent Mexican states of Nuevo Leon and Coahuila, but the most common races in those states differed from the most common races in Texas, indicating that epidemics in Texas and Mexico arise locally and do not depend on cross-border transmission of rust spores. None of the 27 genes for crown rust resistance in oat that we tested will provide long-term protection against crown rust in Mexico. Therefore, Mexican oat breeding programs should incorporate other types of rust resistance that are nonspecific and at least partially effective against all races. Reducing crown rust severity in Mexico will protect oat forage and make dairy farming more profitable.

Pathogenicity of Fungi Associated with the Wheat Crown Rot Complex in Oregon and Washington. Richard W. Smiley, Professor, Jennifer A. Gourlie and Sandra A. Easley, Faculty Research Assistants, and Lisa-Marie Patterson, Senior Faculty Research Assistant, Oregon State University, Columbia Basin Agricultural Research Center, P.O. Box 370, Pendleton 97801. Plant Dis. DOI: 10.1094/PD-89-0949. Accepted for publication 29 April 2005.

Crown rot affects yield of wheat and barley worldwide. Several fungal pathogens causing similar symptoms are associated with the disease. The relative pathogenicity for species in the pathogen complex in the Pacific Northwest had not been defined. We compared the pathogenicity for 178 isolates of Bipolaris sorokiniana, Fusarium avenaceum, F. culmorum, F. pseudograminearum, and Microdochium nivale on winter wheat under greenhouse conditions. Individual isolates were highly variable but, on average, each species reduced plant growth. We also compared the pathogenicity for 24 isolates of B. sorokiniana, F. culmorum, and F. pseudograminearum in four field experiments with spring wheat. Tests were performed over 2 years at two locations in the semiarid wheat belt of eastern Oregon. Results were highly variable over locations and years, particularly when viewed from the perspective of individual isolates for each crown rot species. However, when the effects of isolates were averaged, we found that all three species significantly increased disease incidence and severity over background levels in the noninoculated control treatments. Two species, F. culmorum and F. pseudograminearum, reduced the yield of spring wheat. High variability among isolates requires the use of multiple isolates for experiments with these pathogens.

Suppression of Wheat Growth and Yield by Pratylenchus neglectus in the Pacific Northwest. Richard W. Smiley, Professor, and Ruth G. Whittaker, Jennifer A. Gourlie, and Sandra A. Easley, Faculty Research Assistants, Oregon State University, Columbia Basin Agricultural Research Center, P.O. Box 370, Pendleton 97801. Plant Dis. DOI: 10.1094/PD-89-0958. Accepted for publication 29 April 2005.

Conservation farming systems are becoming more widely practiced in semiarid regions of the Pacific Northwest. Increasing areas of land formerly planted as 2-year rotations of winter wheat and summer fallow are being converted to spring wheat or barley planted annually, often without tillage. Wheat fields planted annually are often infested by high populations of lesion nematodes, Pratylenchus neglectus and P. thornei. Australian wheat cultivars varying in tolerance and resistance to P. neglectus were treated or not treated with aldicarb (Temik 15G) to examine relationships between the nematode and yield of annual no-till wheat. Intolerant cultivars had lower yields than a moderately tolerant cultivar in two rainfed but not in three irrigated experiments. Increasing densities of P. neglectus in soil were more strongly associated with declining growth and yield of intolerant than of moderately tolerant cultivars. Yields for intolerant cultivars were suppressed 8 to 36% by P. neglectus. Aldicarb reduced reproductive rates for P. neglectus and improved wheat growth and yield in irrigated but not rainfed experiments. This demonstration that wheat yields are suppressed by P. neglectus in the Pacific Northwest suggests that breeding wheat for tolerance and resistance could improve productivity in an area where noncereal crops are not profitable.

Temperature and Leaf Wetness Requirements for Pathogen Establishment, Incubation Period, and Sporulation of Phytophthora infestans on Petunia × hybrida. M. C. Becktell, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; M. L. Daughtrey, Long Island Horticultural Research and Extension Center, Cornell University, Riverhead, NY 11901; and W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. DOI: 10.1094/PD-89-0975. Accepted for publication 2 May 2005.

Late blight, which caused the devastating “potato famine” in Ireland in the mid-1800s, is a highly significant, well-studied disease of potato and tomato caused by Phytophthora infestans. Yet virtually nothing is known about late blight disease caused by the same organism on petunia. Recent outbreaks of late blight on petunia plants during greenhouse production triggered our concern that diseased petunia plants might pose a risk to tomato or potato crops. One important question that needed to be answered was whether the environmental requirements for petunia late blight were similar to those for late blight on tomato and potato. We addressed this question by looking at how temperature and moisture affect several aspects of the disease cycle on petunia compared with tomato. Our results showed that the requirements for petunia late blight are indeed very similar to those for tomato and potato late blight. Because petunia and tomato plants often are grown in close proximity in greenhouses during the spring, knowing that similar conditions favor this contagious disease on both plants is important. Now that the environmental requirements for late blight on petunia have been determined, a disease management plan can be devised that will aid flower and vegetable transplant producers as well as their customers.

The Fate of Xylella fastidiosa in Vineyard Weeds and Other Alternate Hosts in California. C. Wistrom and A. H. Purcell, Department of Environmental Science, Policy and Management, University of California, Berkeley 94720-3114. Plant Dis. DOI: 10.1094/PD-89-0994. Accepted for publication 20 April 2005.

Pierce’s disease is caused by Xylella fastidiosa, a bacterium transmitted by sharpshooter leafhoppers and spittlebugs. In response to glassy-winged sharpshooter and Pierce’s disease outbreaks in central California, we assessed the fate of X. fastidiosa in 29 weed species commonly found in California’s San Joaquin Valley. Bacterial populations and movement in greenhouse-grown plants were measured after mechanical and insect inoculation. X. fastidiosa was recovered from 27 of 29 species in greenhouse tests. Sunflower, cocklebur, annual bur-sage, morning glory, horseweed, sacred datura, poison hemlock, and fava bean were most frequently infected. A greater proportion of plants were infected when inoculated by blue-green sharpshooters than by needle inoculation or glassy-winged sharpshooters. X. fastidiosa populations within field-grown plants were consistently at least 10 times lower than populations within plants kept in a greenhouse. Multiplication and systemic movement of X. fastidiosa varied among different plant species and environmental conditions, so weed species in vineyards must be evaluated on an individual basis to determine their potential contribution to Pierce’s disease. Endemic sharpshooter populations and Pierce’s disease in vineyards justify removal of weed hosts as a control measure.

Epidemiology and Management of Petunia and Tomato Late Blight in the Greenhouse. M. C. Becktell, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; M. L. Daughtrey, Department of Plant Pathology, Long Island Horticultural Research and Extension Center, Cornell University, Riverhead, NY 11901; and W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. DOI: 10.1094/PD-89-1000. Accepted for publication 20 May 2005.

Potatoes, tomatoes, and petunias are three well-known crops that serve as hosts to the late blight pathogen, Phytophthora infestans—the pathogen that devastated Irish potato crops in the 1840s. In previous work, we determined that the environmental requirements for petunia late blight are very similar to those for tomato and potato late blight. The next step was to determine whether petunias infected with late blight could be a source of disease to healthy petunias and tomatoes in the greenhouse. We also wanted to know whether watering methods in the greenhouse influenced the development of the disease. Finally, we tested the efficacy of 12 products that included fungicides, bioantagonistic bacteria, and fungi as well as materials that enhance the plant’s natural defense mechanisms. Our results show that late blight–infected petunias can be a source of late blight to healthy petunias and tomatoes in the greenhouse. We also found that disease was more severe on plants that were overhead watered, so that the foliage was wetted, than on plants that were irrigated by directing water to the soil surface. Based on this work, we have identified several cultural control measures that can be used to help prevent and suppress petunia and tomato late blight in the greenhouse: provide adequate airflow, avoid overhead watering, avoid growing petunias and tomatoes in close proximity, scout for disease during cool, wet periods, and dispose of diseased plants away from greenhouse air intake vents. In addition to these cultural control strategies, we have identified five products with potential for assisting in the management of petunia and/or tomato late blight in the greenhouse. These include four fungicides that contain azoxystrobin, dimethomorph/mancozeb, fosetyl-Al, or dipotassium phosphonate/phosphate as their active ingredients and one plant defense activator with acibenzolar-S-methyl as the active ingredient.