Interpretive Summaries

Seedborne Cladosporium variabile and Stemphylium botryosum in Spinach. Pablo Hernandez-Perez, Former Graduate Research Assistant, and Lindsey J. du Toit, Vegetable Seed Pathologist, Washington State University – Northwestern Washington REC, 16650 State Route 536, Mount Vernon, WA 98273-4768. Plant Dis. DOI: 10.1094/PD-90-0137. Accepted for publication 3 September 2005.

Up to 50% of the spinach seed planted in the United States and up to 20% of the world supply of spinach seed is produced in the Pacific Northwest region of the United States. Washington and Oregon are the primary states for spinach seed production in the United States, where Cladosporium and Stemphylium leaf spots, caused by Cladosporium variabile and Stemphylium botryosum, respectively, can cause losses in seed crops during conducive conditions. In addition, these leaf spot diseases can have economic impact on fresh market and processing spinach crops because of additional expenses incurred for hand-sorting symptomatic leaves from healthy leaves. The possibility of infected seed lots serving as a source of inoculum for Cladosporium and Stemphylium leaf spots was investigated. Assays of 77 spinach seed lots produced in the United States, Denmark, the Netherlands, or New Zealand in 2000 to 2003 showed that S. botryosum was present in every lot at a mean incidence of 29.1% per lot. C. variabile, or the morphologically similar species C. macrocarpum, was present in 37 of the 77 lots at a mean incidence of 1.8% per lot. Some seed isolates of S. botryosum and C. variabile proved pathogenic on spinach. Nonpathogenic isolates resembling C. variabile were identified as C. macrocarpum by the absence of torulose aerial hyphae produced by the latter isolates. Further examination demonstrated that differentiation of C. variabile from C. macrocarpum during freeze-blotter seed assays was only possible >5 days after plating the seed, when isolates of C. variabile started forming torulose aerial hyphae from the tips of the conidiophores. Pathogenic isolates of S. botryosum were also detected in each of 12 seed lots stored for up to 11 years at 4.4°C and 60% relative humidity. C. variabile or C. macrocarpum was detected in only 2 of the 11 lots, which had been stored for 3 and 8 years. Component seed assays demonstrated that S. botryosum and C. variabile (or C. macrocarpum) can be internal and external in spinach seed. S. botryosum was detected in 5 to 76% of the embryos of five seed lots, but the two Cladosporium species were only detected in 0 to 1% of the embryos of these lots. This suggests greater potential difficulty at eradicating S. botryosum than C. variabile from infected spinach seed using seed treatments. To our knowledge, this is the first report of S. botryosum on spinach seed produced in Denmark, the Netherlands, and New Zealand. The prevalence of S. botryosum in commercial spinach seed lots might explain the recent first reports of this disease in Arizona, California, Delaware, Maryland, Oregon, and Washington. Development of a polymerase chain reaction molecular seed assay would greatly assist with differentiating S. botryosum and C. variabile from C. macrocarpum and other fungi on or in spinach seed.

Incidence, Spatial Patterns, and Associations Among Viruses in Snap Bean and Alfalfa in New York. Denis A. Shah, Helene R. Dillard, Sudeshna Mazumdar-Leighton, and Dennis Gonsalves, Department of Plant Pathology, and Brian A. Nault, Department of Entomology, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. DOI: 10.1094/PD-90-0203. Accepted for publication 8 September 2005.

In recent years, symptoms assumed to be induced by viruses have been associated with yield losses on a wide scale in processing snap bean in the Midwest and Northeast regions of the United States. Epidemics caused by viruses have coincided with the recent introduction of the soybean aphid, raising the question of how important aphid-transmitted viruses may be in causing disease and yield reductions in snap bean. We surveyed commercial snap bean fields in New York for the aphid-transmitted viruses Alfalfa mosaic virus (AMV), Cucumber mosaic virus (CMV), and the Bean yellow mosaic virus (BYMV)/Clover yellow vein virus (ClYVV) complex in 2002 and 2003, using enzyme-linked immunosorbent assay. Because alfalfa is a possible source of these viruses, we included snap bean fields that were either adjacent to or remote from alfalfa. Viruses were common in both snap bean and alfalfa; mean incidences of plant with AMV, BYMV/ClYVV, and CMV were 41.96, 6.56, and 6.69%, respectively, in alfalfa, and 6.66, 6.38, and 17.20% in snap bean. In 2002, we found more than one virus in 25.9% of snap bean plants that were sampled. The presence of alfalfa next to snap bean did not increase the risk of virus infection in the latter, but incidence of infection by AMV and BYMV/ClYVV was significantly higher in snap bean planted later in the season rather than earlier. In 2002, we found that AMV and CMV tended to occur together in the same snap bean plants.

Fertility and Mating Type Frequency in Indian Isolates of Sclerospora graminicola, the Downy Mildew Pathogen of Pearl Millet. B. Pushpavathi, Department of Plant Pathology, Acharya NG Ranga Agricultural University, Rajendranagar, Hyderabad 500 001, Andhra Pradesh, India; R. P. Thakur, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru 502 324, Andhra Pradesh, India; and K. Chandrashekara Rao, Regional Agricultural Research Station, S. V. Agricultural College Campus, Tirupati 517 502, Andhra Pradesh, India. Plant Dis. DOI: 10.1094/PD-90-0211. Accepted for publication 9 September 2005.

Pearl millet (Pennisetum glaucum) is a staple cereal providing the primary caloric source to millions of people living in semi-arid tropical agroecosystems of Asia and Africa. It is highly productive both as grain and forage, and is grown on about 24 million ha annually, mainly in dry semi-arid tropical environments. Downy mildew, caused by Sclerospora graminicola, is a widely distributed and economically significant disease of pearl millet causing substantial yield loss under favorable weather conditions. The pathogen S. graminicola is an oomycetous fungus that reproduces by both sexual and asexual means and, thus, is genetically highly variable. Because of its highly variable nature, several pathotypes have evolved and resistant hybrid cultivars have been rendered susceptible within a period of 3 to 5 years. To better understand the mechanism of evolution of new virulence in this pathogen, we investigated the fertility status and mating type frequencies of 70 single-zoospore isolates (SZIs) obtained from seven isolates from major pearl millet-growing areas of India. Of the 70 SZIs tested for fertility in terms of oospore production potential, 62 were self-sterile and 8 were self-fertile, indicating the low occurrence of homothallism in the S. graminicola populations. The sexual mating type test of the 70 SZIs revealed the near-equal frequencies of Mat A and Mat B, indicating strong heterothallism in the pathogen populations and, thus, fair chances of producing new genetic recombinants through sexual reproduction. The results indicate that the S. graminicola population has a dynamic genetic structure with great potential for producing new virulence that could match any new resistance gene in a pearl millet cultivar and render it susceptible. Implications of these results in downy mildew resistance breeding program in pearl millet are discussed.

Genetic Analysis of Partial Resistance to Powdery Mildew in Bread Wheat Line Saar. M. Lillemo and H. Skinnes, Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, P.O. Box 5003, N-1432 Ås, Norway; R. P. Singh and M. van Ginkel, International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, 06600 Mexico D.F., Mexico. Plant Dis. DOI: 10.1094/PD-90-0225. Accepted for publication 18 September 2005.

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important disease on bread wheat in many countries. In this paper, we present a genetic study of powdery mildew resistance in the CIMMYT bread wheat line Saar, which has exhibited a good level of partial resistance in field trials across Europe, Asia, and South America. Saar was crossed with a susceptible line, Avocet-YrA, and 114 random inbred lines were field tested at two locations in Norway. The results show that the partial resistance to powdery mildew in Saar is controlled by at least three genes. This line represents a new source of resistance that is different from those previously characterized, and is expected to be durable due to its quantitative inheritance. It also possesses good adult plant resistance to leaf rust and stripe rust based on the Lr34/Yr18 gene complex and is thus a highly valuable source of disease resistance in wheat breeding.

Host Resistance to Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus and Virus Sequence Diversity and Frequency in California. Ryan J. Hayes, William M. Wintermantel, Patricia A. Nicely, and Edward J. Ryder, United States Department of Agriculture, Agricultural Research Service, Crop Improvement and Protection Unit, 1636 E. Alisal St., Salinas, CA 93905. Plant Dis. DOI: 10.1094/PD-90-0233. Accepted for publication 29 September 2005.

Big vein (BV) is an economically damaging disease of lettuce (Lactuca sativa) caused by Mirafiori lettuce big-vein virus (MLBVV). It is spread between plants by the soilborne fungus Olpidium brassicae. Lettuce big-vein associated virus (LBVaV) is an additional virus commonly found in symptomatic plants, but no causal relationship has been demonstrated for LBVaV. Although BV is a perennial problem in the United States, the extent of MLBVV and LBVaV infection in California is unknown. Partially resistant lettuce cultivars have been developed through plant breeding that have lower percentages of symptomatic plants and can reduce the damage caused by BV compared with susceptible cultivars. A wild relative of lettuce, Lactuca virosa, was identified as completely resistant to BV. This wild plant species has not been tested for infection by MLBVV or LBVaV. Several tools are needed to improve the disease management and the process for breeding new lettuce cultivars with higher levels of resistance. These include (i) determining the frequency of virus infection in California field-grown lettuce, (ii) determining the frequency of MLBVV and LBVaV infection in greenhouse-inoculated lettuce cultivars and wild relatives of lettuce, and (iii) determining the deoxyribonucleic acid (DNA) sequence diversity of MLBVV and LBVaV isolates from California. Lettuce cultivars Great Lakes 65, Pavane, Margarita, and L. virosa accession IVT280 were evaluated for BV disease resistance and virus infection in greenhouse trials. Additional lettuce samples were collected from field sites in California, classified for symptom severity, and evaluated for virus infection. Reverse transcription–polymerase chain reaction was used to determine whether plants were infected with MLBVV and LBVaV, and DNA sequencing was used to determine the genetic relatedness among isolates of each virus. Big vein symptom expression in California production areas was dependent on infection by MLBVV/LBVaV, but not by LBVaV alone, and isolates of both viruses were closely related to those found in Europe and Japan. This result is consistent with the previous finding that MLBVV is the virus causing BV. Margarita and Pavane were determined to be partially resistant to BV; however, MLBVV infection was found in asymptomatic plants of these cultivars. This indicates that partially resistant lettuce cultivars can be symptomless carriers. L. virosa IVT280 remained symptomless and virus free, suggesting it is a source of apparent immunity to MLBVV and LBVaV. This trait makes L. virosa accession IVT 280 useful for development of new lettuce cultivars.

Regional Development of Orchardgrass Choke and Estimation of Seed Yield Loss. W. F. Pfender and S. C. Alderman, United Stated Department of Agriculture–Agricultural Research Service National Forage Seed Production Research Center, Corvallis, OR 97331. Plant Dis. DOI: 10.1094/PD-90-0240. Accepted for publication 6 September 2005.

Choke, a disease that interferes with production of seed by orchardgrass, recently was introduced into the Pacific Northwest grass seed-production region. The rate of spread of the disease, and its economic impact, have not been reported previously for U.S. orchardgrass seed production. We conducted a survey for choke disease in orchardgrass seed fields in the Willamette Valley, OR, from 1998 to 2003. In 38% of the 99 fields surveyed, there was a significant increase in the disease and, in 3%, there was a significant decrease. Yearly disease increase was as high as 29% of tillers newly infected, and the average yearly increase was 5 to 8%. In 1998, 60% of orchardgrass fields were infested with choke disease and, by 2003, 90% of the fields were infested. Seed yield loss was found to be directly equivalent to disease level. If 10% of the tillers are diseased, the grower can expect a 10% seed yield loss. We found that disease level can be estimated efficiently simply by counting the number of samples positive for choke disease in a field, instead of exhaustive counting of infected tillers. We estimate regional loss to the 2004 orchardgrass seed crop due to choke disease to be approximately $0.8 million.