Interpretive Summaries

Field Evaluation of Transgenic Papaya Lines Carrying the Coat Protein Gene of Papaya ringspot virus in Taiwan. Huey-Jiunn Bau, Department of Plant Pathology, National Chung Hsing University, Taichung, Taiwan, R. O. C.; Ying-Huey Cheng, Department of Horticulture, Taiwan Agricultural Research Institute, Wufeng, Taiwan, R. O. C.; Tsong-Ann Yu and Jiu-Sherng Yang, Department of Botany, National Chung Hsing University; Pan-Chi Liou, Chi-Hsiung Hsiao, and Chien-Yih Lin, Department of Horticulture, Taiwan Agricultural Research Institute; and Shyi-Dong Yeh, Department of Plant Pathology, National Chung Hsing University. Plant Dis. D-2004-0315-02R, 2004 (online). Accepted for publication 30 December 2003.

Production of papaya in tropical and subtropical areas is limited primarily by susceptibility to aphid-borne Papaya ringspot virus (PRSV). Control by conventional breeding and cultural practices is either in vain or provides only marginal effects. Transgenic approach by introduction of the coat protein (CP) gene of PRSV into transgenic papaya has been proven to be a promising control measure. In Taiwan, four transgenic papaya lines carrying the CP gene of PRSV were evaluated under field conditions for their reaction to PRSV infection and fruit production in 1996 to 1999. None of the transgenic lines showed severe symptoms of PRSV during the test period, whereas control nontransgenic plants were 100% severely infected 3 to 5 months after planting. Some of the transgenic plants showed mild symptoms consisting of confined mottling or chlorotic spots on leaves. The number of transgenic plants with mild symptoms fluctuated according to the season and weather conditions, with a tendency to increase in the winter or rainy season and decrease in the summer. Also, the incidence of the mild symptoms in the third trial increased significantly due to infection by root rot fungi during the rainy season. Interestingly, there was no apparent adverse effect on fruit yield and quality in transgenic plants with mild symptoms. In the first and second experiments, transgenic lines yielded 10.8 to 11.6 and 54.3 to 56.7 times more marketable fruit, respectively, than controls. The results indicate that the CP-gene transgenic papaya lines have a great potential for control of PRSV in Taiwan.

Comparison of Whole-Tissue and Xylem Fluid Collection Techniques to Detect Xylella fastidiosa in Grapevine and Oleander. Blake R. Bextine and Thomas A. Miller, Department of Entomology, University of California, Riverside 92521. Plant Dis. D-2004-0315-03R, 2004 (online). Accepted for publication 14 January 2004.

The bacterium Xylella fastidiosa causes several economically important plant diseases, such as Pierce’s disease of grapevine and oleander leaf scorch. To improve management of these diseases in agronomic and horticultural production systems, detecting the pathogen as early as possible, before visual symptoms develop, is crucial. Two methods of detection, enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) currently are used for routine detection of this pathogen in most diagnostic laboratories across the country. Using both ELISA and PCR as X. fastidiosa detection methods in grapevine and oleander, we compared two sample collection techniques. The traditional technique of whole-tissue maceration was compared with xylem fluid collection. Use of xylem fluid as diagnostic samples improved sensitivity of pathogen detection by ELISA and PCR compared with whole-tissue samples in asymptomatic grapevine, which may allow for earlier detection of X. fastidiosa in California vineyards so that early action can be taken. However, in symptomatic grapevines and all oleander samples, use of xylem fluid as the diagnostic samples did not improve detection by ELISA or PCR compared with the traditional technique of sample preparation.

Species of Pythium from Greenhouses in Pennsylvania Exhibit Resistance to Propamocarb and Mefenoxam. G. W. Moorman, Department of Plant Pathology, The Pennsylvania State University, University Park 16802-4506; and S. H. Kim, Bureau of Plant Industry, Pennsylvania Department of Agriculture, Harrisburg 17110-9408. Plant Dis. D-2004-0315-04R, 2004 (online). Accepted for publication 20 January 2004.

When a plant pathogen is isolated from or observed in a plant, the clinician usually bases the control recommendation on the identity of the pathogen without further analysis. However, it is important to know whether that pathogen is resistant or sensitive to a chemical recommended for protecting plants. Although it is known that some isolates of the root and stem disease-causing organism Pythium are resistant to the commonly used fungicide mefenoxam (Subdue Maxx, Quell) alone, this research is the first to report that isolates of Pythium aphanidermatum, P. irregulare, and P. ultimum from greenhouses are resistant to propamocarb (Banol, Previcure Flex) alone and that there are P. ultimum and P. irregulare isolates resistant to both fungicides. Unfortunately, the sensitivity of an isolate to propamocarb as determined in lab culture plates is not a good predictor of whether propamocarb can protect treated plants or not. Propamocarb protected plants from some isolates that were resistant to the chemical in culture, whereas it did not protect plants from other isolates that were very sensitive to propamocarb in culture. A rapid, simple, and accurate test for predicting propamocarb protection against Pythium spp. is needed.

Identification of Soybean mosaic virus Strains by RT-PCR/RFLP Analysis of Cylindrical Inclusion Coding Region. Yul-Ho Kim, National Institute of Crop Science, RDA, Suwon 441-857 Korea; Ok-Sun Kim, National Seed Management Office, MAF, Suwon 442-400, Korea; Jae-Hwan Roh, Jung-Kyung Moon, and Soo-In Sohn, National Institute of Crop Science; Sang-Chul Lee, Department of Agronomy, Kyungpook National University, Taegu 702-701, Korea; and Jang-Yong Lee, National Institute of Crop Science. Plant Dis. D-2004-0322-02R, 2004 (online). Accepted for publication 21 January 2004.

Soybean mosaic virus (SMV) is the most important viral disease wherever soybean is grown. It causes serious damage and effects on plant growth. Therefore, detection and identification of SMV strains is very important both for soybean cultivation and breeding SMV-resistant cultivars. Nevertheless, the classification of SMV strains mostly depends on conventional methods, which are laborious and time consuming, and uses differential soybean cultivars. A rapid and simple method using reverse-transcriptase polymerase chain reaction/restriction fragment length polymorphism (RT-PCR/RFLP) for identification and differentiation of SMV strains has been developed. In this method, a primer pair amplifying a 1,385-bp fragment of the cylindrical inclusion (CI) coding region at position 4,176 to 5,560 was used for RT-PCR and the RFLP profiles of the RT-PCR products were compared after restriction digestion with RsaI, EcoRI, or AccI restriction endonucleases. These enzymes were chosen based on the nucleotide sequences of SMV strains G2, G5, G5H, G7, and G7H in the CI coding region. These five strains, as well as eight seedborne SMV isolates from local soybean cultivars, could be differentiated by RT-PCR/RFLP analysis. The sensitivity of RT-PCR permitted detection of SMV from plants with necrotic symptoms in which the number of virus particles was too low to be detected by immunological techniques.

Activity of Boscalid, Fenhexamid, Fluazinam, Fludioxonil, and Vinclozolin on Growth of Sclerotinia minor and S. sclerotiorum and Development of Lettuce Drop. M. E. Matheron, Extension Plant Pathologist and Research Scientist, and M. Porchas, Research Specialist, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. D-2004-0412-01R, 2004 (online). Accepted for publication 9 February 2004.

Sclerotinia drop is a major disease of lettuce which is caused by two soilborne fungi, Sclerotinia minor and S. sclerotiorum. Fungicides such as dicloran (Botran), iprodione (Rovral), and vinclozolin (Ronilan) are currently available in the United States to manage this disease. Recently, some new fungicides, including boscalid, fenhexamid, fluazinam, and fludioxonil, have demonstrated efficacy against diseases caused by S. minor and S. sclerotiorum on crops other than lettuce. These studies were conducted to investigate the relative effect of boscalid, fenhexamid, fluazinam, fludioxonil, and vinclozolin on growth of Sclerotinia minor and S. sclerotiorum in agar plate tests as well as control of lettuce drop in the field. In laboratory studies, at a rate of 1.0 µg/ml, all tested fungicides reduced mycelial growth of S. minor and S. sclerotiorum from 87 to 100% and 77 to 100%, respectively. In lettuce plots infested with S. minor, boscalid and fluazinam provided the highest level of disease control, whereas in the presence of S. sclerotiorum, fluazinam, fludioxonil, and vinclozolin were the most efficacious compounds. Boscalid and fluazinam were more effective against lettuce drop caused by S. minor than disease caused by S. sclerotiorum. Boscalid (Endura) is now registered for use on lettuce in the United States.