Interpretive Summaries

Association of Enterobacter cloacae with Rhizome Rot of Edible Ginger in Hawaii. K. A. Nishijima, Pacific Basin Agricultural Research Center (PBARC), USDA-ARS, P.O. Box 4459, Hilo, HI 96720; A. M. Alvarez, University of Hawaii-Manoa, Honolulu 96822; P. R. Hepperly, PBARC, USDA-ARS, Hilo, HI; M. H. Shintaku, University of Hawaii-Hilo, College of Agriculture, Forestry & Natural Resource Management, Hilo 96720; L. M. Keith, PBARC, USDA-ARS, Hilo, HI; D. M. Sato and B. C. Bushe, University of Hawaii-Cooperative Extension Service, Hilo 96720; and J. W. Armstrong and F. T. Zee, PBARC, USDA-ARS, Hilo, HI. Plant Dis. D-2004-1013-01R, 2004. Accepted for publication 17 July 2004.

Edible ginger (Zingiber officinale) is a popular spice crop that is grown in Hawaii primarily for the fresh market, and as such, rhizome quality is important. In our studies, an enteric bacterium identified as Enterobacter cloacae was isolated from decayed as well as healthy ginger rhizomes. Rot symptoms, which usually occurred in the center area (central cylinder) of the rhizome, were characterized by yellowish-brown to brown discolored tissue and firm to spongy texture. In inoculation experiments, ginger strains of the bacterium produced rot symptoms on ginger rhizomes as well as on other hosts such as papaya fruit and onion bulbs. Our studies suggest that E. cloacae can exist in the internal tissue of apparently healthy ginger rhizomes, and under conditions that are favorable for bacterial growth or host susceptibility, including maturity of tissues, rhizome rot may occur. Conditions such as high temperature, high relative humidity, and low oxygen atmospheres may affect the development of decay, and such conditions should be avoided during postharvest handling and storage to ensure good-quality ginger rhizomes. The association of E. cloacae with a rhizome rot of edible ginger is a new finding.

Efficacy of Bioindexing for Graft-Transmissible Citrus Pathogens in Mixed Infections. G. Vidalakis, S. M. Garnsey, J. A. Bash, G. D. Greer, and D. J. Gumpf, Department of Plant Pathology, University of California, Riverside, CA 92521, USA. Plant Dis. D-2004-1004-01R, 2004. Accepted for publication 28 June 2004.

Testing citrus budwood for the presence of graft-transmissible pathogens is important for the maintenance, introduction, and commercial propagation of highly productive citrus. Despite the development of various laboratory diagnostic techniques, biological indexing for citrus pathogens remains the only method available for a number of pathogens and remains a valuable alternative or supplement for pathogens that can be detected by other means. Citrus budwood often is collected from field trees that harbor more than one graft-transmissible pathogen. The effects of multiple infections on the diagnosis of specific pathogens with bioindexing hosts were studied, and their effects on the efficacy of the established bioindexing protocols were evaluated. In addition, alternative indicators were evaluated as bioindexing hosts for some graft-transmissible citrus pathogens. In many cases, the presence of additional pathogens did not affect diagnosis, but evidence of suppression or enhancement of symptoms was noted in certain pathogen–host combinations. Application of this information will help to objectively optimize the use of indicators for biological indexing and minimize the overall cost of pathogen diagnosis.

Detection of Geminiviruses in Sweetpotato by Po­lymerase Chain Reaction. Ruhui Li, Sarbagh Salih, and Suzanne Hurtt, United States Department of Agriculture–Agricultural Research Service, Fruit Laboratory/Plant Germplasm Quarantine Office, Beltsville, MD 20705. Plant Dis. D-2004-0913-02R, 2004 (online). Accepted for publication 2 July 2004.

Several viruses cause important diseases in sweetpotato, and one of them is Sweet potato leaf curl virus (SPLCV), a geminivirus. To prevent introduction of viral pathogens during the importation of germplasm, sweetpotato materials imported into the United States have to be tested by the Plant Germplasm Quarantine Office of the United States Department of Agricul­ture–Agricultural Research Service. The current method to detect geminiviruses is to graft the material onto a susceptible indicator plant, Ipomoea setosa, in summer. The geminiviruses induce leaf curl in the indicator plant 2 to 4 weeks after grafting. To ensure accuracy, this grafting assay has to be repeated. This process is laborious, and takes 3 months. Subsequently, any virus-infected material undergoes meristem tip culture, normally requiring 6 months to a year, to eliminate the virus. To get a virus-free plant, many plantlets generated from meristem tip culture need to be grown out and tested by the grafting assay. Thus, it is necessary to develop a rapid and sensitive virus detection assay suitable for testing large numbers of plants. We have developed a molecular technique to detect the geminiviruses in sweetpotato plantlets and greenhouse-grown plants. This new method is sensi­tive and reliable, and is completed in 2 days instead of 3 months. The method has a broad detection range, and can detect not only SPLCV but also other gemi­niviruses. Moreover, it could be used year round, thereby speeding up dissemination of healthy plant material to the intended users and greatly reducing costs. This method should be of value to other scien­tists interested in detection of geminiviruses.

Suppression of Fusarium Wilt of Watermelon by Soil Amendment with Hairy Vetch. X. G. Zhou, University of Maryland, Lower Eastern Shore Research and Education Center, Salisbury 21801; and K. L. Everts, University of Maryland, Lower Eastern Shore Research and Education Center, Salisbury 21801, with joint appointment with the University of Delaware, Georgetown 19947. Plant Dis. D-2004-1014-02R, 2004 (online). Accepted for publication 17 July 2004.

Fusarium wilt, caused by Fusarium oxysporum f. sp. niveum, is a highly destructive soilborne disease and is a yield-limiting factor in watermelon production worldwide. Few effective, economically feasible, and environmentally benign management options are available for the disease. The objective of this study was to develop a novel soil management approach for Fusarium wilt of watermelon. We evaluated hairy vetch (Vicia villosa Roth) and 12 other organic amendments in the greenhouse, and hairy vetch as a soil amendment in microplots and field condi­tions, for suppression of Fusarium wilt in watermelon and im­pact on the population of F. oxysporum f. sp. niveum in soil. When mixed into a loamy sand soil artificially or naturally infested with race 2 of F. oxysporum f. sp. niveum, pulverized hairy vetch, crab shell, and urea provided the best suppression of Fusarium wilt on watermelon seedlings in greenhouse trials. Hairy vetch amendment in microplots significantly decreased wilt incidence and promoted plant growth. Hairy vetch winter cover crop incorporated into field plots as a green manure and covered with black plastic provided a significant reduction of wilt incidence, a stimulation of plant growth, and an increase in weight of fruit, comparable to improvements achieved by the preplant soil fumigants methyl bromide or 1,3-dichloropropene plus 35% chloropicrin. Hairy vetch also increased the sugar content of watermelon fruit. Significant reductions in the populations of F. oxysporum f. sp. niveum were not observed in hairy vetch-amended soil in microplots and field plots. Incorporating hairy vetch into soil as a green manure may provide an alternative or supplement to cultivar resistance and crop rotation for management of Fusarium wilt of watermelon.

Aspergillus flavus in Soils and Corncobs in South Texas: Im­plications for Management of Aflatoxins in Corn-Cotton Rotations. Ramon Jaime-Garcia and Peter J. Cotty, United States Department of Agriculture–Agricultural Research Service and Division of Plant Pathology and Microbiology, University of Arizona, Tucson 85721. Plant Dis. D-2004-1018-01R, 2004 (online). Accepted for publication 20 July 2004.

Aflatoxins are dangerous toxic chemicals that contaminate many crops. The fungus Aspergillus flavus causes aflatoxin contamination of both cottonseed and corn. Farmers commonly rotate corn and cotton crops in South Texas, where reduced tillage frequently results in long-term residence of corncobs on soil surfaces. A. flavus can grow and survive on corncobs. We studied the potential of corncobs as sources of A. flavus in cotton and corn crops in South Texas from 2001 to 2003 in order to gain insights into potential methods for preventing contamination. The results indicate that corncobs are an important source of crop exposure to A. flavus. Corncobs from the previous sea­son contained, on average, over 192 times more A. flavus propagules than soil from the same field, and 2-year old corncobs still retained 45 times more propagules than soil. The quantity of A. flavus in corncobs decreased with corncob age. The results suggest that aflatoxin management should include prompt harvest and techniques to reduce the period corncobs remain in the field, such as incorporation under the soil.

Detection and Quantification of Fusarium solani f. sp. glycines in Soybean Roots with Real-Time Quantitative Polymerase Chain Reaction. X. Gao, T. A. Jackson, K. N. Lambert, and S. Li, Department of Crop Sciences, University of Illinois at Urbana and Champaign, Urbana 61801-4798; G. L. Hartman, United States Department of Agriculture–Agricultural Research Service, Urbana, IL 61801-4723, and Department of Crop Sciences, University of Illinois at Urbana and Champaign; and T. L. Niblack, Department of Crop Sciences, University of Illinois at Urbana and Champaign. Plant Dis. D-2004-1018-02R, 2004 (online). Accepted for publication 17 August 2004.

Soybean sudden death syndrome (SDS) is a major disease of soybean caused by the fungus Fusarium solani f. sp. glycines. The organism is difficult to detect and measure because it is a slow-growing fungus with variable characteristics. Reliable and fast procedures are important for detection of this soybean pathogen. Procedures for extracting DNA from pure fungal cultures and fresh or dry roots were optimized. A new procedure to test purity of DNA extracts was described. Real-time quantitative polymerase chain reaction (QPCR) assays were developed for both absolute and relative measurement of F. solani f. sp. glycines. The fungus was measured based on detection of a pathogen gene relative to a host plant gene. DNA of F. solani f. sp. glycines was detected in soybean plants both with and without SDS foliar symptoms. The QPCR protocols were specific and sensitive. The relative QPCR assay is reliable if care is taken to avoid reaction inhibition and it may be used to further study the fungus–host interaction in the development of SDS or screen for resistance to the fungus.