Interpretive Summaries

Sterility Mosaic Disease—the “Green Plague” of Pigeonpea: Advances in Understanding the Etiology, Transmission and Control of a Major Virus Disease. A. Teifion Jones, Scottish Crop Research Institute (SCRI), Scotland, UK; P. Lava Kumar, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, India, and SCRI; K. B. Saxena, ICRISAT; N. K. Kulkarni, University of Agriculture Sciences (UAS), Bangalore, India, and ICRISAT; V. Muniyappa, University of Agriculture Sciences, Bangalore, India; and Farid Waliyar, ICRISAT. Plant Dis. D-2004-0308-01F.

Pigeonpea (Cajanus cajan), is a grain legume that is a very important subsistence crop in marginal farming systems adopted by millions of smallholder farmers in the Indian subcontinent. It is grown for its seed for human consumption and for income generation by trading surpluses in local and commercial markets, but is widely used for diverse purposes, including as animal fodder and for soil conservation. Sterility mosaic (SMD) is the most damaging disease of pigeonpea endemic in the Indian subcontinent. It causes yield losses of >US$300 million per annum in India and Nepal alone. SMD-affected plants show severe stunting and mosaic symptoms on leaves, with complete or partial cessation of flowering. The SMD causal agent is spread by the arthropod mite vector Aceria cajani (Acari: Eriophyidae). Cultivating SMD-resistant genotypes is the most viable way to manage this serious disease of pigeonpea. Progress in developing broad-based SMD resistant material has been hindered by the lack of knowledge of the causal agent, the absence of diagnostic tools, and factors influencing host-plant resistance. After seven decades of research, vital breakthroughs made on the identification, detection, transmission, and epidemiology of the SMD causal agent, Pigeonpea sterility mosaic virus (PPSMV), are enabling the development of broad-based durable resistant pigeonpea cultivars. These breakthroughs will contribute greatly to sustainable pigeonpea production and enhance the income and livelihood of poor farmers in the semi-arid tropics of the Indian subcontinent.

Distribution and Incidence of Iris yellow spot virus in Colorado and its Relation to Onion Plant Population and Yield. David H. Gent and Howard F. Schwartz, Department of Bioagricultural Sciences and Pest Management, and Rajiv Khosla, Department of Soil and Crop Sciences, Colorado State University, Fort Collins 80523. Plant Dis. D-2004-0225-01R, 2004 (online). Accepted for publication 15 December 2003.

Iris yellow spot virus (IYSV) is transmitted by thrips and has emerged as a potentially devastating and widespread disease of onion in the western United States. In annual surveys in Colorado, IYSV was confirmed in one of 18 fields (5.6%) in 2001, four of 24 (16.7%) in 2002, and 41 of 56 (73.2%) in 2003. IYSV was confirmed on volunteer onions in 2003 at all four locations where IYSV was observed in the onion crop the previous year. The spatial variability of disease incidence, yield, and plant population also were mapped in two fields in 2003 using the global positioning system and a geographic information system. Disease incidence varied among cultivars, plant population, fields, and location in the field. Distinct disease gradients were observed in both fields with susceptible cultivars Teton and Granero, but not in the moderately resistant cv. Sterling. In fields planted to the susceptible cultivars, disease incidence was highest on the field edges and lowest near the field centers. Yield of jumbo market class onions, but not total yield, was negatively correlated with increasing IYSV incidence in cultivar Teton. Colossal market class yield, but not other yield components, was negatively correlated with IYSV incidence in cultivar Sterling. The results of these studies indicate the distribution of IYSV is rapidly expanding in Colorado and is associated with a general reduction in bulb size.

Timing of Preharvest Infection of Pear Fruit by Botrytis cinerea and the Relationship to Postharvest Decay. C. L. Lennox, ARC-Plant Protection Research Institute, Weeds Pathology Unit, Private Bag X5017, Stellenbosch 7599, South Africa, and R. A. Spotts, Oregon State University Mid-Columbia Agricultural Research and Extension Center, 3005 Experiment Station Drive, Hood River 97031. Plant Dis. D-2004-0217-02R, 2004 (online). Accepted for publication 28 November 2003.

Botrytis cinerea causes significant levels of postharvest decay in the winter pear cultivar d’Anjou. The objectives of this study were to determine the timing of B. cinerea infection of pear stems and calyxes in the orchard during the growing season, to investigate the development of gray mold in storage, and to determine whether preharvest levels of B. cinerea in pear stems and calyxes can be used as predictors of gray mold levels observed in storage. Stem tissue showed very low levels of infection by B. cinerea prior to harvest. Little or no stem end gray mold was detected in fruit after 3 months in air storage; however, incidence increased between 6 and 8 months. Calyx end gray mold was detected at low levels in fruit stored for up to 8 months. Calyxes were susceptible to infection soon after full bloom; however, inoculation of calyxes in April or May did not result in higher levels of calyx end gray mold in storage. We concluded that preharvest level of calyx infection is a poor predictor of calyx end gray mold in storage. In addition, application of benomyl in the orchard reduced the level of B. cinerea in blossoms but had no effect on levels of calyx end gray mold of fruit in storage. Packing and shipping fruit within 3 to 6 months of harvest may reduce losses due to gray mold.

Incidence of Postharvest Decay of ‘d’Anjou’ Pear and Control with a Thiabendazole Drench. Cheryl L. Lennox, ARC-Plant Protection Research Institute, Weeds Pathology Unit, Private Bag X5017, Stellen­bosch 7599, South Africa; Robert A. Spotts, Oregon State University Mid-Columbia Agricultural Research and Extension Center, Hood River 97031; and Mardé Booyse, ARC-Biometry Unit, Private Bag X5013, Stellenbosch 7599, South Africa. Plant Dis. D-2004-0217-01R, 2004 (online). Accepted for publication 1 December 2003.

Postharvest decay causes economic losses in the pear industry. Knowing which decay is the most prevalent enables growers and packinghouses to implement effective control strategies. In this study, decay in air and controlled-atmosphere (CA)-stored ‘d’Anjou’ pear fruit was investigated, as was the effect of a prestorage thiabendazole drench. In air storage, decay varied according to the year. Bull’s-eye rot (31.37%) was most prevalent in 1996, whereas gray and blue mold (1996 and 1997), and bull’s-eye rot (1997) were simi­lar. Mucor, Alternaria, and Coprinus rot levels were low. Stem-end gray mold (2.58%) was higher than calyx-end (0.73%) and puncture gray mold (0.61%). Incidence of gray mold (2.26%) was higher than all other decay in nondrenched CA storage, and incidence of other decay types was similar. Incidence of punc­ture gray mold (1.13%) was higher than stem-end gray mold (0.84%), which in turn was higher than calyx-end gray mold (0.36%) in nondrenched CA storage. Incidence of gray mold (1.04%) in CA-stored fruit was reduced by a prestorage thiabendazole drench. Drenching reduced stem-end (0.34%) and puncture gray mold (0.40%) but had no effect on all other decay or the total decay incidence. These results support the current recommendations of a single postharvest appli­cation of thiabendazole to control gray mold in d’Anjou pear fruit.

Conventional and Real-Time PCR-Based Assay for Detecting Pathogenic Alternaria brassicae in Cruci­ferous Seed. Thomas Guillemette, UMR 77 Patholo­gie Végétale, Faculté des Sciences, Angers, France; Béatrice Iacomi-Vasilescu, UMR 77 Pathologie Végétale, France, and USAMV, Department of Plant Protection, Bucharest, Romania; and Philippe Simoneau, UMR 77 Pathologie Végétale, France. Plant Dis. D-2004-0301-01R, 2004 (online). Accepted for publication 19 December 2003.

Alternaria brassicae is an important and widely distributed pathogen of crucifers. This fungus is responsible for the black spot disease that results in serious reductions in crop yields. Genetic control of the pathogen is not possible because most commercial cultivars are susceptible. Consequently, the use of pathogen-free seed is essential to limit the spread and incidence of the disease and also to reduce fungicide applications. Therefore, sanitary certification pro­grams for commercial cruciferous seed include A. brassicae detection. Diagnosis currently is obtained after plating seed on nutritive media, using incubation and morphological characterization of the fungus. This procedure is time-consuming—requiring at least 1 week to obtain a diagnostic result—and often not very accurate due to potential confusion with nonpatho­genic Alternaria spp. Therefore, there is an urgent need to develop alternative diagnostic tools such as molecular techniques based on polymerase chain reac­tion (PCR). In this article, we describe two molecular assays for detecting A. brassicae in cruciferous seed using conventional or real-time PCR. The two meth­ods perform equally well in terms of specificity, sensi­tivity, and speed. However, the real-time PCR assay is better suited for routine detection because no post-amplification manipulations are required. Further­more, this real-time PCR diagnosis method may be readily amenable to automation.

Biocontrol of Postharvest Diseases of Jujube Fruit by Cryptococcus laurentii Combined with a Low Dosage of Fungicides under Different Storage Conditions. Guo Zheng Qin and Shi Ping Tian, Key Laboratory of Photosynthesis and Environmental Mo­lecular Physiology, Institute of Botany, Chinese Acad­emy of Sciences, Xiangshan Nanxincun 20, Haidian District, Beijing 100093, China. Plant Dis. D-2004-0309-01R, 2004 (online). Accepted for publication 21 December 2003.

Jujube fruit is susceptible to postharvest decay caused by various pathogenic fungi. Synthetic chemical fungi­cide is the primary means to control postharvest dis­eases of jujube fruit. However, concerns about public health and the environment and the development of resistant pathogens have increased the search for alter­native methods. In this article, an integrated control strategy against Alternaria alternata (Fr.:Fr.) Keissl. and Monilinia fructicola (G. Wint.) Honey of jujube fruit was obtained by combining Cryptococcus lauren­tii (Kuff.) C. E.Skinner with a low dose of fungicide and controlled-atmosphere (CA) storage. The result showed that the biological control activity of C. laurentii was enhanced significantly when combined with the fungicides imazalil (25 µg a.i./ml) or kresoxim-methyl (50 µg a.i./ml) against both patho­gens under CA storage. C. laurentii was resistant to low rates of fungicides and was adapted to CA stor­age. C. laurentii grew rapidly in the wounds of jujube fruit under all storage conditions, including CA stor­age at 0°C, regardless of whether the fungicides were used or not. This indicates that C. laurentii has great commercial potential as a biological control product.

Dose Curves of Disinfestants Applied to Plant Production Surfaces to Control Botrytis cinerea. W. E. Copes, USDA/ARS Small Fruit Experiment Station, Poplarville, MS 39470. Plant Dis. D-2004-0223-01R, 2004 (online). Accepted for publication 22 December 2003.

Percent spore mortality of the gray mold fungus (Botrytis cinerea) was compared in response to multiple concentrations of six disinfestants on various materials (pine lumber [natural, pressure-treated, exterior latex-painted], polyethylene [ground fabric, pot plastic], and metal [galvanized, stainless steel]). The spore mortality responses were used to calculate predictive lethal dose ranges. The predictive lethal doses of three commercially available disinfestants (hydrogen dioxide, quaternary ammonium chloride, sodium hypochlorite) were additionally compared with concentrations that resulted in zero growth of B. cinerea for the same substrates. Results show that the material being disinfested affects the concentration of a disinfestant needed to rid the surface of B. cinerea spores. In general, higher rates were required on natural and pressure-treated pine lumber, and lower rates were required on latex-painted pine lumber and stainless steel. The rates presented are to disinfest only clean surfaces of the specific fungus tested. This research increases our knowledge about the selectivity of disinfestant rates.

Engineered Resistance Against Papaya ringspot virus in Venezuelan Transgenic Papayas. Gustavo Fermin, Valentina Inglessis, Cesar Garboza, Sairo Rangel, and Manuel Dagert, Department of Biology, Universidad de Los Andes, Mérida, Venezuela; and Dennis Gonsalves, USDA - Pacific West Area, Pacific Basin Agricultural Research Center, Hilo, HI 96720. Plant Dis. D-2004-0303-01R, 2004 (online). Accepted for publication 24 December 2003.

Papaya ringspot virus causes the most important virus disease of papaya worldwide, including Venezuela. In a technology transfer program between Venezuela and Cornell University, genetically engineered papaya that express the coat protein gene of Papaya ringspot virus was developed and found to be resistant to Papaya ringspot virus isolates from Venezuela, Hawaii, and Thailand. Besides being the first virus-resistant genetically engineered papaya produced for Venezuela, the papaya is of the horticultural type that is preferred in Venezuela. This transgenic papaya will be used for further tests in Venezuela and could provide a practical solution to the Papaya ringspot virus problem in Venezuela.

Assessment of the Potential Year-Round Establish­ment of Soybean Rust Throughout the World. S. Pivonia and X. B. Yang, Iowa State University, De­partment of Plant Pathology, Ames 50011. Plant Dis. D-2004-0301-03R, 2004 (online). Accepted for publi­cation 2 January 2004.

Soybean rust has occurred in eastern Asia and Austra­lia for decades. During the past few years, the fungus had spread into new areas in Africa and South Amer­ica. A modeling approach, taking into account the fungus temperature and moisture constraints for repro­duction, was applied to study the potential year-round persistence of soybean rust. The data is important to assess (i) the potential threat of soybean rust in new regions and (ii) the potential dissemination into soy­bean production regions from an overwintering area. Our study shows that, globally, soybean production regions can be divided into two types: type 1 areas, where the disease can survive year-round on suitable hosts, and type 2 areas, where disease occurrence de­pends on an external source of inoculum after long-distance dispersal from a source area. Most regions where soybean rust is known to occur are in areas of type 1. After soybean rust enters the United States, the fungus is likely to overwinter in parts of Florida and southern Texas. Major soybean production regions in the United States and central Argentina are, like Cen­tral China, type 2 areas. Further study is needed to determine the potential build-up of soybean rust in the south and the establishment of a northward spore path­way during a soybean growing season.

A Computer Program to Improve the Efficiency and Accuracy of Postulating Race-Specific Resistance Genes. Yeshi A. Wamishe, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; Kevin C. Thompson, Agricultural Statistics Laboratory, University of Arkansas, Fayetteville 72701; and Eugene A. Milus, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. D-2004-0311-01S, 2004 (online). Accepted for publication 5 January 2004.

Race-specific resistance genes that are expressed in the seedling stage have been used to protect wheat from leaf rust caused by Puccinia triticina. Combinations of several genes provide better protection than one or a few genes. Knowing which resistance genes are in particular wheat lines (cultivars and breeding lines) would allow breeders to choose parents for developing new cultivars with particular combinations of resistance genes. The objective of this study was to develop a computer program to facilitate identification of race-specific resistance genes in wheat lines. For each line, step 1 of the program used data from P. triticina races that attacked the line, and these data definitively excluded genes that cannot be present in the line. Step 2 of the program utilized data from races that did not attack the line and produced a concise table of data that was then examined visually to determine which resistance genes were present and which may be present in the line. This program would be especially useful for identifying a large number of resistance genes in a large number of lines with a large number of races and also could be adapted to host–pathogen systems other than leaf rust of wheat.