Interpretive Summaries

Cultivar and Seedling Susceptibility to Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa and Graft Transmission of the Pathogen. R. S. Sanderlin, Louisiana State University Agricultural Center, Pecan Research-Extension Station, P.O. Box 5519, Shreveport 71135. Plant Dis. DOI: 10.1094/PD-89-0446, 2005 (online). Accepted for publication 2 December 2004.

A bacterium (Xylella fastidiosa) that infects the water carrying tissue of many plants is known to cause a severe leaf scorch disease in the pecan cultivar Cape Fear. A survey of pecan orchards in Louisiana revealed that the bacterium and the disease also occur commonly in numerous of the other commercially grown cultivars. In addition, the disease also was found in some ungrafted trees in orchards. Experimental tests indicated that the bacterium can easily be transmitted through pecan scion wood collected from infected trees. Because all pecan cultivars are clonally propagated, graft-transmission may be a significant source of infection of trees. However, the presence of the disease in ungrafted trees in orchards indicates that there is at least one additional way that the pathogen can infect trees besides through scion wood transmission. Efforts to identify natural means of pecan tree infection by the bacterium are ongoing.

Seedling and Adult Plant Resistance to Powdery Mildew in Chinese Bread Wheat Cultivars and Lines. Z. L. Wang, Institute of Crop Breeding and Cultivation/National Wheat Improvement Center, Chinese Academy of Agricultural Sciences (CAAS), No. 12 Zhongguancun South Street, 100081, Beijing, China and Northwest Sci-Tech University of Agriculture and Forestry, Yangling, 712100, Shaanxi, China; L. H. Li, Institute of Crop Breeding and Cultivation/National Wheat Improvement Center, CAAS; Z. H. He, Institute of Crop Breeding and Cultivation/National Wheat Improvement Center, CAAS and CIMMYT China Office, C/O CAAS, No. 12 Zhongguancun South Street, 100081, Beijing, China; X. Y. Duan and Y. L. Zhou, Institute of Plant Protection, CAAS, No. 2 Yuanmingyuan West Road, 100094, Beijing, China; X. M. Chen, Institute of Crop Breeding and Cultivation/National Wheat Improvement Center, CAAS; M. Lillemo and R. P. Singh, International Maize and Wheat Improvement Center (CIMMYT), Apdo Postal 6-641, 06600, Mexico D.F., Mexico; H. Wang, Northwest Sci-Tech University of Agriculture and Forestry; and X. C. Xia, Institute of Crop Breeding and Cultivation/National Wheat Improvement Center, CAAS. Plant Dis. DOI: 10.1094/PD-89-0457, 2005 (online). Accepted for publication 4 December 2004.

Powdery mildew, caused by the fungus Blumeria graminis f. sp. tritici, is a major wheat disease worldwide, and causes tremendous loss of wheat production every year. Use of resistant wheat cultivars is the most efficient way to control the disease. In this study, 192 Chinese wheat cultivars and lines were tested with 20 Chinese isolates of B. graminis f. sp. tritici. It was found that resistance genes Pm12, Pm16, and Pm20 are effective against powdery mildew in China. These resistance genes should be used in wheat breeding programs to replace Pm8, which exists in a great number of Chinese wheat cultivars. In addition, we identified 22 wheat lines with good adult plant resistance to powdery mildew in the field. Adult plant resistance may confer durable resistance to powdery mildew in wheat. We also determined that maximum disease severity on the penultimate leaf (F-1 leaf) is a good indicator for identifying adult plant resistance, with a single scoring at an appropriate time.

Occurrence of Resistance-Breaking Beet necrotic yellow vein virus of Sugar Beet. H.-Y. Liu, J. L. Sears, and R. T. Lewellen, USDA-ARS, Salinas, CA 93905. Plant Dis. DOI: 10.1094/PD-89-0464, 2005 (online). Accepted for publication 8 December 2004.

Rhizomania is a serious disease of sugar beet. This disease is caused by Beet necrotic yellow vein virus (BNYVV) and vectored by the soilborne fungus Polymyxa betae. The major problem for control of rhizomania disease is that the resting spore of the viruliferous fungal vector is able to survive in the soil for at least 15 years. Hence, use of resistant cultivars has been the only efficient method by which to control this devastating disease. Partially resistant sugar beet cultivars based upon single dominant genes have been developed and are widely used by the sugar beet industry. In the summer of 2002, three sugar beet fields planted with a BNYVV-resistant cultivar in the Imperial Valley of California were observed to exhibit severe rhizomania symptoms, suggesting that resistance had been compromised. Standard soil baiting with sugar beet plants followed by enzyme-linked immunosorbent assays (ELISA) were used to diagnose virus occurrence and reaction. Resistant varieties grown in BNYVV-infested soil from Salinas, CA, remained resistant. In contrast, when grown in Imperial Valley BNYVV-infested soil from pertinent fields, all resistant varieties tested susceptible according to elevated ELISA values. Inoculum potential did not change these relationships. Based on host reaction, eight distinct BNYVV isolates have been identified from Imperial Valley soil (IV-BNYVV) by single local lesion isolation. IV-BNYVV isolates did not contain RNA-5 as determined by reverse transcription–polymerase chain reaction (RT-PCR). In single-strand conformation polymorphism analyses for all of the isolates, the banding patterns were identical to A-type and different from P-type strains. Our results indicate that the resistance-breaking BNYVV isolates from Imperial Valley likely evolved from existing A-type.

Pathogenicity of Xanthomonas translucens from Annual Bluegrass on Golf Course Putting Greens. N. A. Mitkowski, M. Browning, C. Basu, K. Jordan, and N. Jackson, Department of Plant Sciences, University of Rhode Island, 9 E. Alumni Ave., Suite 7, Kingston 02881. Plant Dis. DOI: 10.1094/PD-89-0469, 2005 (online). Accepted for publication 12 December 2004.

Bacterial wilt of annual bluegrass has become a significant problem on golf courses putting greens in the northeastern United States. The disease can kill significant amounts of annual bluegrass, the dominant grass in most putting greens, leaving large unsightly patches of dead grass. The disease is difficult to manage, in part because the identity of the causal agent has been in doubt. The purpose of this study was to isolate the causal agent from diseased annual bluegrass, prove that it had been isolated by demonstrating that the bacterial isolates obtained were able to cause disease through repeated inoculations, and investigate its taxonomic identity. Identifying the causal agent of bacterial wilt of annual bluegrass will enable plant pathologists and turf professionals to diagnose this disease more effectively in the future. Bacteria were isolated from diseased annual bluegrass from Connecticut, Massachusetts, Rhode Island, New York, and Canada, then tested to determine if they could cause disease on annual bluegrass in the greenhouse. Bacterial isolates colonies from two locations in Rhode Island and one location in Connecticut were obtained that could incite disease. All three isolates produced were rod-shaped, gram-negative bacteria that produced mucoid, yellow colonies on a specialized growth medium (YDC), indicative of a Xanthomonad. Through pathogenicity experiments, the isolates were shown to be restricted to attacking mainly annual bluegrass. Some variation in the ability of the three isolates to cause disease in the greenhouse was noted. Fatty acid analysis was undertaken to further identify the causal agent, but results were inconclusive. DNA analysis of the isolated bacteria and type specimens from of the genus Xanthomonas suggest that the causal agent is Xanthomonas translucens pv. poae. Further study is warranted to conclusively determine to which pathovar the organism belongs.

Experimental Bin Drenching System for Testing Biocontrol Agents to Control Postharvest Decay of Apples. W. J. Janisiewicz and D. L. Peterson, USDA-ARS, AFRS, Kearneysville, WV; K. S. Yoder, VPI&SU, Winchester, VA; and S. S. Miller, USDA-ARS, AFRS, Kearneysville, WV. Plant Dis. DOI: 10.1094/PD-89-0487, 2005 (online). Accepted for publication 20 December 2004.

Beneficial bacteria and yeasts that can control fruit decays are applied to apples after harvest in commercial operations by spraying, dipping, and drenching. Method of application can have a significant effect on efficacy. Spraying or dipping applications have been used often in the past by researchers evaluating potential biological control agents because bin drenching required large amounts of fruit and a considerable amount of treatment suspension with the beneficial bacteria or yeast. Thus, drenching tests have been very expensive and also often technically prohibitive for most laboratories. A portable drencher capable of drenching a single bin of fruit was built to simulate the commercial application of chemicals to harvested apples in small orchard operations in the central and eastern United States. The drencher required as little as 125 liters of the treatment solution, permitted various bin travel speeds, and allowed for frequent change of treatments. Wounded apples that were placed midway between the bottom and top of the bin and were drenched with suspension containing fungus causing blue mold decay had a high incidence of decay after 3 months in cold storage. Treatment with the beneficial yeast resulted in significant reduction of this fruit decay, and the addition of sodium bicarbonate (baking soda) further reduced decay to negligible levels on ‘Golden Delicious’ apples. This portable drencher can be very useful in evaluating beneficial organisms for ability to control fruit decay by drenching application, and it may also be used for treating fruit with beneficial microorganisms and chemicals in small commercial operations.

Molecular Identification of Oidium neolycopersici as the Causal Agent of the Recent Tomato Powdery Mildew Epidemics in North America. Levente Kiss, Plant Protection Institute of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 102, Hungary; Susumu Takamatsu, Laboratory of Plant Pathology, Faculty of Bioresources, Mie University, Tsu 514-8507, Japan; and James H. Cunnington, Department of Primary Industries – Knoxfield, Private Bag 15, Ferntree Gully Delivery Centre, Victoria, 3156, Australia. Plant Dis. DOI: 10.1094/PD-89-0491, 2005 (online). Accepted for publication 29 December 2004.

In the late 1970s, an apparently new powdery mildew disease appeared on tomato (Lycopersicon esculentum) in Japan and Australia. Later, outbreaks of a similar disease were reported from many parts of Europe and North America. In all these reports, the causal agent of the novel epidemics, causing economic damage on tomato, has been described as an Oidium anamorph and has always been clearly distinguished from Oidiopsis, the anamorph stage of Leveillula taurica, long known to affect tomato in warmer regions of the world. However, the reported data on the conidiogenesis of the novel pathogens were contradictory. Some authors described an Oidium anamorph on tomato that produces conidia in chains, while others reported a pathogen that produced conidia singly. The sexual stage of these pathogens has not been found so far. As the nature of the conidiogenesis is a stable pattern within powdery mildew species, the contradictions in the reported data suggested that the tomato powdery mildew epidemics might have been caused by different species in different parts of the world or even in the same geographical region. A comprehensive morphological study of Oidium anamorphs responsible for the recent tomato powdery mildew outbreaks worldwide suggested that, despite controversial data in the literature, the North American epidemics were caused solely by a newly erected species, O. neolycopersici. We report here the first molecular evidence that the North American anamorphs do belong to O. neolycopersici. The rDNA internal transcribed spacer sequences of the North American anamorphs determined in this study were identical with those of three Japanese and four European specimens of O. neolycopersici. A morphological study confirmed that all the North American Oidium anamorphs included in this study produced conidia singly, similar to O. neolycopersici. These fungi were readily distinguished from O. lycopersici, which produces conidia in chains and is known to infect tomato only in Australia. The phylogenetic analysis showed that O. neolycopersici is a distinct powdery mildew species, and it is neither identical to nor closely related to any known polyphagous species of the Erysiphaceae. Apparently, it was introduced into the United States and Canada only in the 1990s, but its origin is still unknown.

Partial Resistance of Pepper to Bacterial Wilt Is Oligogenic and Stable under Tropical Conditions. Denis Lafortune and Michel Béramis, INRA-URPV, Domaine Duclos, Prise d’eau, 97170 Petit Bourg, France; and Anne-Marie Daubèze, Nathalie Boissot, and Alain Palloix, INRA-GAFL, BP 94, 84143 Montfavet Cedex, France. Plant Dis. DOI: 10.1094/PD-89-0501, 2005 (online). Accepted for publication 8 January 2005.

Bacterial wilt, caused by Ralstonia solanacearum, is a widespread and devastating disease of many crops, ornamentals, and weeds in the tropic and subtropic areas. Host plant resistance to the disease offers a long-term, inexpensive, and environmentally safe control measure. A high level of resistance to bacterial wilt is common in small-fruited, pungent peppers, but absent in large-fruited (bell pepper) cultivars. The inheritance of resistance was analyzed in the progeny from a cross between a resistant Indonesian line (PI 322719) and a susceptible bell pepper line (Yolo Wonder). Resistance to bacterial wilt was stable against local isolates from the French Lesser Antilles (Guadeloupe) over 2 years during the hot and rainy season. Resistance was shown to be controlled by a few genes in the small-fruited parent, with quantitative and additive effects as previously shown in tomato. Therefore, the transfer of resistance to bell pepper cultivars should be relatively easy. However, resistance to bacterial wilt was also shown to be linked to susceptibility to Tobacco mosaic virus and to root-knot nematodes; therefore, additional crosses will be required in the breeding program to deliver multi-resistant cultivars.

Inoculum Sources and Survival of Xanthomonas axonopodis pv. allii in Colorado. David H. Gent, National Forage Seed Production Research Center, United States Department of Agriculture–Agricultural Research Service, Corvallis, OR 97331; Jillian M. Lang, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523-1177; Michael E. Bartolo, Department of Horticulture and Landscape Architecture, Colorado State University; and Howard F. Schwartz, Department of Bioagricultural Sciences, Colorado State University. Plant Dis. DOI: 10.1094/PD-89-0507, 2005 (online). Accepted for publication 4 January 2005.

Xanthomonas leaf blight, caused by Xanthomonas axonopodis pv. allii, was first observed in Colorado in 1996, and annual occurrences of the disease since its appearance suggest that it has become endemic in southern Colorado. Management of Xanthomonas leaf blight largely has been limited to copper bactericide applications because basic elements of its epidemiology are unknown and inoculum sources have not been identified. In this study, we have identified and quantified several primary inoculum sources of X. axonopodis pv. allii. Reservoirs of the pathogen were identified in or on other crop and weed species, volunteer onion plants, contaminated irrigation water, and infested onion crop debris. The relative importance of these inoculum sources to the epidemiology and management of Xanthomonas leaf blight development is unknown, but the design of onion production and pest management systems may need to eliminate multiple X. axonopodis pv. allii inoculum sources to reduce recurring losses from Xanthomonas leaf blight. Integrated management of Xanthomonas leaf blight in Colorado may need to consider multiple inoculum sources of X. axonopodis pv. allii, including contaminated seed, weeds, leguminous crops, infested crop debris, irrigation water, and volunteer onion. The contribution of these potential inoculum sources to Xanthomonas leaf blight appearance or severity remain speculative, and require more investigation to elucidate their relative importance to disease epidemiology in Colorado and elsewhere. Nonetheless, onion production systems that practice strict sanitation of weed and volunteer onion plants, follow a 2-year or longer rotation to nonhosts such as small grains, avoid reuse of irrigation tail water, and promote rapid breakdown of crop debris by deep tillage should reduce X. axonopodis pv. allii survival and minimize reliance upon copper bactericides for disease management.

Effect of Cowpea severe mosaic virus on Crop Growth Characteristics and Yield of Cowpea. H. M. Booker, P. Umaharan, and C. R. McDavid, Department of Life Sciences, The University of the West Indies, St. Augustine, Republic of Trinidad & Tobago, W.I. Plant Dis. DOI: 10.1094/PD-89-0515, 2005 (online). Accepted for publication 26 November 2004.

Field experiments were carried out in St. Augustine, Trinidad and Tobago, West Indies in the wet and dry seasons to determine the effects of time of inoculation of Cowpea severe mosaic virus (CPSMV) and cultivar on crop growth and yield in cowpea. Yield losses associated with CPSMV inoculation can vary from as little as 2% to as much as 85%, compared with uninoculated plots, depending on the time of inoculation, season, and cultivar. Time of inoculation with CPSMV had the most profound impact on yield. Inoculation during the seedling growth stage 12 days after seeding (DAS) consistently had the greatest impact (48 to 84% yield loss) followed by inoculation at 24 DAS (22 to 66% yield loss). Interestingly, the lower and upper limits of yield loss associated with inoculation at any particular stage corresponded to the wet and dry season, respectively. At any given time of inoculation, yield losses were much higher during the dry season than the wet season. The results suggest that the warmer and wetter conditions that exist during the wet season may allow recovery growth. The more determinate cultivar H8-8-27 (48 to 88%) suffered more than the semi-determinate cultivars Green Arrow (21 to 72%) and Bush Sitao (3 to 57%). We recommend control measures aimed at delaying CPSMV epidemics, such as sowing with “clean” seed, rouging of infected plants, and reducing vector transmission early in the crop lifecycle. Our results show that cultivation of semi-determinant varieties also can reduce the impact of CPSMV on crop yield, particularly in the wet season.