Interpretive Summaries

Long-Term Prediction of Soybean Rust Entry into the Continental United States. Z. Pan, Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, MO 63103; X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50011; S. Pivonia, Arava R&D, Sapir, P.O. Box. Arava, 86825 ISRAEL; L. Xue and R. Pasken, Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, MO 63103; and J. Roads, Scripps Institution of Oceanography, UCSD, 0224, La Jolla, CA 92093. Plant Dis. DOI: 10.1094/PD-90-0840. Accepted for publication 13 February 2006.

The Asian soybean rust, a devastating fungus disease, has invaded all major soybean production regions of the world. Its major outbreaks have occurred in Asia, Africa, and South America, and have caused severe yield damage in recent years. It is not unusual for the rust pathogen to reduce soybean yields by half in those invaded regions. The rust first entered the continental United States in November 2004. It is projected that hundreds of millions of dollars could potentially be lost in yield damage annually if severe epidemics occur in the United States. We have integrated a climate model and a particle dispersal model into a soybean rust spore prediction system. This paper describes the model system, demonstrates the feasibility of predicting long-distance dispersal of rust spores, and describes the dispersal mechanisms that were responsible for the rust entry into the United States. The validation of multiple year and location predictions showed that this model system is capable of predicting, a month in advance, the likely pathways of the rust spores based on meteorological conditions and pathogen source distribution. The results also indicated that the abnormally large number of hurricanes in 2004 disrupted the normal zonal flow, permitting meridional flow to transport rust spores from Central America into the United States. The preliminary results from 2005 growing season predictions indicated that the model system can serve as an effective tool for predicting rust or other airborne disease dispersal within the United States in the future.

Aggressiveness of Puccinia striiformis f. sp. tritici Isolates in the South-Central United States. E. A. Milus and E. Seyran, Department of Plant Pathology, and R. McNew, Agriculture Statistics Laboratory, University of Arkansas, Fayetteville 72701. Plant Dis. DOI: 10.1094/PD-90-0847. Accepted for publication 11 February 2006.

Although stripe rust, caused by Puccinia striiformis f. sp. tritici, has been an occasional problem on wheat in the south-central United States from 1941 until 1999, the disease has been consistently severe in the region since 2000. Furthermore, since 2000, the geographic range of stripe rust in the eastern United States has expanded, and the old population of the pathogen has been replaced by a new population. The objective of this study was to determine whether new isolates (collected since 2000) of the pathogen were more aggressive (able to cause disease more quickly) and better adapted to warmer temperatures than old isolates (collected before 2000). Representative old and new isolates were evaluated at 12ºC (favorable) and 18ºC (near upper limit) for spore germination on agar medium and for latent period (time from inoculation until new spores are produced) on wheat seedlings. On average, old and new isolates were similar at 12ºC. However, at 18ºC, new isolates had double the germination rate and produced spores 2 days (18%) sooner than old isolates. These results indicate that the current population is more aggressive at warmer temperatures than the previous population and helps explain why stripe rust has been a problem in the south-central United States since 2000. This profound change in the pathogen population has impacted wheat breeding and management in the south-central United States. Although resistance to stripe rust and applying a fungicide for stripe rust were not priorities before 2000, incorporating stripe rust resistance has become a high priority for wheat breeders developing cultivars for the Great Plains and eastern United States, and recommending a fungicide application specifically for stripe rust has become common.

Almond Replant Disease and Its Management with Alternative Pre-Plant Soil Fumigation Treatments and Rootstocks. G. T. Browne, USDA-ARS CPGRU, Department of Plant Pathology, University of California, Davis 95616; J. H. Connell, UC Cooperative Extension, Butte County, 2279-B Del Oro Avenue, Oroville, CA; and S. M. Schneider, USDA-ARS, SJVASC, 9611 S. Riverbend Ave., Parlier, CA 93648. Plant Dis. DOI: 10.1094/PD-90-0869. Accepted for publication 22 January 2006.

Young orchard trees can suffer from diverse replant problems when planted after other crops. In California’s Central Valley, we repeatedly observed a replant disease (RD) on almond, typified by poor tree growth and high incidences of tree mortality in the first year after planting on land with a history of almond production. Our objectives were to characterize the RD and evaluate potential contributions of different pre-plant soil fumigation treatments and almond rootstocks to its control. In trials at orchard and microplot sites with a history of RD, trees replanted without pre-plant soil fumigation grew poorly and sometimes died. Parasitic nematodes were not associated with the RD, but the affected trees had relatively little length of fine roots (<1 mm diameter), suggesting attack by root pathogen(s). Almond developed RD on all rootstocks evaluated (Marianna 2624, Lovell, and Nemaguard), but Marianna 2624 was the most severely affected. Pre-plant tree-site (spot) fumigation treatments with methyl bromide (MB), chloropicrin (CP), 1,3-dichloropropene (1,3-D), 1,3-D + CP, iodomethane, and iodomethane + CP all prevented severe RD. Spot fumigation treatments, once optimized, offer fumigant use savings. Broadcast soil fumigation with CP also was effective, but broadcast MB and 1,3-D were ineffective. CP generally was more potent than MB for prevention of the RD. The RD apparently is mediated by a biological agent(s) other than nematodes and can be prevented by appropriate fumigation with CP or other MB alternatives.

Geocenamus brevidens Associated with Reduced Yield of No-Till Annual Spring Wheat in Oregon. Richard W. Smiley, Professor, and Ruth G. Whittaker, Jennifer A. Gourlie, and Sandra A. Easley, Faculty Research Assistants, Oregon State University, Columbia Basin Agricultural Research Center, P.O. Box 370, Pendleton 97801. Plant Dis. DOI: 10.1094/PD-90-0885. Accepted for publication 14 February 2006.

Conservation farming systems are becoming more widely practiced in semiarid regions of the Pacific Northwest. Increasing areas of land formerly planted as 2-year rotations of winter wheat and summer fallow are being converted to spring wheat or barley planted annually, often without tillage. While studying damage caused by cereal cyst and root-lesion nematodes in no-till annual spring wheat, we found two locations where yield reductions were suspected of being caused by stunt nematodes. This paper reports four experiments at those locations. Each field was infested with Geocenamus brevidens (=Merlinius brevidens) either alone or mixed with Tylenchorhynchus clarus. Grain yields were inversely correlated with preplant numbers of stunt nematodes at both locations. In 19 previously unreported experiments, wheat yield was unrelated to populations of stunt nematodes at T. clarus–infested sites, or of G. brevidens at sites where yield suppression was caused by the more aggressive cereal cyst or root-lesion nematodes. This is the first report associating G. brevidens with suppression of wheat yield in the Pacific Northwest. Additional studies are needed to define cropping systems and locations where populations of G. brevidens flourish and determine threshold populations that cause economic damage.

Effect of Two Allexivirus Isolates on Garlic Yield. E. E. Cafrune, M. C. Perotto, and V. C. Conci, Instituto de Fitopatología y Fisiología Vegetal, Instituto Nacional de Tecnología Agropecuaria (IFFIVE-INTA) Camino 60 cuadras km 5 ½ (5119) Córdoba, Argentina. Plant Dis. DOI: 10.1094/PD-90-0898. Accepted for publication 14 February 2006.

Garlic is infected by aphid-borne viruses and mite-borne viruses. The greatest economic losses in garlic have been attributed to Onion yellow dwarf virus (OYDV), and to a lesser degree, to Leek yellow stripe virus (LYSV) (aphid-borne viruses). This work is the first attempt to compare the effect on yield of garlic production by two mite-borne virus isolates (Allexivirus), Garlic virus A (GarV-A) and Garlic virus C (GarV-C), in Argentina. They were compared in field assays and in anti-aphid cages during 2002 and 2003. Virus-free garlic plants were used as healthy controls and garlic plants chronically infected with the virus complex were used as negative controls. GarV-A produced significant reductions with respect to virus-free plants in bulb weight (14 to 32%) and diameter (6 to 11%) in the two cultivars under both assay conditions throughout the study period. GarV-C reduced weight 15% and diameter 5% with respect to virus-free plants in cv. Blanco-IFFIVE and did not produce significant yield losses in cv. Morado-INTA in the two cultivars under both assay conditions.

Formation of Conidial Pseudochains by Tomato Powdery Mildew Oidium neolycopersici. W. Oichi, Y. Matsuda, T. Nonomura, and H. Toyoda, Laboratory of Plant Protection and Biotechnology, Faculty of Agriculture, Kinki University, 3327-204 Nakamachi, Nara 631-8505, Japan; L. Xu, School of Life Science, East China Normal University, Shanghai 200062, China; and S. Kusakari, Agricultural, Food and Environmental Sciences Research Center of Osaka Prefecture, Osaka, 583-0862, Japan. Plant Dis. DOI: 10.1094/PD-90-0915. Accepted for publication 27 February 2006.

The formation of conidial pseudochains by the tomato powdery mildew Oidium neolycopersici on tomato leaves was monitored using a high-fidelity digital microscope. Individual living conidiophores that formed mature conidial cells at their apex were selected for observation. The conidial cells were produced during repeated division and elongation by the generative cells of the conidiophores. Under weak wind conditions (0.1 m/s), these conidial cells did not separate from each other to produce a chain of conidial cells (pseudochain). The pseudochains dropped from the conidiophores once four conidial cells were connected. The conidiophores resumed conidium production, followed by another cycle of pseudochain formation. The formation of pseudochains by tomato powdery mildew was not influenced by the ambient relative humidity. On the other hand, the conidial cells produced were wind dispersed easily without forming pseudochains when conidiophores were exposed to stronger winds (1.0 m/s). The present study successfully demonstrated that the pathogen required wind to disperse progeny conidia from the conidiophores and produced conidial pseudochains when the wind was below a critical level, independent of high relative humidity as reported previously.

Biological Significance of Mefenoxam Resistance in Phytophthora erythroseptica and Its Implications for the Management of Pink Rot of Potato. Raymond J. Taylor, Julie S. Pasche, and Neil C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. DOI: 10.1094/PD-90-0927. Accepted for publication 1 March 2006.

Pink rot is an important soilborne disease of potato caused by Phytophthora erythroseptica. It usually is associated with conditions of high moisture, generally develops late in the growing season, and can result in significant losses at harvest and during storage. In the United States, pink rot infestation in fields can range as high as 75% and, in very severe cases, tubers in infested areas or sometimes entire fields will not be harvested. Although certain cultural practices may be employed to control pink rot, mefenoxam is the primary fungicide currently in use. Resistance to mefenoxam has developed in the pathogen populations in some potato-growing areas and has important implications for disease management. Based upon laboratory testing, isolates of P. erythroseptica can be classified as being sensitive or resistant to mefenoxam. Isolates falling between these categories are deemed to have intermediate resistance to the fungicide. The purpose of these experiments was to determine the “biological significance” of these intermediate isolates and the level of control attainable when mefenoxam is applied under standard field conditions. After harvest, tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of P. erythroseptica having varying levels of sensitivity to the fungicide. Mefenoxam reduced the amount of disease caused by sensitive isolates by as much as 37%. Mefenoxam was unable to control any of the intermediate and resistant isolates and these isolates consistently caused significantly more pink rot in the mefenoxam treatments. Intermediate isolates having the lowest levels of resistance in laboratory assays were associated with the greatest increase (84.9%) in disease under mefenoxam pressure. Intermediate and resistant isolates caused as much disease as sensitive isolates in the absence of mefenoxam and isolates with the highest level of resistance caused the greatest amount of disease under these conditions. Resistance to the fungicide did not reduce an isolate’s fitness as a pathogen. The results suggest that pink rot may become more severe in fields known to contain populations with resistance to mefenoxam if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.

Influence of Temperature, Relative Humidity, Ascospore Concentration, and Length of Drying of Colonized Dry Bean Flowers on White Mold Development. R. Harikrishnan and L. E. del Río, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. DOI: 10.1094/PD-90-0946. Accepted for publication 10 March 2006.

North Dakota is the main producer of dry bean in the United States. White mold (WM) of dry bean, caused by the fungus Sclerotinia sclerotiorum, is a common disease in North Dakota. This study evaluates the impact of temperature (18 or 22ºC) and relative humidity (25 or 90%) on WM development using bean blossoms inoculated with two types of inoculum and three ascospore concentrations as a source of the pathogen. Results indicate a linear increase in WM with increase in ascospore concentration. Mycelium is a better source of inoculum than ascospores under dry conditions. WM mold can develop at 25% relative humidity although it develops faster at 90%. Pre-colonized bean blossoms can be a viable source of inoculum even after 144 h of drying under the temperature and moisture conditions tested. These findings will help develop more accurate forecasting models for WM management.