Interpretive Summaries

Vegetative Compatibility Groups of Fusarium oxysporum f. sp. lactucae from Lettuce. Matias Pasquali, Flavia Dematheis, Giovanna Gilardi, Maria Lodovica Gullino, and Angelo Garibaldi, Di.Va.P.R.A. and Centre of Competence for the Innovation in the Agro-Environmental Field (AGROINNOVA), via Leonardo da Vinci, 44 10095 Grugliasco (TO), Italy. Plant Dis. DOI: 10.1094/PD-89-0237, 2005 (online). Accepted for publication 28 September 2004.

Fusarium oxysporum f. sp. lactucae, the causal agent of Fusarium wilt of lettuce, has been reported in three continents (Asia, North America, and Europe) in the last 10 years and is one of the major diseases on this important horticultural crop. In 2001 and 2002, an epidemic spread in the Lumbardy region, the main producing area in Italy. Similar epidemics were reported in some areas of the United States. This work aims to study genetic characteristics of the pathogen. The vegetative compatibility group analysis, based on the fact that two fungal strains can “help” themselves by complementing gene pathways if they are genetically similar, has been adopted to compare the level of similarity of pathogens in Italy, the United States, Taiwan, and Japan. We have identified one population that is distributed worldwide and another group that is present only in Taiwan. This information may be useful to growers and breeders: studies on resistant cultivars carried out in Japan, where the disease was first reported more than 40 years ago, may be applied by Italian or American growers adopting appropriate resistant cultivars. Moreover, our results may suggest a common approach by breeders, selecting resistant cultivars from all over the world because one common population is colonizing lettuce fields.

Effect of the F129L mutation in Alternaria solani on Fungicides Affecting Mitochondrial Respiration. J. S. Pasche, L. M. Piche, and N. C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. DOI: 10.1094/PD-89-0269, 2005 (online). Accepted for publication 14 October 2004.

Early blight, caused by Alternaria solani, is an important foliar disease of potato in the Midwestern United States. Alternating wet and dry periods provided by consistent dew formation are particularly favorable for the development of the disease. As a result, yield losses of 30% caused by early blight have been reported. Few potato cultivars are resistant to the disease; therefore, potato early blight is managed primarily through the use of foliar fungicides. Broad-spectrum fungicides such as chlorothalonil and mancozeb control early blight disease under moderate disease pressure but not always under irrigated potato production. As a result, in the late 1990s, potato producers took advantage of the high level of disease control (>90%) provided by strobilurin fungicide chemistry (Q(o)I, group 11 fungicides) such as azoxystrobin (Quadris) and pyraclostrobin (Headline). In 2000, 1 year after the registration of the first Q(o)I fungicide, isolated potato fields were identified with significant levels of early blight despite several applications of Quadris made on the crop. Isolates of A. solani recovered from these fields, and from other fields throughout the Midwest in 2001, confirmed that the early blight fungus had reduced-sensitivity to azoxystrobin in due to the presence of the F129L mutation. Previous studies confirmed that azoxystrobin reduced-sensitive isolates are also reduced in sensitivity to pyraclostrobin (Headline) but less so to trifloxystrobin (Gem). In current studies, we also demonstrated that the F129L mutation does not appreciably affect disease control provided by nonstrobilurin Q(o)I fungicides famoxadone (Tanos) and fenamidone (Reason). Early blight disease control provided by famoxadone is significantly better than the level of control provided by fenamidone, primarily due to higher intrinsic activity on the early blight fungus. Interestingly, A. solani isolates possessing the F129L mutation are significantly more sensitive to boscalid (Endura) in vitro than the sensitive wild-type. Fungicide rotations of famoxadone and boscalid may be effective in slowing the development of resistance in A. solani to both chemistries.

Resistance to Leaf Rust, Stripe Rust, and Stem Rust in Aegilops spp. in Israel. Y. Anikster and J. Manisterski, Institute for Cereal Crops Improvement, Tel Aviv University, Ramat Aviv 69978, Israel; D. L. Long, United States Department of Agriculture–Agricultural Research Service, Cereal Disease Laboratory, University of Minnesota, St. Paul 55108; and K. J. Leonard, Plant Pathology Department, University of Minnesota, St. Paul 55108. Plant Dis. DOI: 10.1094/PD-89-0303, 2005 (online). Accepted for publication 25 October 2004.

Leaf rust is one of the most serious diseases of wheat in the United States and worldwide. The most effective and economical way to control leaf rust is through the development of resistant wheat cultivars. Unfortunately, most resistance to leaf rust is effective against only some races of the leaf rust fungus but not others. Such resistance usually remains effective for only a few years, because new rust races arise that are not affected by the resistance. The available supply of rust resistance genes in cultivated wheat cultivars and breeding lines is nearly exhausted. We collected seed from 1,323 plants of wild wheat relatives in the genus Aegilops from natural habitats in Israel and tested the Aegilops lines for resistance to leaf rust, stripe rust, and stem rust in Israel and the United States. Nearly all lines of Aegilops speltoides, a close relative of bread wheat, were resistant to all three rusts. Many lines of A. longissima, A. sharonensis, and A. variabilis also were resistant to wheat leaf rust. These lines can be crossed to cultivated wheat cultivars and may provide combinations of resistance genes with long-lasting protection against leaf rust in U.S. wheat production. This would prevent wheat leaf epidemics that periodically reduce yields by 15% or more in major wheat-producing states in the United States.

Impact of Postharvest Hot Water or Ethanol Treatment of Table Grapes on Gray Mold Incidence, Quality, and Ethanol Content. F. Mlikota Gabler, Institute for Adriatic Crops, Put Duilova 11, 21000 Split, Croatia; and J. L. Smilanick, J. M. Ghosoph, and D. A. Margosan, United States Department of Agriculture–Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center, Parlier, CA 93648. Plant Dis. DOI: 10.1094/PD-89-0309, 2005 (online). Accepted for publication 26 October 2004.

The influence of a brief immersion of table grape berries in water or 35% ethanol at 25 or 50°C on the incidence of postharvest gray mold and grape quality was evaluated. Immersion of grape berries inoculated with Botrytis cinerea in ethanol solutions effectively controlled gray mold. These treatments could be applied up to 24 h after inoculation and remain effective. Effectiveness of heated-water treatment was inadequate and significantly inferior to heated ethanol. Treatments did not influence berry bloom, but slightly increased the susceptibility of berries to subsequent infections with B. cinerea. Crimson Seedless clusters were inoculated, immersed for 1 min in 35% ethanol at 25 or 50°C, and stored for 1 month at 0.5°C and 2 days at 25°C. Infected berries were 78.7, 26.2, and 3.4 per kilogram among untreated grapes and those immersed in ethanol at 25 or 50°C, respectively. Rachis appearance and berry shatter were not significantly changed by these treatments whereas berry color was slightly changed after treatment by heated ethanol. Sanitizing grape berries in ethanol solutions could be particularly useful for the postharvest treatment of grape fruit marketed under “organic” classifications, where sulfur dioxide treatments are prohibited and grape berries are stored and marketed without any protection from postharvest decay.

Co-infection of Beet mosaic virus with Beet Yellowing Viruses Leads to Increased Symptom Expression on Sugar Beet. William M. Wintermantel, United States Department of Agriculture, Agricultural Research Service, 1636 E. Alisal Street, Salinas, CA 93905. Plant Dis. DOI: 10.1094/PD-89-0325, 2005 (online). Accepted for publication 1 November 2004.

Three distinct aphid-transmitted viruses associated with a yellowing disease on sugar beet were examined in single and mixed infections for the effects of virus interactions on whole-plant fresh weight, rate of symptom appearance, and virus concentration. Sugar beet breeding lines exhibiting different degrees of susceptibility to the virus yellows complex were inoculated with either one, two, or all three viruses. Severe stunting, as measured by whole-plant fresh weight, was observed during mixed infections with Beet yellows virus (BYV) and Beet mosaic virus (BtMV) compared with single infections of these viruses, particularly in highly susceptible sugar beet. In addition, the overall rate of appearance of Beet western yellows virus (BWYV) symptoms increased during co-infection with BtMV. The effect of virus interactions on stunting severity, as measured by whole-plant fresh weight, were more pronounced in susceptible beet lines, but similar patterns also were observed in lines exhibiting tolerance to virus yellows. Relative amounts of viruses in leaf tissue were compared among single and mixed infections using nucleic acid hybridization (Northern blotting) with virus-specific probes. Levels of all three viruses increased as a result of co-infection compared with single infections.