Interpretive Summaries

Yield Losses Associated with Virus-Infected Garlic Plants During Five Successive Years. Vilma C. Conci, Ana Canavelli, and P. Lunello, Instituto de Fitopatología y Fisiología Vegetal INTA, Camino 60 Cuadras km 5 y 1/2, (5119), Córdoba, Argentina; J. Di Rienzo, Facultad de Ciencias Agropecuarias, Universidad Nacional de Córdoba, Av. Valparaiso s/n Ciudad Universitaria, Córdoba, Argentina; S. F. Nome, Instituto de Fitopatología y Fisiología Vegetal INTA, Camino 60 Cuadras km 5 y 1/2, (5119), Córdoba, Argentina; G. Zumelzu, Facultad de Ciencias Agropecuarias, Universidad Nacional de Córdoba, Av. Valparaiso s/n Ciudad Universitaria, Córdoba, Argentina; Rusell Italia, Agencia de Extensión Rural INTA Jesús María, Tucumán 255 (5220) Jesús María, Córdoba, Argentina. Plant Dis. D-2003-0922-03R, 2003 (online). Accepted for publication 12 June 2003.

Garlic is naturally infected with a complex of viruses. Virus-free garlic plants, when planted in the field without protection, are quickly infected with viruses, but it is not known to what extent this affects yield over successive crop cycles. Highly significant differences were detected in the weight and perimeter of bulbs in relation to years of exposure to virus infection. There is a progressive decline in yield until the third crop year, and then there is a plateau that does not yet reach the levels of chronically infected plants, which are 44% in weight and 13% in perimeter below plants with 5 years cultivation in the field. The results show that it is possible to keep these plants for several years (at least three) in the commercial production field with good yield, but periodic renewal of “seed cloves” with new virus-free material is necessary to maintain the profitability of the crop.

Potential Primary Inoculum Sources of Bean pod mottle virus in Iowa. Rayda K. Krell and Larry P. Pedigo, Department of Entomology, John H. Hill, Department of Plant Pathology, and Marlin E. Rice, Department of Entomology, Iowa State University, Ames 50011. Plant Dis. D-2003-1002-02R, 2003 (online). Accepted for publication 16 June 2003.

Bean pod mottle virus (BPMV) has become a common soybean disease in Iowa since approximately 1999. Most research on BPMV had been done in the southern United States and little was known about this virus in the north-central region. In this study, we tested three potential primary inoculum sources of BPMV including (i) soybean seed, (ii) overwintered bean leaf beetles, and (iii) alternate host plant species. Less than 1% of the soybean seed transmitted BPMV and <2% of the beetles we tested transmitted BPMV after overwintering. A single plant, Desmodium canadense, tested positive for BPMV from our field-collected samples. This study identified three potential primary inoculum sources of BPMV in Iowa, but more work is needed to determine the epidemiological significance of each source.

Peach Rusty Spot Epidemics: Management with Fungicide, Effect on Fruit Growth, and the Incidence-Lesion Density Relationship. Laura A. Furman and Norman Lalancette, Rutgers University, Agricultural Research and Extension Center, Bridgeton, NJ 08302-5919; and James F. White, Jr., Rutgers University, Cook College, Department of Plant Biology and Pathology, New Brunswick, NJ 08901-8520. Plant Dis. D-2003-1010-01R, 2003 (online). Accepted for publication 26 July 2003.

Rusty spot of peach, reportedly caused by the powdery mildew fungus Podosphaera leucotricha, can cause significant yield loss on susceptible cultivars. Infection of fruit results in the development of orange to brown lesions; death of the epidermal cells causes the surface to become russeted, rendering the fruit unmarketable. Although the disease is managed with fungicides, the most effective timing is not known, particularly when applications should be terminated. A recent analysis of rusty spot development indicated that control need be applied only during the early-season phase of the epidemic. In the present study, further analysis of rusty spot epidemics showed that, on average, a total of four fungicide applications provided adequate control. The timings of these applications were late (>95%) petal fall (PF), early shuck-split (PF + 8 days), first cover (PF + 18 days), and second cover (PF + 30 days). Application of fungicide during the mid- and late-season epidemic phases provided no benefit in reducing disease intensity. A single application at PF provided significant control in 1 of 3 years; additional study is needed to confirm the importance of this timing. Analysis of disease intensity at harvest revealed that further reduction in fungicide usage may be possible by omitting the second cover application. Unlike earlier findings, rusty spot did not significantly decrease fruit volume or weight at midseason or at harvest. The relationship between disease incidence and lesion density (number of lesions per fruit) was fairly consistent when less than 50% of the fruit were infected. Thus, prediction of density from the simpler incidence assessments may allow for more efficient scouting procedures.

Simultaneous Detection of the Defoliating and Nondefoliating Verticillium dahliae Pathotypes in Infected Olive Plants by Duplex, Nested Polymerase Chain Reaction. J. Mercado-Blanco, D. Rodríguez-Jurado and S. Parrilla-Araujo, Instituto de Agricultura Sostenible (IAS), Consejo Superior de Investigaciones Científicas (CSIC), Apdo. 4084, 14080 Córdoba, Spain; and R. M. Jiménez-Díaz, IAS-CSIC, and Escuela Técnica Superior de Ingenieros Agrónomos y Montes, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain. Plant Dis. D-2003-1007-04R, 2003 (online). Accepted for publication 27 July 2003.

Olive is a mayor crop in the Mediterranean Basin, as well as in areas of Australia, Africa, and North and South America. Its main economic interest is for oil production but there is also a considerable market for table consumption. In Andalucia, southern Spain, many new orchards are being established in the interests of variety introduction, new cultivation strategies, or simply to increase production. This trend is aided by the recent innovation and improvement of plant propagation techniques which facilitate the rapid production of homogeneous commercial planting stocks, as well as by the establishment of high-tree-density, irrigated orchards. During the last few years, Verticillium wilt of olive, caused by the defoliating (D) and nondefoliating (ND) pathotypes of Verticillium dahliae, has spread throughout the olive-growing areas of Andalucia which, created great concern among growers. Of particular concern is the spread of the highly virulent D pathotype, which can cause infections lethal to olive, whereas plants infected by ND V. dahliae can show complete remission from symptoms. The early and rapid detection of V. dahliae pathotypes is of importance for the management of Verticillium wilt. Early detection would help to avoid spread of the pathogen to new areas, especially if they are free from V. dahliae or the D pathotype. Detection by means of isolation suffers from the inconsistency of recovering V. dahliae from affected woody tissues, and does not reveal the infecting pathotype. In this study, we developed detection procedures based on polymerase chain reaction (PCR) that are effective in the rapid and unequivocal characterization and detection of D and ND V. dahliae using DNA extracted from fungal mycelia or from artificially and naturally infected olive plants. For characterizing fungal cultures, a three-primer PCR assay was developed that allows simultaneous identification of V. dahliae and of D and ND pathotypes. D V. dahliae is characterized by a two-PCR-band pattern of 539 and 334 bp in size in agarose gels, and ND V. dahliae produces a single 523-bp band in PCR assays. The 539- and 523-bp bands are polymorphic of a species-specific marker of V. dahliae. For detection and characterization of the pathogen in infected plants, a duplex, nested-PCR procedure was developed, in which template DNA extracted from an infected olive was amplified using two primer pairs; from this first reaction, 0.5 µl of product was submitted to a second round of amplification using two different primer pairs. This duplex, nested-PCR procedure can detect simultaneously the D and ND pathotypes in adult olive trees naturally infected by both pathotypes and in young olive plants that were double-inoculated with D and ND V. dahliae isolates under controlled conditions.

Genetic Analysis of Adult-Plant Resistance to Leaf Rust in Five Spring Wheat Genotypes. A. Navabi, Department of Agricultural, Food, and Nutritional Science, 4-10 Agriculture/Forestry Center, University of Alberta, Edmonton, AB, T6G 2P5, Canada; R. P. Singh, CIMMYT, Apdo, 6-641, 06600 Mexico, DF, Mexico; J. P. Tewari and K. G. Briggs, Department of Agricultural, Food, and Nutritional Science, University of Alberta, Canada. Plant Dis. D-2003-1009-03R, 2003 (online). Accepted for publication 22 August 2003.

Inheritance of adult-plant resistance to leaf rust, caused by Puccinia triticina Eriks., was studied in the progenies of crosses involving five adult-plant resistant wheat (Triticum aestivum L.) genotypes and a susceptible wheat ‘Avocet-YrA’. F(1) progenies, F(2) populations, F(2)-derived F(3), and F(4)-derived F(5) lines were field evaluated under artificial epidemics with leaf rust race MCJ/SP in two locations in Mexico. Adult-plant resistance to leaf rust was incompletely dominant in crosses with the susceptible parent and was found to be controlled by additive interactions of Lr34 with at least two to tree additional genes. Additive interactions of genes were found to be the major components of variation. Adult-plant resistance to leaf rust in the genotypes studied was found to be highly heritable. Plants or lines with higher and lower levels of resistance than the parents were observed in all F(2) and F(5) derivatives of the resistant-parent intercrosses and suggested that, apart from Lr34, some of the other additive genes were different. This indicated the possibility of incorporating these genes into new breeding lines. High levels of resistance in the resistant parents included in this study are expected to be durable.