Interpretive Summaries

Nature, Incidence, and Symptomatology of Viruses Infecting Vanilla tahitensis in French Polynesia. M. Grisoni, Centre de Coopération Internationale en Recherche Agronomique pour le Développement, CIRAD/SDR, BP 467, 98713 Papeete, French Polynesia; F. Davidson and C. Hyrondelle, Service du Développement Rural, BP 13, 98735 Raiatea, French Polynesia; K. Farreyrol, School of Biological Sciences, The University of Auckland, PB 92019, Auckland, New Zealand; M. L. Caruana, Centre de Coopération Internationale en Recherche Agronomique pour le Développement, AMIS/PC TA 40/02, 34398 Montpellier Cedex 5, France; and M. Pearson, School of Biological Sciences, The University of Auckland, New Zealand. Plant Dis. D-2003-1208-03R, 2004 (online). Accepted for publication 1 July 2003.

Vanilla is the most-used flavor in the food industry. Natural vanilla produced in French Polynesia from Vanilla tahitensis is particularly appreciated on the market, and intensive cultivation is being developed in the Society Islands. Several viruses have been reported that represent a potentially serious threat for vanilla plantations. This study was carried out to confirm and identify the viruses infecting vanilla plants and the extent of their spread in the Society Islands. Virus identification was based on enzyme-linked immunosorbent assay (ELISA) and, for potyviruses, also on the sequence of part of the coat protein and inoculation assays. In all, 3,610 vanilla plants from 49 plots were indexed between 1998 and 1999. Cymbidium mosaic virus (CymMV) was detected in 500 vines from 10 plots on the islands of Tahaa, Raiatea, and Huahine. The data indicates that this virus has spread widely since it was first detected in Raiatea in 1986, most likely through the dissemination of symptomless infected cuttings. Viruses belonging to the Potyvirus genus were found in 674 plants from 27 plots in Tahaa, Raiatea, Huahine, and Moorea. Three distinct potyviruses have been identified: (i) Vanilla mosaic virus, (ii) Watermelon mosaic virus, and (iii) and a virus related to Bean common mosaic virus. The mosaic symptoms induced on V. tahitensis by the three potyviruses can be differentiated from each other and from those due to CymMV. A significant proportion of the plants surveyed (97/476) were symptomatic but tested negative by ELISA for CymMV and the Potyvirus group. Odontoglossum ringspot virus was not detected in any samples tested. The information obtained regarding the detection, identification, and dissemination of vanilla viruses in the field will allow the development of control measures to limit their economic impact on the crop.

Effects of Timing of Copper Sprays, Defoliation, Rainfall, and Inoculum Concentration on Incidence of Olive Knot Disease. Beth L. Teviotdale, Department of Plant Pathology, University of California-Davis, Kearney Agricultural Center, Parlier 93648; and William H. Krueger, University of California-UCCE, Orland 95963. Plant Dis. D-2003-1124-01R, 2004 (online). Accepted for publication 31 August 2003.

The olive knot bacterium, Pseudomonas savastanoi, causes galls on shoots, branches, fruit, and leaves. Shoots with multiple galls often are girdled and die, which reduces the fruiting potential of the tree. Severely infected large branches also may die. Infection may occur at any time of year if climatic conditions are favorable. Any fresh wound is susceptible but the most common entry sites are leaf scars. Olive trees are evergreen, and healthy leaves remain attached for 2 or 3 years. Although olive trees lose leaves all year, most are shed in spring. Leaf scars are most susceptible to infection during the first 2 days after leaf fall and remain susceptible for 7 more days. In rainy weather, bacteria exude from existing galls and are spread by wind and rain to new infection sites. Galls develop only when the tree is growing; thus, leaf scars or wounds infected in winter do not develop symptoms until the following spring. The disease is managed by spray treatments with copper-containing bactericides. We tested the timing and number of spray treatments of copper-containing bactericide for efficacy in controlling the disease. Identification and monitoring of natural leaf scars was impossible; therefore, we created simulated leaf scars on ‘Manzanillo’ olive trees by removing leaves from healthy shoots at approximately monthly intervals from December through June 1997-98, 1998-99, and 1999-2000. Trees were treated with a water suspension of cupric hydroxide (Kocide DF40) at 3 g/liter one, two, or three times in 1998-99 and 1999-2000 with a hand-gun sprayer. Generally, disease control improved with increasing numbers of applications. Three applications, one in winter and two in spring, provided the most consistent season-long control. Disease incidence was greatest on shoots that were defoliated in March 1998, April and June 1999, and March and May 2000. We also related disease occurrence to rainfall. Cumulative rainfall 2 days (the most susceptible period for leaf scars) and 9 days (the maximum susceptible period for leaf scars) after each defoliation date was recorded. Disease incidence was positively correlated with rainfall in spring (March through June) but not winter (December through February). The effect of inoculum concentration on disease incidence was tested. Comparable simulated leaf scars were treated with a water suspension of 2.4 or 3.0 g/liter Kocide DF40 in 1997-98 and 1998-99, respectively. The material was allowed to dry, and shoots were inoculated with suspensions containing the pathogen at 10(^4), 10(^6), and 10(^8) CFU/ml. Inoculated and noninoculated, nontreated shoots were included. More disease developed from April than from December inoculations in both years. Disease incidence increased with increasing inoculum concentration in treated and nontreated shoots in both years and generally was lower in shoots treated with Kocide DF40. Our work demonstrated that the common grower practice of one postharvest (late fall) application of copper bactericide provides only minimal protection against olive knot, and additional sprays in spring are needed to substantially improve disease control.

Seedling Resistance Genes to Leaf Rust in Soft Red Winter Wheat. Yeshi A. Wamishe and Eugene A. Milus, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. D-2003-1119-03R, 2004 (online). Accepted for publication 27 August 2003.

Leaf rust caused by Puccinia triticina is one of the most important and widespread diseases of wheat. Growing resistant varieties is the most cost-effective means of managing leaf rust and has no negative impact on the environment. Breeders have developed resistant wheat varieties by incorporating genes for resistance, but the pathogen population has the ability to overcome all of the resistance genes that have been used. Combining more than one resistance gene in varieties is more effective than using the genes singly. The objective of this research was to determine which genes for leaf rust resistance are in 116 contemporary soft red winter wheat varieties and breeding lines. Fourteen resistance genes and 44 different combinations of these genes were identified among the 116 lines. Genes Lr3, 10, and 11 were the most frequent and present in 23.3, 25.8, and 34.5% of the lines, respectively. Only four lines had a combination of three genes (Lr9, 24, and 26) that conferred resistance to all isolates of the pathogen used in the study. Knowing which genes are in each of the lines provides an understanding of the genetic basis for leaf rust resistance in soft red winter wheat and will allow breeders to develop new varieties with more effective combinations of resistance genes.

Analysis of Pathotypes of Colletotrichum lindemuthianum Found in the Central Region of Mexico and Resistance in Elite Germ Plasm of Phaseolus vulgaris. Mario González-Chavira and Raúl Rodríguez Guerra, Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Campo Experimental del Bajío, Celaya, Guanajuato, Mexico; Fernando Hernández-Godínez, Department of Genetic Engineering, CINVESTAV, Unidad Irapuato, Apdo. Postal 629, Irapuato, Guanajuato, Mexico; Jorge A. Acosta-Gallegos, Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Campo Experimental del Bajío, Celaya, Guanajuato, Mexico; and Octavio Martínez de la Vega and June Simpson, Department of Genetic Engineering, CINVESTAV, Unidad Irapuato, Apdo. Postal 629, Irapuato, Guanajuato, Mexico. Plant Dis. D-2003-1124-02R, 2004 (online). Accepted for publication 3 September 2003.

Anthracnose of common bean (Phaseolus vulgaris) caused by Colletotrichum lindemuthianum is widespread throughout the world and can cause partial or total loss of susceptible crops. The objectives of this research were to determine the pathotypes of isolates of C. lindemuthianum obtained from the central region of Mexico, to establish the genetic relationships between isolates from this region and isolates from other regions of Mexico, and to evaluate the resistance present in the National Research Institute for Forestry, Agriculture, and Animal Husbandry (INIFAP) elite collection of germ plasm of P. vulgaris to five different pathotypes of C. lindemuthianum found in Mexico. The pathotypes of 17 isolates of C. lindemuthianum from the central region of Mexico were determined. Eight pathotypes were identified, including pathotype 292, which is reported for the first time in Mexico. The lack of isolates infecting P. vulgaris cultivar TU carrying the Co-5 resistance gene suggests that this cultivar is a useful source of resistance. Analysis of five different pathotypes of C. lindemuthianum on 21 elite genotypes of P. vulgaris identified four genotypes from different races of P. vulgaris that were resistant to all five pathotypes. This information will allow breeders and farmers to select the resistant genotypes most suited to their needs.

Comparison of Reduced-Application and Sulfur-Based Fungicide Programs on Scab Intensity, Fruit Quality, and Cost of Disease Control on Peach. Guido Schnabel, Department of Entomology, Soils, and Plant Sciences, and Desmond R. Layne, Department of Horticulture, Clemson University, Clemson SC 29634. Plant Dis. D-2003-1118-03R, 2004 (online). Accepted for publication 6 September 2003.

Commercial peach production in the southeastern United States is a profitable business requiring high-quality, blemish-free fruit. Diseases and insects need to be controlled during the growing season for high-quality fruit. Fungal diseases, such as peach scab, are a recurring problem and many commercial peach growers spray fungicides on a weekly schedule starting at bloom and ending just prior to harvest. In late-season cultivars the total number of applications may be 15 or more. This study compared reduced-application fungicide programs with sulfur-based fungicide programs for peach scab control. We found that 5 applications of nonsulfur-based products can control peach scab as effectively as 11 applications of a sulfur-based product during high levels of disease incidence. Also, the reduced-fungicide programs were cheaper than applying sulfur 11 times and have the additional benefit of controlling other summer diseases. Sulfur-based fungicide programs may be a viable scab control strategy for growers wishing to produce organic peaches. In our study, we did not see a consistent impact of sulfur-based programs on fruit quality.

Shift in Sensitivity of Alternaria solani in Response to Q(o)I Fungicides. J. S. Pasche, C. M. Wharam, and N. C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. D-2003-1119-01R, 2004 (online). Accepted for publication 17 September 2003.

Early blight, caused by Alternaria solani, is an important foliar disease of potato in the midwestern United States. Alternating wet and dry periods in potato crops in the Midwest are provided by consistent dew formation and irrigation. These conditions are particularly conducive for the development of this disease. As a result of this relatively high disease pressure, yield losses caused by early blight of nearly 40% have been reported. Since few potato cultivars possess resistance to early blight, management is primarily accomplished through the use of foliar fungicides. While older fungicide chemistries control early blight disease under moderate disease pressure, producers of irrigated potatoes have taken advantage of the high level of disease control (>90%) provided by Q(o)I fungicide chemistries such as azoxystrobin (Quadris) and pyraclostrobin (Headline). Quadris was the first Q(o)I fungicide labeled for potato; an emergency use label was issued in North Dakota, Minnesota, Nebraska, and Wisconsin in July 1998, and a full label was granted in the rest of the states in 1999. In 2000, isolated potato fields were identified with significant levels of early blight despite several applications of Quadris made to the crop. Isolates of A. solani recovered from these fields, and from other fields throughout the Midwest in 2001, confirm the presence of azoxystrobin reduced-sensitivity in the early blight fungus. Results of these studies confirm that azoxystrobin reduced-sensitive isolates are highly cross-sensitive with pyraclostrobin (Headline) and less so with trifloxystrobin (Gem). We do not regard these isolates as being resistant primarily because the research reported here demonstrates a 40 to 50% loss of disease control of A. solani isolates possessing reduced sensitivity to Q(o)I fungicides such as Quadris and Headline. Potato producers need to determine if reduced sensitivity to Q(o)I fungicides exists in their A. solani population before using this chemistry for the control of early blight.

Activity of Chlorine Dioxide in a Solution of Ions and pH Against Thielaviopsis basicola and Fusarium oxysporum. W. E. Copes, Small Fruit Research Station, USDA-ARS, Poplarville, MS 39470; and G. A. Chastaganer and R. L. Hummel, Puyallup Research and Extension Center, Washington State University, Puyallup 98371. Plant Dis. D-2003-1119-02R, 2004. Accepted for publication 21 September 2003.

Chlorine dioxide (ClO(2)) is a disinfectant with low potential for damage of many ornamental plants and could be used to treat irrigation water. Chlorine dioxide was mixed in a water solution in test tubes, with different levels of nutrients, pH, and synthetic hard water for 10 minutes. Spores from two fungi, Fusarium oxysporum f. sp. narcissi (causal agent of Fusarium of daffodils) and Thielaviopsis basicola (causal agent of black root rot) were then injected into the solution for 30 seconds, filtered off, and tested for percent spore germination. Micronutrients (divalent metal ions), pH, and water hardness affected ClO(2) activity, reducing biocidal activity similarly for both fungi. High doses of ClO(2) (37 to 62 ppm ClO(2)) were required to kill 90% of the thick-walled spore type (aleuriospores) of T. basicola. In the presence of 5 ppm of divalent metal ions, high doses of ClO(2) were required to kill 90% of the spores (conidia) of T. basicola and F. oxysporum f. sp. narcissi (4.8 to 18.4 ppm ClO(2)). However, low doses killed 90% of the thin-walled spores (conidia) of T. basicola (1.0 to 2.7 ppm ClO(2)) and F. oxysporum f. sp. narcissi (0.8 to 2.3 ppm ClO(2)) at a pH of 4 to 8 and up to 200 ppm nitrogen, 100 ppm hard water, and 1 ppm each of copper, iron, manganese, and zinc. Chlorine dioxide can be used at rates likely to be economical in treating irrigation water, but water properties affect the rates needed to maximize biocidal activity.

Effect of Cercosporella rubi on Blackberry Floral Bud Development. Melinda R. Lyman and Kenneth J. Curry, Department of Biological Sciences, University of Southern Mississippi, Hattiesburg 39406; Barbara J. Smith, United States Department of Agriculture-Agricultural Research Service, Small Fruit Research Station, Poplarville, MS 39470; and Susan V. Diehl, Department of Forest Products, Mississippi State University, Mississippi State 39762. Plant Dis. D-2003-1208-04R, 2004 (online). Accepted for publication 21 October 2003.

Rosette disease of blackberry is a serious problem for growers in the southeastern United States because it often severely reduces fruit production. The pathogen, Cercosporella rubi, is always found in symptomatic flowers that develop on rosetted shoots in early spring; however, contrary to most fungal pathogens, it proliferates on the surfaces of developing vegetative and floral organs without penetrating host tissues. We sought evidence using electron microscopy that would indicate whether or not fungal penetration of host cells occurred anytime during floral development. C. rubi was observed in floral buds from 0.08 to 6.0 mm in diameter, and persisted in opened flowers. Although C. rubi appears to induce rosette formation, its presence does not appear to alter the early stages of floral development morphologically. The fungus seems to induce floral abortion by accelerating senescence. In 6.0-mm-diameter floral buds, hyphae growing through the style appeared to interfere with pollen tube development and cause the ovule to abort. These findings directly benefit blackberry growers in the southeastern United States because they indicate the necessity of continuing fungicide applications until midsummer. Other research scientists can use this information as a basis for in-depth studies of the early infection events.

Response of Chile Pepper to Phytophthora capsici in Relation to Soil Salinity. S. Sanogo, Department of Entomology, Plant Pathology, and Weed Science, New Mexico State University, Las Cruces 88003-0003. Plant Dis. D-2003-1124-04R, 2004 (online). Accepted for publication 26 September 2003.

Soil salinity and Phytophthora capsici are major concerns in chile pepper production in New Mexico. P. capsici may infect roots, stems, fruit, and leaves of chili pepper plants. Soil salinity has been shown to increase plant susceptibility to other species of Phytophthora, but it is not known how salinity affects infection of chile pepper by P. capsici. This study showed that plant infection by P. capsici is enhanced by salinity levels that may be encountered in chile pepper fields. Additionally, this study indicates that salinity promotes disease development in chile pepper plants susceptible to P. capsici, but not in those that are resistant to P. capsici. Results suggest that an effective management of P. capsici under saline conditions should include the selection of cultivars with tolerance to salinity and resistance to P. capsici.

Chemical Alternatives to Methyl Bromide in Spanish Strawberry Nurseries. A. De Cal, Department of Plant Protection. SGIT-INIA. 28040 Madrid; A. Martinez-Treceño, Ministerio de Agricultura, Pesca y Alimentación, Madrid; J. M. Lopez-Aranda, CIFA Málaga, CAP Junta de Andalucía, Churriana, Málaga, Spain; and P. Melgarejo, Department of Plant Protection. SGIT-INIA. 28040 Madrid. Plant Dis. D-2003-1201-01R, 2004 (online). Accepted for publication 25 September 2003.

Strawberry runners are a high-value cash crop in Spain, which is the most important runner production area in the European Union (EU). Vigorous pathogen-free transplants are required, and preplant soil fumigation with mixtures of methyl bromide (MB) and chloropicrin (Pic) is a standard practice for controlling soilborne plant pathogens such as Phytophthora cactorum and Verticillium spp. However, by the year, 2005 the use of MB will be banned in EU countries. The objective of this research was to evaluate certain soil fumigants in combination with different plastic films as possible alternatives for MB for controlling soilborne pathogens in Spanish strawberry nurseries. Soil fumigants Pic, 1,3-dichloropropene (1,3-D), dazomet, metam sodium, metam potassium, and dimethyl disulfide were evaluated in combination with different plastic films as alternatives for MB soil fumigation of strawberry nurseries. The studies were conducted over a 4-year period, with fumigant applications prior to planting. Verticillium wilt (caused by Verticillium spp.) and crown rot (caused by P. cactorum) were the main diseases. Chloropicrin, 1,3-D, and dazomet compared well with MB fumigation for control of strawberry nursery diseases. Furthermore, 1,3-D and MB, applied at a 50% rate under virtually impermeable film, provided effective disease control in strawberry nurseries. The use of VIF films with reduced dosages of MB will help Spanish strawberry nurseries to conform to Montreal Protocol requirements in the short term, but reliable and long-term solutions such as 1,3-D:Pic, Pic, or dazomet under VIF films are possible. Although VIF films are costly, the increase in cost is offset by the use of reduced doses of fumigants.