Diversity of Phytophthora capsici in Northwest Spain: Analysis
of Virulence, Metalaxyl Response, and Molecular Characterization. C.
Silvar, F. Merino, and J. Díaz, Departamento de Bioloxía Animal, Bioloxía
Vexetal e Ecoloxía. Universidade da Coruña. A Coruña, Spain. Plant Dis.
DOI: 10.1094/PD-90-1135. Accepted for publication 15 March 2006.
Phytophthora crown rot, caused by the pathogen Phytophthora capsici,
is potentially the most destructive disease of pepper in Spain. Phenotypic
and genetic diversity of 16 P. capsici isolates collected from 11
farms in northwest Spain was characterized based on virulence, mating
type, sensitivity to the fungicide metalaxyl, and genetic analysis using
random amplified polymorphic DNA (RAPD) methods. Low variability was
observed among the isolates in their metalaxyl response, with most of them
being highly sensitive. Metalaxyl has been an important tool worldwide for
managing crown rot; however, in some areas the pathogen has developed
resistance to this fungicide which could affect metalaxyl efficacy. All
the isolates were A2 mating type. Existence of both mating types A1 and A2
together is necessary for sexual reproduction and formation of specialized
survival structures (oospores). More variability among isolates was
revealed by the virulence assay: isolates were classified into two groups
according to their pathogenicity on a set of four pepper cultivar
differentials. Analysis of genetic variation also revealed the existence
of differences among isolates and was used to classify these Spanish
isolates and a representative worldwide group of isolates into different
groups. No correlation was found between groups obtained by genetic
analysis and groups defined by virulence or metalaxyl response.
Inheritance of White Mold Resistance in Phaseolus vulgaris ×
P. coccineus Crosses. Howard F. Schwartz and Kristen Otto,
Department of Bioagricultural Sciences and Pest Management, Colorado State
University, Fort Collins 80523-1177; and Henry Terán, Margarita Lema, and
Shree P. Singh, Department of Plant, Soil and Entomological Sciences,
University of Idaho, Kimberly 83341-5076. Plant Dis. DOI:
10.1094/PD-90-1167. Accepted for publication 27 April 2006.
The fungus Sclerotinia sclerotiorum, cause of white mold, is
known to attack >400 plant species. It is a widespread problem in dry bean
(Phaseolus vulgaris) in the United States, causing >30% average
yield losses. Low to moderate levels of resistance are found in dry bean.
However, some accessions of P. coccineus (commonly known as scarlet
runner bean) possess a relatively higher level of resistance. Our
objective was to verify the reaction of 13 known white mold-resistant
P. coccineus germ plasms and determine inheritance of resistance in
accessions PI 433246 and PI 439534. Pinto Othello was crossed with PI
433246, and the resulting interspecific F(1) was backcrossed onto Othello
and allowed to produce F(2) seed. Similarly, pinto UI 320 was crossed with
PI 439534. The F(1) was backcrossed onto UI 320 and allowed to produce F(2)
seed. The two parents, F(1), F(2), and backcross to dry bean of each set were
evaluated in the greenhouse using the straw test at Fort Collins, CO in
2004. All 13 P. coccineus accessions and the two F(2) also were
evaluated using the modified petiole test at Kimberly, ID in 2005. The
white mold reaction of PI 433246 and PI 439534 was
dominant in their respective F(1). The F(2) segregation further indicated that
white mold resistance in PI 433246 and PI 439534 was controlled by a
single dominant gene. Future work should investigate the performance of
our resistant P. coccineus and interspecific breeding lines when
exposed at different growth stages to different types of inoculum under
varying environmental conditions. These resistant and intermediate parents
and selected interspecific breeding lines should be useful for future
improvement of white mold resistance of pinto and other market classes of
dry and green bean.
Identification of Sources of Multiple Disease Resistance in Mini-core
Collection of Chickpea. S. Pande, G. Krishna Kishore, H. D. Upadhyaya
and J. Narayana Rao, International Crops Research Institute for the
Semi-Arid Tropics (ICRISAT), Patancheru 502 324, Andhra Pradesh, India.
Plant Dis. DOI: 10.1094/PD-90-1214. Accepted for publication 12 May 2006.
Chickpea (Cicer arietinum L.) is used extensively in developing
countries for human nutrition due to its high protein content, for cattle
feed, and for improving soil fertility due to its ability to fix
atmospheric nitrogen into a form useable by plants. However, its yields
are very low (approximately 0.8 t ha–1) due to several fungal diseases
attacking the crop throughout the growing season. Among these diseases,
Ascochyta blight (AB), Botrytis grey mold (BGM), Fusarium wilt (FW), and
dry root rot (DRR) are destructive and cause severe losses in grain yield
worldwide. Growing resistant cultivars is the most economical way to
manage these diseases. Cultivars resistant to FW are available, but they
are susceptible to other diseases. Similarly, the available cultivars
moderately resistant to AB, BGM, and DRR are susceptible to FW. Therefore,
our quest continues to identify chickpea accessions with multiple disease
resistance that could be used in breeding resistant cultivars to
successfully combat these four diseases. Through this study, 211 chickpea
accessions were screened for resistance to these diseases using standard
protocol under the controlled environment of a greenhouse. The accessions
were selected to represent the diversity of the entire collection of
chickpea germ plasm. Combined resistance to BGM and FW was found in 11
accessions, 3 were moderately resistant to both FW and DRR, 2 were
moderately resistant to both BGM and DRR, 1 was moderately resistant to
both AB and BGM, 46 were resistant to FW, and 6, 3, and 55 accessions were
moderately resistant to DRR, AB, and BGM, respectively.
Interaction of Fungicide Physical Modes of Action and Plant Phenology
in Control of Stem Rust of Perennial Ryegrass Grown for Seed. W. F.
Pfender, United States Department of Agriculture–Agricultural Research
Service National Forage Seed Production Research Center and Oregon State
University Department of Botany and Plant Pathology, Corvallis 97331.
Plant Dis. DOI: 10.1094/PD-90-1225. Accepted for publication 19 May 2006.
Stem rust is the most economically damaging disease or pest problem for
farmers producing seed of perennial ryegrass or tall fescue. The disease
can be managed with the use of fungicides; however, the number of
fungicide sprays needed to maintain yield varies from year to year. Better
information is needed about the effectiveness of fungicides against stem
rust, particularly the persistence of the fungicide in the plant after it
is applied, and the fungicide’s capability to inhibit infections that
started before application. The two fungicides most commonly used in the
grass seed crop were found to have protective and curative activity, but
to differ in their duration of activity. One of the fungicides also was
found to reduce fungus spore production and viability. A major finding of
this research is that the fungicides differ in their ability to interfere
with a special type of disease spread that occurs within each grass plant
when it is infected with stem rust. The fungicide that reduces spore
viability is capable of inhibiting this within-plant disease spread
regardless of the plant growth stage during which it is applied. In
contrast, the other fungicide has a much narrower time window during plant
growth when it is effective at inhibiting the within-plant disease spread.
Results of these experiments will allow effects of fungicide application
to be incorporated into predictive disease management models, reducing
cost and environmental impact from unneeded sprays.
Root Diseases of Wheat and Barley During the Transition from
Conventional Tillage to Direct Seeding. K. L. Schroeder and T. C.
Paulitz, USDA-ARS Root Disease and Biological Control Unit, Washington
State University, Pullman, WA 99164-6430. Plant Dis. DOI:
10.1094/PD-90-1247. Accepted for publication 2 May 2006.
Root diseases of cereals can limit yields in dryland production areas
of the Pacific Northwest. Wind and water erosion of the soil can also be
problematic, especially in areas of eastern Washington with steep
hillsides. More growers are adopting direct-seeding or no-till practices,
where the crop is seeded directly into the last year’s stubble. This
practice reduces erosion, improves soil quality and organic matter, and
reduces fossil fuel inputs. However, many growers report more root disease
when tillage is reduced or stopped, during the transition from
conventional tillage to direct seeding. This research investigates the
dynamics of root diseases in the conversion from conventional tillage to
direct seeding, using replicated field plots over a 4-year period. Another
study site examined the changes in a long-term direct-seeded plot where
conventional tillage was resumed. During the first 2 years of the
transition from conventional tillage to direct seeding, there were no
major differences in yield and diseases between the two tillage
treatments. However, in the third and fourth years, yields were
significantly reduced in the direct-seeded plots, and Rhizoctonia
solani was in higher population densities compared with the
conventionally tilled plots. However, R. oryzae, Pythium spp., and
Gaeumannomyces graminis var. tritici were not affected by
tillage. In the plots that had been direct seeded for the past 12 years,
bringing back the plow and using conventional tillage did not give any
yield advantage or disease reduction. Further work is needed to understand
the mechanisms behind the shift in disease dynamics.
