An Accurate In Vitro Assay for High-Throughput Disease
Testing of Phytophthora infestans in Potato. Sanwen
Huang, Vivianne G. A. A. Vleeshouwers, Richard G. F. Visser, and Evert
Jacobsen, Graduate School of Experimental Plant Sciences, Laboratory of
Plant Breeding, Department of Plant Sciences, Wageningen University, P.O.
Box 386, 6700 AJ Wageningen, the Netherlands. Plant Dis. DOI:
10.1094/PD-89-1263. Accepted for publication 27 June 2005.
Research and breeding for resistance to Phytophthora infestans, the
causal agent of the devastating late blight disease in potato (Solanum
tuberosum), requires extensive disease assessments. Especially for
large-scale experiments such as recombinant screenings for genetic mapping
of novel resistance genes, a quick and space-effective method is required.
Potato breeding clones and collections of wild Solanum germ plasm
are often maintained in vitro, and this environment is extremely favorable
for late blight disease development. In this paper, we describe the
development of a high-throughput in vitro disease testing assay. We
compared the new method with the well-established detached-leaf assay and
proved that the in vitro assay is accurate and reliable, and can routinely
be used for investigating the qualitative interaction between potato and
P. infestans.
Two Convenient Methods to Evaluate Soybean for
Resistance to Sclerotinia sclerotiorum. Y. Chen,
Research Associate, and D. Wang, Assistant Professor, Department of Crop
and Soil Sciences, Michigan State University, East Lansing 48824. Plant
Dis. DOI: 10.1094/PD-89-1268. Accepted for publication 5 July 2005.
Sclerotinia stem rot is a major soybean disease in north-central regions
of the United States and in southern Canada. It can cause substantial
yield losses of 170 to 330 kg/ha for every 10% increase in Sclerotinia
stem rot incidence. Using resistant soybean cultivars is the most
economical and efficient method to control the disease. Evaluation of
soybean for resistance to the disease with currently available methods is
labor intensive and the results can be inconsistent among repeated tests.
The objective of this research was to develop a low-cost and
high-efficiency greenhouse inoculation method that can generate a
consistent result. We developed a spray-mycelium method in which mycelia
were cultured in liquid potato dextrose broth and homogenized before
spraying onto the soybean leaves. We also developed a drop-mycelium method
in which a drop of homogenized mycelium suspension was dropped on the tips
of main stems. The results obtained with the spray-mycelium and
drop-mycelium inoculation methods were significantly correlated with the
results obtained with a commonly used method, the cut-petiole inoculation
method. The spray-mycelium and the drop-mycelium methods used less than
20% of the inoculation time used by the cut-petiole method and cost less
than 30% of the cost of the cut-petiole method in terms of materials used.
Both of these new methods are low cost, efficient, and reliable and can be
valuable for large-scale evaluation of germ plasm and breeding lines for
resistance to Sclerotinia stem rot in a greenhouse or other similar
facilities.
Sensitivity of Brazilian Isolates of Phytophthora
infestans to Commonly Used Fungicides in Tomato and Potato Crops.
Ailton Reis, Fabiana H. S. Ribeiro, Luiz A. Maffia, and Eduardo S. G.
Mizubuti, Departamento de Fitopatologia, Universidade Federal de Viçosa,
36570-000, Viçosa-MG, Brazil. Plant Dis. DOI: 10.1094/PD-89-1279. Accepted
for publication 22 June 2005.
Late blight, caused by Phytophthora infestans, is one of the most
devastating foliar diseases of potato and tomato in Brazil. Disease
management is based heavily on both systemic and protectant fungicide
applications. The population of P. infestans in Brazil
comprises two clonal lineages with high host specificity, US-1 and BR-1.
US-1 causes late blight on tomato, whereas BR-1 affects potato. Lately,
growers have noticed reduction in chemical control efficacy which could be
associated with shifts in fungicide sensitivity in P. infestans
populations. However, no thorough study on fungicide sensitivity was
carried out in Brazil. Isolates of the US-1 and BR-1 lineages were
collected from tomato and potato fields of two main producing regions of
Brazil (south and southeast) and tested for sensitivity to the systemic
fungicide metalaxyl, plus the three protectant fungicides mancozeb,
chlorothalonil, and cymoxanil. For metalaxyl, one agar test and two
leaf-disc tests were carried out. For all tests, metalaxyl-insensitive,
-intermediately insensitive, and -sensitive isolates, of both lineages,
were detected. There was no association among metalaxyl sensitivity and
region, lineage, or host. All isolates were sensitive to protectant
fungicides. Late blight management should be carried out carefully in
order to avoid selection of insensitive individuals.
Managing the Race Structure of Phytophthora
parasitica var. nicotianae with Cultivar Rotation. M.
J. Sullivan, National Weed Management Laboratory, United States Department
of Agriculture APHIS PPQ CPHST, Fort Collins, CO 80526; T. A. Melton,
Associate State Program Leader, AgNR/CRD, Cooperative Extension Service,
North Carolina State University, Raleigh 27695-7602; and H. D. Shew,
Department of Plant Pathology, North Carolina State University, Raleigh
27695. Plant Dis. DOI: 10.1094/PD-89-1285. Accepted for publication 10
July 2005.
Black shank, caused by the soilborne pathogen Phytophthora parasitica
var. nicotianae, is one of the most destructive diseases of tobacco
(Nicotiana tabacum) in the southeastern United States. Races 0 and
1 of the black shank pathogen have been reported in North Carolina,
but race 0 has been the predominant race present in North Carolina since
the 1930s. The recent widespread deployment of tobacco cultivars with
single-gene (Ph) resistance to race 0 has resulted in a rapid
increase in the occurrence of race 1. Moderate to high levels of partial
resistance also are available to growers. Cultivar-rotation studies were
conducted in grower fields to assess how different levels and types of
resistance affected the race structure of the pathogen. In two fields
where race 1 initially was present, a high level of partial resistance was
most effective in reducing disease and decreasing the proportion of race 1
in the pathogen population. In a field with both races present, the
deployment of single-gene resistance resulted in intermediate levels of
disease control and race 1 became the predominate race. Single-gene
resistance was very effective in suppressing disease in a field where race
1 was not initially detected; however, race 1 was recovered after only one
growing season and rapidly increased throughout the field. A rotation of
cultivars with single-gene resistance and cultivars with a high level of
partial resistance provides the most effective approach to black shank
management by reducing disease incidence and minimizing race shifts in the
pathogen.
Influence of Epidemiological Factors on the
Bioherbicidal Efficacy of Phomopsis amaranthicola on Amaranthus
hybridus. E. N. Rosskopf and C. B. Yandoc, United
States Department of Agriculture–Agricultural Research Service, United
States Horticultural Laboratory, Fort Pierce, FL 34945; and R. Charudattan
and J. T. DeValerio, Plant Pathology Department, University of Florida,
Gainesville 32611. Plant Dis. DOI: 10.1094/PD-89-1295. Accepted for
publication 12 July 2005.
Weeds belonging to the genus Amaranthus, known as pigweeds and
amaranths, cause significant losses to a number of agronomic and
horticultural crops. Many weeds in this group have resistance to commonly
used herbicides. A host-specific fungal plant pathogen, Phomopsis
amaranthicola, is under investigation for development as a biological
control agent for these weeds. Previous studies have found that the host
range of the pathogen is restricted to the genus. The studies presented
here establish the environmental conditions most conducive for the disease
to develop, as well as evaluating additives for potential enhancement of
disease.
Efficacy of Hot Water and Chlorine for Eradication of
Cladosporium variabile, Stemphylium botryosum, and
Verticillium dahliae from Spinach Seed. Lindsey J. du Toit,
Vegetable Seed Pathologist, and Pablo Hernandez-Perez, Former Graduate
Research Assistant, Washington State University–Northwestern Washington
REC, Mount Vernon 98273-4768. Plant Dis. DOI: 10.1094/PD-89-1305. Accepted
for publication 15 July 2005.
Cladosporium variabile, Stemphylium botryosum, and
Verticillium dahliae are seedborne and seed-transmitted fungal
pathogens of spinach. Spinach seed treatments in chlorine (1.2% NaOCl) for
10 to 60 min or in hot water (40, 45, 50, 55, and 60ºC) for 10 to 40 min
were evaluated for their potential to eradicate these fungi from spinach
seed. C. variabile and V. dahliae were eradicated by
chlorine treatment for as little as 10 min. Chlorine treatment reduced the
incidence of S. botryosum, but this fungus was not eradicated even
after 60 min in chlorine. Seed germination was not affected adversely by
chlorine treatment. However, germination was reduced by hot water
treatment at 50ºC for 30 min or longer, or at 55 or 60ºC for 10 min or
longer. C. variabile was eradicated from seed treated at 40ºC for
only 10 min. V. dahliae was eradicated from seed treated at 55ºC
for at least 30 min or at 60ºC for 10 min or longer. S. botryosum
was eradicated from a lightly infected seed lot by hot water treatment at
55 or 60ºC for 10 min or longer, but not from two heavily infected lots,
even when heated to 60ºC for 40 min. Using precisely controlled
parameters, chlorine or hot water seed treatments can be used to eradicate
C. variabile and reduce the incidence of S. botryosum and
V. dahliae in spinach seed without damaging germination.
Quantitative Assessment of Anguina sp. and
Rathayibacter rathayi in Dactylis glomerata Seed Production
Fields in Oregon and Estimates of Yield Loss. Steve C. Alderman,
United States Department of Agriculture–Agricultural Research Service,
National Forage Seed Production Research Center; Cynthia M. Ocamb,
Department of Botany and Plant Pathology, Oregon State University,
Corvallis, 97331, and Mark E. Mellbye, Oregon State University, Linn
County Extension, Albany 97321. Plant Dis. DOI: 10.1094/PD-89-1313.
Accepted for publication 15 July 2005.
Anguina sp. is a nematode that infests the seed head of orchardgrass
and forms galls that replace the seed. When Anguina sp. infects a
plant, it can carry with it the bacterial pathogen Rathayibacter
rathayi. R. rathayi also infests the orchardgrass seed head and
forms galls that replace the seed or, more often, it proliferates to form
a sticky yellow growth that exudes from infected flowers. Disease
assessments were conducted in five orchardgrass fields in Oregon in 2003
and 2004 to estimate the yield loss associated with the nematode and
bacterial pathogens alone and in combination. Seed loss from Anguina
sp. was less than 0.2%. However, in combination with Rathayibacter
spp., seed losses from 0.1 to 8% were recorded, indicating that the
association of Anguina and Rathayibacter spp. can cause
significant seed yield losses in orchardgrass. In addition, the number of
Anguina galls remaining in the field after harvest was estimated.
As many as 40 galls of Anguina per square meter were returned to
the field during harvest, equating to about 48,000 nematodes per square
meter.
Suppression of Grapevine Powdery Mildew by a
Mycophagous Mite. Heather S. Melidossian and Robert C. Seem,
Department of Plant Pathology, Greg English-Loeb, Department of
Entomology, and Wayne F. Wilcox and David M. Gadoury, Department of Plant
Pathology, Cornell University, New York State Agricultural Experiment
Station, Geneva 14456. Plant Dis. DOI: 10.1094/PD-89-1331. Accepted for
publication 10 August 2005.
Powdery mildew is the most widespread and destructive disease of
cultivated grapevines worldwide. Although wild Riverbank Grape is
susceptible to the powdery mildew pathogen (Uncinula necator),
these vines rarely become diseased in natural settings. We found that
these wild vines harbored large populations of mildew-eating (mycophagous)
mites (Orthotydeus lambi) that grazed upon the mildewed leaf
surfaces and provided a surprising degree of natural disease suppression.
In an experimental vineyard of mildew-susceptible European wine grapes,
the mites were able to suppress powdery mildew on grape berries by 50 to
80%. While this does not equal the degree of suppression of the best
chemical fungicides, the mites do this for free. A potential limiting
factor is the harmful effect of certain fungicides upon the mites.
However, we have developed “mite-friendly” fungicide programs that
preserve the natural populations of beneficial mites and allow them to
significantly augment control of powdery mildew provided by deployment of
fungicides.
Molecular, Physiological, and Host-Range
Characterization of Acidovorax avenae subsp. citrulli
Isolates from Watermelon and Melon in Israel. Saul Burdman and
Nadia Kots, Department of Plant Pathology and Microbiology, The Hebrew
University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel; Giora
Kritzman, Department of Plant Pathology, Agricultural Research
Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel; and
June Kopelowitz, Savyon Diagnostics Ltd., 3 Habosem St., Ashdod 77610,
Israel. Plant Dis. DOI: 10.1094/PD-89-1339. Accepted for publication 9
July 2005.
Acidovorax avenae subsp. citrulli is the causal agent of
bacterial fruit blotch (BFB) of cucurbits. BFB gained importance after
devastating outbreaks in watermelon fields in the United States in the
late 1980s. Since then, BFB has emerged as a major threat to the
watermelon industry. In recent years, significant economic losses caused
by the bacterium on other cucurbits, including melon, cucumber, squash and
pumpkin, have been reported in different parts of the world. A. avenae
subsp. citrulli strains can be divided into two differentiated
groups: group I includes strains that are mainly associated with
nonwatermelon cucurbits; whereas group II includes the typical watermelon
BFB-causing strains. In Israel, natural BFB outbreaks occurred in several
watermelon and melon fields and greenhouses from different regions during
2000 to 2003, causing economic losses. The objective of this work was to
characterize A. avenae subsp. citrulli isolates associated
with these outbreaks. Twelve isolates were characterized in terms of their
aggressiveness on watermelon, melon, and cucumber by pathogenicity assays
of seedlings and fruits as well as by seed transmission assays. The
isolates were also analyzed by GC-FAME, substrate-utilization profiles,
and DNA-fingerprinting techniques. Results from this study support the
existence of two well-distinguished groups of A. avenae subsp.
citrulli and demonstrate that isolates from both groups have been
introduced to Israel. Pulse-field gel electrophoresis revealed that the 12
analyzed isolates can be divided into five different haplotypes, of which
four were previously unreported. Although economic losses caused by BFB
were limited to a few of the affected fields, it is too early to predict
future impact of the disease on watermelon and melon production in Israel.
Biological Control of Pierce’s Disease in the
Vineyard with Strains of Xylella fastidiosa Benign to Grapevine.
D. L. Hopkins, University of Florida, Mid-Florida Research and
Education Center, Apopka, 32703. Plant Dis. DOI: 10.1094/PD-89-1348.
Accepted for publication 9 August 2005.
Pierce’ disease (PD) is the primary factor in the southeastern United
States limiting the development of a grape industry based on the
high-quality European grapes that are grown in California and Europe. In
the past, PD epidemics erupted sporadically to cause severe losses in
California vineyards. The recent introduction and development of the
glassy-winged sharpshooter in California makes PD an increased threat to
California’s grape industry. The only effective control of PD is
resistance, which is not present in the high-quality, European-type bunch
grapes. Earlier research had shown that inoculations of European-type
grapes with strains of the PD bacterium that were weakly virulent
protected these plants against virulent PD strains in the greenhouse. In
this study, these benign strains were evaluated for biological control of
PD in the vineyard in Florida. One benign strain, which occurred naturally
in elderberry in Florida, provided excellent biological control of PD in
the highly susceptible grape ‘Cabernet Sauvignon’ for 4 years in a central
Florida vineyard. The nontreated vines were all dead by the end of the
trial. Inoculation with benign strains of the PD bacterium, or closely
related strains, appears to have the potential for the biological control
of PD in commercial vineyard in Florida and other areas.
