Distribution and Fungicide Sensitivity of Fungal Pathogens Causing
Anthracnose-like Lesions on Tomatoes Grown in Ohio. Laura J. Gutierrez
Chapin, Ying Wang, Elizabeth Lutton, and Brian B. McSpadden Gardener,
Department of Plant Pathology, The Ohio State University, Ohio
Agricultural Research and Development Center, Wooster 44691. Plant Dis.
DOI: 10.1094/PD-90-0397. Accepted for publication 24 October 2005.
On tomatoes, anthracnose is a disease that is characterized by
circular, sunken lesions on fruit. The size and number of visible lesions
indicative of anthracnose are used as the basis for rejecting shipments of
tomatoes for processing. Thus, the disease needs to be controlled in order
to maximize marketable yield of a tomato crop. While a number of practices
can be used to reduce tomato fruit rots, applying fungicides is most
effective and thus widely used. In this study, the primary causal agents
of anthracnose-like fruit rots and their sensitivity to commonly used
fungicides were investigated. In 2002 and 2003, tomato fields throughout
the state were sampled, and fungi were isolated from fruits
displaying anthracnose-like lesions. Colletotrichum spp., including
Colletotrichum coccodes, were most abundant in our collection,
representing 136 of the 187 isolates. This fungus is generally considered
to be the causal agent of anthracnose. In addition, there were 23
Alternaria, 12 Fusarium, 12 Phomopsis, and 4 Mucor
isolates. In a laboratory investigation, a subset of the Colletotrichum,
Alternaria, and Fusarium isolates caused symptoms similar to
early development of anthracnose on wounded tomato fruit. In vitro
inhibition assays indicated that most Colletotrichum isolates were
sensitive to labeled rates of the fungicides azoxystrobin, chlorothalonil,
and mancozeb. However, some Alternaria isolates were relatively
insensitive to azoxystrobin and chlorothalonil compared to the
Colletotrichum isolates. In addition, most Fusarium isolates
were also more insensitive to azoxystrobin and mancozeb, and most
Phomopsis isolates were not inhibited by azoxystrobin at the levels
tested. The patterns of insensitivity to azoxystrobin and chlorothalonil
were also observed using an excised fruit assay. The fungicides tested are
labeled for control of Colletotrichum and Alternaria, but
not for Fusarium nor Phomopsis on tomatoes. Our results
indicate that multiple pathogen species may cause anthracnose-like
symptoms and that some of these pathogens may not be entirely sensitive to
fungicides commonly used in tomato production.
Control of Benzimidazole- and DMI-Resistant Strains of Cercospora
beticola with Strobilurin Fungicides. G. S. Karaoglanidis,
Hellenic Sugar Industry S.A., Plant Protection
Department, Sugar Factory of Platy, 59032,
Platy Imathias, Greece; and G. Bardas,
Aristotelian University of Thessaloniki, Faculty of
Agriculture, Plant Pathology Laboratory, POB 269, 54006, Thessaloniki,
Greece. Plant Dis. DOI: 10.1094/PD-90-0419. Accepted for publication 19
October 2005.
Cercospora leaf spot, caused by Cercospora beticola, is the most
important foliar disease of sugar beet in warm and humid areas worldwide.
Its control is largely dependent on fungicide applications. However, the
extensive use of fungicides has lead to resistance development to
benzimidazole and to sterol demethylation-inhibiting (DMI) fungicides.
Consequently, there is a need for new compounds that could be introduced
into the spray application programs. Strobilurin fungicides
constitute a relatively novel fungicide class with a new mechanism of
action that possesses an extremely broad spectrum of activity, including
C. beticola. The control efficacy of two new strobilurin
fungicides, trifloxystrobin and pyraclostrobin, against C. beticola
isolates resistant and sensitive to DMI and benzimidazole fungicides, and
the effects on evolution of resistance, were tested in the current study.
We found that both pyraclostrobin and trifloxystrobin provided high levels
of control of all the three isolates used in the study when applied at 10
µg ml(^–1). Control was independent of the isolates’ sensitivity to benomyl
and difenoconazole. In contrast, benomyl and difenoconazole applied at 10
µg ml(^–1) failed to control sufficiently the benzimidazole- and
DMI-resistant isolate, respectively. Moreover, the effects of strobilurin
fungicide applications on the evolution of resistance to benzimidazole and
DMI fungicides were investigated under field conditions. Measurements of
resistance frequencies to benomyl and to difenoconazole showed that
successive applications of benomyl or difenoconazole tended to select for
high frequencies of benzimidazole- or DMI-resistant phenotypes,
respectively. In contrast, applications of either trifloxystrobin or
pyraclostrobin prevented an increase of benzimidazole- or DMI-resistant
phenotypes compared with the plots treated with benomyl or difenoconazole,
respectively, and decreased frequency of resistance compared with
untreated control plots. Such results suggest that the two strobilurin
fungicides tested could be used for the control of benzimidazole- and
DMI-resistant strains of the pathogen. However, further research is
required to determine the efficacy, the optimal application dosages, and
the optimal treatment schedules under field conditions.
Factors Affecting Infection of Yellow Starthistle (Centaurea
solstitialis) by Synchytrium solstitiale, Causal Agent of False
Rust Disease. T. L. Widmer, Research Plant Pathologist, and F.
Guermache, Technician, European Biological Control Laboratory, United
States Department of Agriculture–Agricultural Research Service, Campus
International de Baillarguet, CS 90013, Montferrier sur Lez, 34988 St.
Gely du Fesc Cedex, France. Plant Dis. DOI: 10.1094/PD-90-0425. Accepted
for publication 25 October 2005.
Yellow starthistle is a noxious weed that has invaded the United States
from the Mediterranean region. It is a serious pest of pastures,
rangelands, croplands, natural areas, and recreational areas. Chemicals
can manage yellow starthistle; however, because of economic and
environmental issues, other control methods are sought. A new fungus,
causing orange to red galls, was isolated from leaves of yellow
starthistle in France. The disease caused by this fungus on yellow
starthistle is described and environmental conditions for best infection
are described. In addition, other plants that are infected by this fungus
are mentioned. The result is a new biological agent that potentially can
reduce the growth and spread of yellow starthistle, thus allowing areas to
be restored to their native vegetation.
Cryopreservation of Synchytrium solstitiale In Planta. T. L.
Widmer, Research Plant Pathologist, European Biological Control
Laboratory, United States Department of Agriculture–Agricultural Research
Service, Campus International de Baillarguet, CS 90013, Montferrier sur
Lez, 34988 St. Gely du Fesc CEDEX, France. Plant Dis. DOI:
10.1094/PD-90-0429. Accepted for publication 4 October 2005.
Yellow starthistle is a noxious weed that has invaded the United States
from the Mediterranean region. It is a serious pest of pastures,
rangelands, croplands, natural areas, and recreational areas. Chemicals
can manage YST, but because of economic and environmental issues, other
control methods are sought. A new fungus, causing orange to red galls, was
isolated from leaves of yellow starthistle in France, and this fungus is
being investigated as a biological control agent against this weed. This
article describes a method to store the fungus for a longer period of time
below a freezing temperature. The impact of this study allows research on
this fungus to continue even when the fungus is no longer found in the
field.
Preharvest Chitosan and Postharvest UV Irradiation Treatments Suppress
Gray Mold of Table Grapes. G. Romanazzi, Department of Environmental
and Crop Sciences, Marche Polytechnic University, 60131 Ancona, Italy; F.
Mlikota Gabler, Institute for Adriatic Crops, 21000 Split, Croatia; and J.
L. Smilanick, United States Department of Agriculture–Agricultural
Research Service, Horticultural Crops Research Laboratory, San Joaquin
Valley Agricultural Sciences Center, Parlier, CA 93648. Plant Dis. DOI:
10.1094/PD-90-0445. Accepted for publication 4 November 2005.
Gray mold, caused by Botrytis cinerea Pers., is the most
economically important postharvest disease of table grapes. Currently, in
California, it is controlled primarily by sulfur dioxide fumigation,
although there is a growing consumer demand for food without chemical
preservatives. Chitosan is a natural polymer with a disease-suppressive
activity, resulting from both physical and biochemical mechanisms. The
effectiveness of chitosan treatment, alone or in combination with
ultraviolet-C (UV-C) radiation, to control postharvest gray mold of table
grapes was determined and the influence of such treatments to catechin and
resveratrol contents in berry skins was assessed. Clusters of cvs.
Thompson Seedless, Autumn Black, and Emperor sprayed in the vineyard with
1% chitosan, then harvested daily and, soon after, inoculated with B.
cinerea showed a reduced decay incidence and disease severity.
In another experiment, grape berries sprayed with
chitosan in the vineyard were irradiated with UV-C, then inoculated with
B. cinerea 2 days later. Combined chitosan and UV-C treatments
applied to cv. Autumn Black or selection B36-55 were synergistic in the
reduction of gray mold incidence and severity compared with either
treatment alone. Preharvest chitosan treatment did not increase
concentration of catechin or resveratrol in berry skin. Conversely, UV-C
irradiation, alone or combined with chitosan treatment, induced catechin
in Autumn Black berries and trans-resveratrol in both cv. Autumn
Black and selection B36-55.
Use of Entomopathogenic Nematodes and Thyme Oil to Suppress
Plant-Parasitic Nematodes on English Boxwood. Enrique E. Pérez,
Research Scientist, Monsanto Company, 700 Chesterfield Parkway West,
Chesterfield, MO 63017; and Edwin E. Lewis, Associate Professor,
Departments of Nematology and Entomology, University of California, Davis,
CA 95616. Plant Dis. DOI: 10.1094/PD-90-0471. Accepted for publication 25
July 2005.
English boxwood is one of the most common woody ornamental plants found
in landscapes. Boxwood decline is a serious problem in field-grown nursery
stock and landscape plantings, and has been attributed to unfavorable
environmental conditions combined with plant-parasitic nematodes and
certain root rot fungi. Boxwood plants may require three or four growing
seasons to develop decline symptoms, which can include bronzing of
foliage, stunting, and root necrosis. By the time these symptoms are
obvious, up to 50% of the root system may be necrotic. The options for
controlling the nematodes that contribute to boxwood decline are very
limited, especially if chemical nematicides are not available. Our
objective was to determine the level of nematode control offered by three
commercially available, environmentally friendly alternatives to chemical
nematicides: two species of entomopathogenic nematodes and a nematicide
based on the essential oil of common thyme. The two entomopathogenic
nematode species were Steinernema feltiae and S. riobrave.
The test was conducted during the summers of 2001 and 2002 on
field-planted English boxwoods that were between 45 and 55 years old,
exhibited severe symptoms of boxwood decline, and had significant
populations of four species of plant-parasitic nematodes in the root zone
of each plant. Plant-parasitic nematode populations were evaluated 7, 30,
and 60 days after treatment. Both the essential oil of thyme and S.
riobrave reduced the population growth rates of most of the
plant-parasitic nematode species for up to 30 days after treatment, and
some of the species for up to 60 days after treatment. S. feltiae
was less effective than the other treatments. None of the treatments
eliminated plant-parasitic nematode populations. Because the plants were
severely damaged at the time of treatment, their recovery was very
unlikely regardless of the efficacy of treatments used. Therefore, we were
unable to determine whether these treatments would be effective when used
curatively. Their best use might be as preplant treatments when boxwoods
are replaced in areas where boxwood decline has occurred previously.
