Interactions Between Herbicides and Cotton Seedling Damping-off in the Field
A. Heydari, Plant Pathology Department
I. J. Misaghi, Plant Pathology Department
We studied the impact of three pre-plant herbicides, trifluralin, pendimethalin
and prometryn on the incidence and the development of Rhizoctonia
solani-induced cotton seedling damping-off in the field. In a field experiment
conducted in Safford, Arizona, pre-plant application of pendimethalin or
prometryn but not trifluralin caused significant (P <0.05) increases in
disease incidence. In another field experiment in Tucson, Arizona,
significant (P <0.05) increase in disease incidence was observed in plots
treated with prometryn and not in those treated with pendimethalin and
trifluralin . In Tucson field experiment, application of herbicides also
affected disease development as judged by the slope of disease progress curves.
Environmental factors play important roles in the incidence and development
of soilborne plant diseases (Agrios, 1988). Soil factors that can potentially
affect soilborne plant diseases in the field include: moisture, temperature,
texture, pH, soil atmospheric composition and presence of agricultural chemicals
such as herbicides.
Herbicides are reported to affect the incidence and severity of plant
diseases (Altman and Campbell, 1977; Altman and Rovira, 1989; El-Khadem and
Papavizas, 1984; El-Khadem et al., 1984; El-Khadem et al., 1979; Rovira
and McDonald, 1986). The cereal cyst disease caused by a nematode (Altman
and Rovira, 1989), take-all of wheat caused by fungus Gaeumannomyces graminis
var tritici (Rovira and McDonald, 1986) and sugar beet damping-off
caused by fungus Rhizoctonia solani (Altman and Campbell, 1977) increased
after application of herbicides trifluralin, chlorsulfuron and cycolate,
respectively. Application of herbicides trifluralin and dinitramine
to the field soil also increased cotton seedling damping-off caused
by R. solani (El-Khadem et al., 1979).
In contrast the incidence of cotton seedling disease caused by fungus
Fusarium oxysporum vasinfectum decreased after application of
trifluralin, dinitramine, fluometuron, diuron, dalapon and prometryn to the
field soil , while the incidence of R. solani -induced cotton seedling
damping-off was not significantly affected by these herbicides
(El-Khadem et al., 1984). Application of herbicides EPTC and linuron
also decreased the incidence of post emergence but not pre-emergence cotton
seedling damping-off (El-Khadem and Papavizas, 1984).
We investigated the impact of three widely used pre-plant herbicides,
pendimethalin, prometryn, and trifluralin on the incidence and the development of
R. solani - induced cotton seedling damping-off. The disease is important
on cotton (Brown and McCarter 1976), often resulting in a substantial
reduction in cotton stand.
Materials and Methods
Three herbicides, trifluralin (treflan) [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)-benzenamine], pendimethalin (prowl) [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], and prometryn (caparol) [N,N-bis(1- methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine] were tested
in this study. These herbicides are currently registered for use as pre-emergence
herbicides on many crops including cotton.
We established two field experiments, with plots arranged in a
randomized block design, and managed them according to the procedures
followed by cotton growers in Safford and Tucson, Arizona in April 1996.
The soil at the Safford and Tucson experimental sites were loam soils
containing 14% clay, 36% silt, 50% sand and 15.1% clay, 33.0% silt, and
51.9% sand, respectively. There were four treatments each with four
replicates in each experiment. Each replicate plot was a 10-m long section of a
bed. Beds were 100 cm apart. Trifluralin, pendimethalin or prometryn was
sprayed in a 60-cm band on the top of each plot at the recommended field
concentration using a CO2 -pressurized back pack sprayer. Herbicides were
incorporated into the top 5 to 7 cm of soil with a rotomulcher shortly after
application. Plots were infested with R. solani inoculum. according to the
following procedures: One kg of barley seeds were wetted with 500 ml of water
and autoclaved at 15 psi for 60 min. Seeds were inoculated with R. solani and
incubated at 25 C for three weeks. Seeds were air dried for 24 h and ground
with a mill through a 3-mm sieve and were stored in a paper bag at 25-27 C in
the laboratory. Eight g of the fungal inoculum were mixed with 200 g of the
field soil and sprinkled into the furrow of each 10-m-long replicate plot shortly
before sowing. The incidence and the development of R. solani-induced cotton
seedling damping-off were assessed by counting the number of emerged seedlings
(stand count ) in each plot 15, 25 and 50 days after planting.
The stand count data in herbicide-treated and untreated plots for two field
experiments were analyzed by analysis of variances (ANOVA) and means were
compared by Duncan multiple range test using COSTAT (Cohort Software,
Berkeley, CA). Slopes of disease progress curves (change in stand or
mortality over time) was determined by regression analysis using COSTAT.
Slopes of disease progress curves for different treatments in each experiment
were compared using procedures described by Campbell and Madden.
On all sampled, non-emerged and emerged seedlings, typical
R. solani -induced damping-off
symptoms were observed in both field experiments. Radicles, roots and lower
stems of all
sampled seedlings yielded R. solani on PDA. Other cotton seedling
were not recovered.
The cotton seedling damping-off incidence was affected by two of three
test herbicides in
both field experiments (Table 1 and
2). The stand count in plots treated with
prometryn in the Safford field experiment was significantly (P <0.05) reduced
by 29.6, 38.9,
46.7% and 27.2, 30.2, 39.1%, relative to the control (no herbicide), 15, 25
and 50 days after
sowing, respectively (Table 1). The change in the stand count in plots
trifluralin in the Safford experiment was not significant (P >0.05).
The stand counts in
plots treated with prometryn in the Tucson field experiment was significantly
reduced by 40.6, 49.5 and 54.4% relative to the control, 15, 25 and 50
days after sowing,
respectively (Table 2). Pendimethalin
and trifluralin did
not cause significant changes in the stand count in the Tucson field
Test herbicides also affected disease development in one field experiment as
judged by the
slopes of disease progress curves (Table 1 and
2). In the Tucson field experiment
trifluralin but not prometryn affected disease development
significantly (Table 2). Disease
development was not affected by any herbicide in the Safford experiment.
It is concluded that pendimethalin and prometryn, that are currently being
used on cotton, may cause significant (P<0.05) increases in the
incidence of R. solani -induced cotton seedling damping-off in the field.
These findings are in agreement with the results of previous studies on
the effect of herbicides on the incidence and severity of some plant diseases
including cotton seedling diseases (Altman and Campbell, 1977; Altman and
Rovira, 1989; El-Khadem and Papavizas, 1984; Miller et al., 1979;
Moustafa-Mahmoud et al., 1993; Rovira and McDonald, 1986).
We do not know how herbicides cause changes in damping-off incidence.
The phenomenon could be due to the effect of herbicides on plant
(Neubauer and Avizohar-Hershenson, 1973; Pinckard and Standifer, 1966),
on pathogen (El-Khadem and Papavizas, 1984; El-Khadem et al., 1984), on
antagonistic microorganisms (Heydari et al., 1997) and/or on
the interactions among these entities. Herbicide-mediated changes in the
plant may occur both at the physical and biochemical levels
(Altman and Campbell, 1977).
Herbicides may affect pathogens directly (Altman and Campbell, 1977;
El-Khadem et al., 1984; El-Khadem et al., 1979). Herbicides are
to encourage (El-Khadem et al., 1979) or discourage the pathogenic
activity of R. solani. The effect could be stimulatory or inhibitory.
For example, in R. solani-sugar beet combination, herbicide cycolate may
interfere with the growth of the fungus and at the same time may enhance root
exudates. In such cases, the impact of the herbicide on the disease is
determined by the balance of stimulatory and inhibitory effects (Altman and
Campbell, 1977). In our study, the growth of R. solani in vitro was not
significantly affected by any of three test herbicides.
Alterations in the outcome of plant-pathogen interactions in the
presence of herbicides may also be due to the effect of herbicides on
naturally occurring microbial antagonists of pathogens (Katan and Eshel, 1973).
Herbicides tested in this study caused a shift in bacterial communities in
cotton rhizosphere including some biocontrol-active bacteria
(Heydari et al., 1997).
Trifluralin and prometryn effects on cotton seedling damping-off have been
variable. For example prometryn which increased cotton seedling damping-off
in this study did not change the incidence of this disease in a previous study
(El-Khadem et al., 1984). Moreover, trifluralin that has been reported
to increase cotton seedling damping-off (Moustafa-Mahmoud et al., 1993;
Neubauer and Avizohar-Hershenson, 1973) did not affect the disease in our study
and in a previous study (El-Khadem et al., 1984). Differential responses
due to the differences in soil moisture, soil temperature, herbicide
concentration, R. solani races, cotton varieties, the composition of
rhizosphere microflora and the rate of herbicide inactivation. The
development of tolerance to herbicides by pathogens as a result of long-term
herbicide use may also be responsible.
Results of the present study provide additional evidences that a pesticide
which is being used against a specific pest may encourage the activity of
another pest and argue in favor of Integrated pest management (IPM)
strategies for managing pest problems. However, the selection of an ideal
pesticide (one which is not harmful to the crop and to non-target beneficial
organisms) is difficult because of the number and diversity of pests involved
in any crop in a region. For example, in addition to insects and weeds, cotton
seedlings are damaged by a number of soilborne pathogens besides R. solani.
Although R. solani-induced cotton seedling damping-off was shown in this
study to increase in the presence of pendimethalin and prometryn, it is not
known how other soilborne cotton diseases respond to these herbicides. The
development of IPM requires a knowledge of the impact of a selected pesticide
not only on its intended target but also on plants as well as on beneficial
and harmful microorganisms and insects.
We thank W. B. McCloskey of the Department of Plant Sciences for providing
herbicides and valuable advices in field treatment, and L. J. Clark and E. W.
Carpenter from Safford Experimental Station for their valuable cooperations in
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This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture, The University of
Arizona, Tucson, Arizona, 85721.
Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az100610b.html
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