SCREENING OF BIOPESTICIDES AND CONVENTIONAL FUNGICIDES
FOR THE CONTROL OF PHYTOPHTHORA ROOT
AND CROWN ROT OF SQUASH IN GEORGIA
Kenneth W. Seebold, Jr.
Plant Pathologist
University of Georgia
160 Plant Science Building
Tifton, GA 31793
Introduction
Phytophthora root and crown rot, caused by Phytophthora capsici, is responsible for
serious losses to growers of summer squash each year. No effective fungicides are currently
labeled for control of this disease, and recommended control practices have had only limited
success. Several biopesticides and synthetic active ingredients with potential to control
Phytophthora crown and fruit rot have been introduced recently and should be evaluated for
efficacy against P. capsici. The objectives of the project were to 1) identify new biopesticides
and fungicides that are efficacious against P. capsici on yellow squash and test materials alone
and in combination to determine potential for disease control, and 2) determine any phytotoxic
effects that new fungicides may incite. The biopesticides evaluated were: ABM-127, strain G41
of Gliocladium virens (BioWorks, Geneva NY), and FNX-100 (Foliar Nutrients, Inc.). Evidence
to suggest that these materials have potential to control Phytophthora crown and fruit rot on
yellow squash has been published. Smith et al. (1990) reported that G. virens, when added to
potting mix, suppressed Phytophthora cactorum on apple seedlings. Rosskopf et al. (2002)
applied PCC-1210 (also known as FNX-100), a formulation of phosphonic acid, to greenhousegrown pepper seedlings on a 10-day schedule and reduced the severity of P. capsici-induced
blight by as much as 100%, depending upon the rate of inoculum used. The fungicides evaluated
included pyraclostrobin, cyazofamid, fenamidone, dimethomorph plus maneb, and mefenoxam.
The majority of these fungicides were reported previously as being active against P. capsici on
yellow squash (Seebold et al., 2002).
Materials and Methods
Summer squash (cv. ‘Dixie’) was direct-seeded with a vacuum planter April 21, 2003
(spring trial) on bare ground at the Blackshank Research Farm in Tifton, GA. The cultivar
‘Destiny III’ was transplanted September 9, 2003 for the fall trial. The field where the trials were
conducted is known to have a high level of P. capsici in the soil. Land was prepared according
to guidelines recommended by the Georgia Cooperative Extension service. Plots were laid out
in a split-plot design with 5 replications. The whole-plot factors were biopesticide type, and subplot factors were fungicide type (Table 1). The treatment total was 18, including untreated
checks. Whole-plot size was 3 beds (2 rows per bed on 36-inch centers) by 15 feet for the spring
trial; single rows were transplanted on each bed for the fall trial. Sub-plots consisted of single
beds within whole-plots. Materials were applied with a CO2-powered backpack sprayer set to
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deliver 60 gallons per acre at 40 psi. With the exception of seed-applied ABM-127, materials
were applied a 7-10 schedule day beginning 7 days after seeding (spring trial) or 14 days after
transplanting (fall trial) for a total of 6 applications. FNX-100 was applied to whole plots before
the application of individual fungicide treatments. ABM-127 was applied at a rate of 2 oz per
cwt of seed before direct seeding (spring) or seeding of transplants (fall).
Plant populations for each plot were recorded at the time of the first
biopesticide/fungicide application. The number of P. capsici-infected plants was counted on a
weekly basis following the first appearance of symptoms, and continued until maximum
mortality was observed in check plots. Weekly evaluations of mortality were used to construct
disease progress curves, from which the area beneath the disease progress curve (AUDPC) was
calculated. Squash were be harvested twice for the spring trial; however, severe infection of fruit
by papaya ringspot virus prevented harvest in the fall trial. Data were be analyzed with the
general linear model procedure (SAS Institute, Cary NC) and means were separated with
Fisher’s protected least significant difference test (P=0.05).
Results and Discussion
No interaction was observed in either trial between biopesticide (whole plots) and
fungicide (sub-plots). Means are therefore presented as averages across biopesticide type or
fungicide type.
Spring trial. The severity of Phytophthora crown and fruit rot (PCFR) was high due to
warm temperatures and high moisture. In terms of efficacy, Ridomil Gold (mefenoxam), Cabrio
(pyraclostrobin), and Ranman (cyazofamid) reduced the severity of PCFR relative to the
untreated control (no fungicide, no biopesticide) (Table 1). Differences in yield between these
treatments and the untreated check were not observed. The onset of disease occurred after the
first harvest of squash and probably played a role in the lack of differences in yield. No effects
on PCFR were observed from either biopesticide product, ABM-127 or FNX-100, in terms of
disease severity or yield.
Fall trial. Disease onset was delayed due to lack of natural rainfall; however,
supplemental overhead irrigation was employed to create conditions favorable to infection by P.
capsici. All fungicide treatments significantly reduced the severity of PCFR compared to the
check (Table 1). Ridomil Gold 4EC was the most effective fungicide treatment. No difference
in severity of disease was found between the untreated control (no fungicide, no biopesticide)
and ABM-127; however, FNX-100 significantly reduced the AUDPC of PCFR. Although the
onset of disease was delayed due to dry weather, the application of overhead irrigation resulted
in higher severity of disease than was seen in the spring trial.
In general, Cabrio, Ranman, and Ridomil gave consistent suppression of PCFR in the two
trials conducted in 2003, and the addition of FNX-100 was beneficial in the fall only. Data from
a single year of testing are inconclusive and indicate that additional testing with FNX-100 is
warranted. Increased rates of FNX-100 or earlier initiation of application could result in
increased efficacy against PCFR when applied alone or with synthetic fungicides, and could
allow for reduced rates or fewer applications of the synthetic compounds.
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Table 1. Effect of foliar-applied compounds and biopesticides on the severity of Phytophthora crown and fruit rot of ‘Dixie’
summer squash – 2003, Tifton GA (PCAP03002).
Spring trial
Fall Trial
Fungicide
Fruit yield per plot
Treatment
Rate / Acre
AUDPCa
No. marketable
Weight (lbs)
AUDPCa
SUB-PLOT EFFECT (FUNGICIDE)
Untreated check
-3.52 a
83.1 abc
42.1 a
12.6 a
Cabrio 20EG
12 oz
1.98 b
81.7 abc
43.1 a
9.0 b
Ranman 400SC +
Silwet L-77
2.75 fl oz
2 fl oz
1.61 b
93.0 ab
42.1 a
9.3 b
Reason 500 SC
7 fl oz
2.61 ab
78.8 bc
36.1 ab
9.5 b
0.06 c
95.8 a
43.9 a
6.5 c
Acrobat 50WP +
6.4 oz
3.26 a
Maneb 75WP
2 lb
WHOLE PLOT EFFECT (BIOPESTICIDE)
ABM-127
2 oz/cwt
2.49 a
68.5 c
31.9 b
9.0 b
84.7 a
41.1 a
10.5 a
No Biopesticide
--
2.29 a
84.8 a
41.1 a
10.3 a
FNX-100
2% v/v
1.70 a
81.0 a
37.4 a
7.2 b
Ridomil Gold 480EC 1 pt
Means followed by the same letter do not differ significantly as determined by Fisher’s protected least significant difference test (P=0.05).
a
AUDPC=area under the disease progress curve, constructed from evaluations taken on 5/23/03, 5/30/03, 6/04/03, and 6/13/03 (spring test); or
10/10/03, 10/17/03, 10/24/03, 10/31/06, and 11/06/03 (fall test).
b
Yield data are the total of two harvests - 5/27/03, and 6/02/03.
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