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Homestead TREC Research Report SB84-1 21 September 1984
Efficacy of Selected Fungicides Against Snap Bean Rust:
a Preliminary Report on Alternative Control Programs
Compatible with Canadian Fungicide Tolerances
KEN POHRONEZNY, Associate Professor (Pest Management), JOYCE
FRANCIS, Biologist, and JAMES S. REYNOLDS, Agricultural Technician,
IFAS, University of Florida, Tropical Research and Education Center
In south Florida, rust, caused by the fungus Uromyces phaseoli
(Reben) Wint., is a serious and recurring problem on snap beans,
Phaseolus vulgaris L.(3,4). Both bush and pole types are affected.
In the Homestead area, rust usually first appears in early to mid
January, and the disease becomes progressively more severe during
the February to May spring crop. Less frequently, powdery mildew,
caused by Oidium spp, also results in economic losses in the spring
Growers commonly spray every 5-7 days with one of the ethylene
bis-dithiocarbamate (EBDC) fungicides. Commonly available EBDC
fungicides include maneb, mancozeb, and zineb. The EBDC's are
usually tank-mixed with sulfur for increase efficacy in "rust"
Recently, Canadian regulatory office ls d to Florida
winter beans entering their country with more th n pm EBDC
residue. This extremely low tolerance irtuady el A EBDC
field use on Florida beans, especially "rust ~e son 4/ any
produce destined for Canada. Since gro e' cannot ,agmat c lly
determine final destination after selling ~ okers,6 and A
substantial portion of south Florida beans go. Cnada ~ -13%),
registered alternatives to regular spraying wit and sulfur
were sought. In addition, several non-registered, se'qa -inhibitor
fungicides (bitertanol, triadimefon) were tested for t control
MATERIAL AND METHODS
Beans,cv Sprite (Expt. I), or cv Savor (Expt. II), were
direct-seeded into a Rockdale series soil (pH approximately 7.8) at
the TREC, Homestead. Planting dates were 8 Dec 1983 (Expt. I) and
1 March 1984 (Expt. II). Test plots consisted of 4 rows, 20 ft (6.1
m) long on 3 ft (.91 m) centers. Ten treatments were replicated 4
times in a randomized complete block design. Thirty ft (9.1 m) were
left between blocks to reduce interplot interference.
Table 1. Fungicides treatments, snap bean rust control experiments, Homestead
1) water control
Manzate 200 80WP
Manzate 200 80WP
XWhenever mancozeb was sprayed, 8 oz/acre (545.4 ml/ha) Nu-film-17
adjuvant was used.
YBitertanol and Triadimefon applications were made with 1 pt/acre
(1.17 L/ha) Penetrator as an adjuvant.
SStarted at first detectable symptoms.
All chemicals were applied with a2tractor-mounted hydraulic
boom sprayer at a pressure of 275 lb/in (190 nt/cm ) delivering 100
gal finished spray/acre (932 L/ha). The arrangement of cone nozzles
was 5 per 2 bean rows, 1 over the top of the plants and 4 on
drop-nozzle extensions. Treatment spray dates for Expt. I were
January 5, 12, 19, 26, and February 2 and 9, 1984; Expt. II plots
were sprayed March 22, 30, April 5, 12, 20, and 27, 1984.
Bitertanol-on-demand plots received one treatment (Expt. I) 9
February 1984 and 4 treatments (Expt. II) 5, 12, 20, 27 April 1984.
Weed control consisted of a pre-emergence application of
trifluralin at 0.75 lb a.i./acre (0.84 kg/ha) and metolachlor at 1.15
Ib a.i./acre (1.29 kg a.i./ha), and periodic mechanical cultivation
and hand-weeding. Benomyl applications were made as recommended for
Sclerotinia control (4), and applications of insecticide, primarily
acephate, methomyl, and endosulfan, were made as needed, based on
field scouting results (3,4).
Once rust appeared, weekly ratings were made of disease
severity by counting uredinia on the underside of 5 trifoliate
leaves/plot taken from the middle of the the plant canopy. Count
data were tested for homogeneity of variances (1) and appropriate
transformations were used when needed in the analysis of variance.
Plots were harvested once, 16 Feb 1984 (Expt. I) and 1 May 1984
(Expt. II). Forty row-ft (12.2 row-m) were harvested from the middle
of the plots for yield determinations. Records were taken of the
weight of marketable beans and culls. The culls were further sorted
into several defect categories and the percent culls were calculated.
For both yield data and uredinia counts, where F-tests indicated a
significant difference in an array, means were separated by Waller-
Duncan's method at P<0.05.
RESULTS AND DISCUSSION
Rust pressure was very light in Expt. I, making it difficult to
evaluate fungicide efficacy. Rust did not appear until 6 Feb 1984
and levels remained low until harvest. Therefore, no evaluations of
relative efficacy of the test materials could be made for Expt. I.
In both experiments powdery mildew levels were too low to assess
fungicide efficacy for this disease.
In Expt. II, rust pressure became intense by the end of the
crop, near harvest reaching an average of about 3600 uredinia per 5
trifoliate leaves in the control plots. All fungicide treatments re-
duced rust levels (Table 2). For the most part, using mancozeb and
sulfur up to flowering and then switching to chlorothalonil was as
effective for control of rust as using mancozeb and sulfur throughout
the entire crop. Chlorothalonil as a weekly spray was statisically
equal to mancozeb and sulfur treatments. For the mancozeb/sulfur
full-season treatments, Canadian tolerances of 0.1 ppm EBDC were
exceeded for every residue sample (personal communication, V.
Middlehem, Fla. Dept. Agriculture, Chemistry Div.). When
mancozeb/sulfur was discontinued in favor of 2.5 lb a.i/acre
chlorothalonil at flowering, the Canadian tolerances were never
exceeded (V. Middlehem, personal communication). Based on 1984 price
quotes from a local chemical supply house, a grower might expect the
following comparative costs per crop per season for several of the
2.5 lb a.i. chlorothalonil only $76.14
1.25 lb a.i. chlorothalonil only $38.07
mancozeb/sulfur tank-mix only $25.56
mancozeb/sulfur switched to
2.5 lb a.i. chlorothalonil at
We feel that more local testing is needed to ensure that the 1.25 lb
a.i. chlorothalonil treatment will be consistently efficacious.
Therefore, BASED ON CANADIAN RESIDUE REQUIREMENTS, RUST CONTROL
EFFICACY, AND ECONOMICS, USE OF AN EBDC COMPOUND (SUCH AS MANEB)
TANK-MIXED WITH SULFUR UNTIL FLOWERING, FOLLOWED BY 2.5 LB A.I.
CHLOROTHALONIL UNTIL THE PHI, PRELIMINARILY REPRESENTS THE MOST
VIABLE ALTERNATIVE FUNGICIDE PROGRAM FOR BEAN GROWERS SHIPPING TO
The sterol inhibitor fungicides, bitertanol and triadimefon,
were substantially superior to the other treatments for control of
rust (Table 2). Even near harvest, no rust uredinia were seen in
plots treated with 0.125 or 0.250 lb a.i./acre bitertanol. However,
applications of triadimefon, especially at the higher rate, resulted
in substantial reductions in yield (Table 3), as has been reported
elsewhere (2). Besides losses in gross yields, triadimefon plots
showed an increase in percentage of distorted, twisted, and misshapen
pods, resulting in large cull rates for these treatments.
The systemic fungicide bitertanol used on-demand (i.e., weekly
applications initiated at first sign of rust, based on twice-a-week
scouting visits ), was as efficacious for rust control as
calendar-determined sprays. This suggests that growers whose fields
are scouted by well-qualified integrated pest management personnel
could benefit economically by avoiding sprays thatmmight be applied
prior to attack by rust. The combination of excellent rust control
S and lack of any phytotoxicity and yield-reduction suggests that
bitertanol has promise as a bean rust fungicide, pending
Environmental Protection Agency registration.
Table 2. Mean number of uredinia per 5 trifoliate leavesy on 3 sampling dates, rust
efficacy Expt. II, 1984, Homestead TREC.
rates (lb a.i./acre) 9 April 16 April 23 April
Water control 35.25 az 320.25 a 3584.8 a
Bitertanol-on-demand, 0.25 21.75 ab 00.00 b 0.3 c
Chlorothalonil, 1.25 19.50 ab 179.50 ac 180.0 b
Mancozeb, 1.2 + 1 qt 9.75 abc 58.50 c 212.8 b
Mancozeb, 1.2 + 1 qt 7.50 abc 54.00 cd 193.3 b
Chlorothalonil, 2.50 6.75 abc 49.00 bc 157.0 b
Triadimefon, 0.125 6.00 bc 0.25 b 1.0 c
Triadimefon, 0.250 2.25 cd 11.75 de 1.5 c
Bitertanol, 0.125 0000 d 00.00 b 0.0 c
Bitertanol, 0.250 0000 d 00.00 b 0.0 c
YUredinia counted on underside of leaves.
ZMeans in a column followed by the same letter are not significantly different at
P<0.05 according to Waller-Duncan's mean separation test. Data were converted to
log10 values before analysis.
Table 3. Mean market
rates (lb a.i./
+ 1 qt S formulation
Mancozeb, 1.2 + 1 qt
S switched to
table and total snap
bean yields in rust fungicide efficacy
ZMeans in a column followed by the same letter
P<0.05, according to Waller-Duncan's mean
are not significantly different
1. Little, T. M., and F. J. Hills. 1972. Statistical methods in
agricultural research. Univ. Cal. Coop. Ext. Ser., Berkeley.
2. McMillan, R. T., G. Ellal, and H. H. Bryan. 1982. Fungicides
for the control of squash powdery mildew and bean rust. Proc.
Fla. State Hort. Soc. 95:305-307.
3. Pohronezny, K., R. McSorley, and V. H. Waddill. 1981.
Integrated management of pests of snap bean in Florida. Proc.
Fla. State Hort. Soc. 94:137-140.
4. Pohronezny, K., and W. M. Stall. 1984. Integratd pest
management for Florida snap beans. Ext. Plant Path Rept. No.
36. 14 pp.