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BIG BEND HORTICULTURAL LABORATORY
Mimeo Report BBL 68-4 January 22, 1969
SCAB CONTROL ON SEVERAL PECAN VARIETIES
by William J. French
Assistant Plant Pathologist
In 1967 Large (1) reported that Brestan 60W was a promising fungicide for
pecan scab control. He reported that Brestan 60W at 1 and 3 lbs, and Du-Ter 50W
at 0.5 Ibs. and 1.5 lbs. gave comparable scab control (95-98%) on the Mahan variety
and that no phytotoxicity was noted at any of the concentrations tested. In 1968
Brestan 60W was included among the fungicides evaluated for control of pecan scab.
Severe phytotoxicity in the form of leaf stunting, curling, and defoliation was
first observed on Frotscher variety following the May 28-29 and June 25-27 appli-
cations of Brestan 60W at 1 lb/100. The damage was most severe on young leaves.
No injury was noted on Moneymaker or Schley pecans. No further damage was noticed
when the rate was reduced to 1/2 lb/100.
The tests were conducted in the Rainey orchard three miles west of Monticello,
Florida. Eight treatments consisting of six applications were applied to three
varieties, viz., Schley, Frotscher, and Moneymaker. Each treatment was applied to
one tree plots with four replicates per treatment. The sprays were applied 10-15
gallons per tree using an airblast sprayer. The prepollination sprays were applied
on April 8 and 9 with the exception of GS 16306 which was not available until
April 18. The first cover spray was applied on May 6 and 7 and the four subsequent
applications were at approximately 21 days intervals. Malathion 25W 3 lbs/100 was
added to each application.
Dormant sprays were applied March 3 to four Schley trees per treatment. Du-Ter,
Difolatan and Daconil were applied at high rates to determine if rain would redis-
tribute and dilute the materials and thereby protect new foliage during the early
part of the growing season. The dormant sprays were followed by two cover sprays
at lower rates on July 22 and August 13-14.
Scab observations were made throughout the summer and final scab samples were
taken September 16. A total of approximately 150 leaves were examined from the top,
middle, and bottom of each tree. Scab infection was graded according to percent of
leaf area affected; 0=no disease, 1=1-24%, 2=25-49%, 3=50-74%, 4=75-100% affected.
Each leaf was graded, the frequency of each grade was weighted by the grade number,
the figures summed and divided by 4 (maximum grade) and the total divided by the
sample size, This gave the mean percentage of scab for the treatment. Scab eval-
uations on nuts could not be made due to poor fruit set. The results are recorded
in Table I and II.
Weighted Mean %
Weighted Mean %
Treatment Rate Scab Leaf Blotch & Brown Spot
Formulation lbs/100 Schley MM Frotscher Schley MM Frotscher
Daconil 75W 1 341 0.8 0.2 0.9 9.3 4.8
Daconil 75W 1.5 2.0 0.8 0.0 4.3 2.5 5.7
Benlate 50W 0.36 10 0.8 0.0 0.8 0.4 2.9
Bordeaux(2)4-l(4) 6-1 1.5 247 0.5 1.5 1.6 4.6
Du-Ter 47*5W i020 0.2 1.9 3.6 0*2 5.1 0.6
Brestan 60W(4)l.0,(2)05S li4 0.7 0,1 0.3 2.1 0.9
Cyprex 65W 1.0 340 0.2 0.0 0.2 1.5 5.3
Difolatan 80W 1.0 3.4 1.2 0.0 0.5 4.7 10.0
GS 16306 95W 1.0 4.3 0.2
GS 16306 95W 2.0 1.1 0.8
Check 1.9 0.5 0 21.5 28.3 26.2
DORMANT SPRAY MASSIVE DOSAGE
Treatment Rate Foliar Leaf Blotch
Formulation lb/100 Scab Index Brown Spot Index
Du-Ter 3.0 1.2 2.6
Difolatan + Volck oil(2 gal) 6.25 3.8 1.9
Daconil 6.0 5.9 1.9
Unsprayed 1.9 21.5
The data represent the results of only 1 year of testing.
Tradenames, chemical name and manufacturer
Daconil (Tetrachloroisophthalonitrile) Diamond Alkali Co.
Benlate (1991)(Methyl l-(butylcarbamoyl)-2-benzimidazole carbamate DuPont
Du-Ter (Triphenyl Tin hydroxide) Thompson Hayward Chemical Co.
Brestan (Triphenyl Tin Acetate) American Hoechst Corp.
Cyprex (N-dodecylguanidine acetate) American Cyanamid Chemical Company
Difolatan (cis-N-E(l,1,2,2-tetrachloroethyl) thio]-4-cyclohexene-l,2 dicarboximide
Chevron Chemical Company
GS16306 (4-tertiarybutyl-7-decylamino-3,4,5 ,6-tetrahydro-2H-azepine-chlorohydrate)
Geigy Chemical Co.
Scab was a minor problem due to the dry season. Summer diseases such as
leaf blotch, brown spot and mildew were prevalent but did not cause any appreciable
defoliation. Mites and black aphids caused moderate to severe damage from late
July to October.
The total rainfall for the period April through September was 23.6 inches.
The monthly rainfall in inches was; April 5.33, May 4.39, June 2.50, July 4.07,
August 5.32, September 1.99.
(1) Large, J. R. Brestan 60*(Triphenyl-Tin Acetate) A Promising Fungicide
for Pecan Scab Control. Big Bend Hort. Lab. Mimeo Report BBL 67-8.