Mimeograph Series"No. 22 November 
1945
AGRICULTURAL EXPERIMENT STATION
of The Alabama Polytechnic 
Institute, Auburn, Ala.
. . . . . M. J. Funchess, Director ? .
STUDIES ON TUE CONTROL OF LLA EBLIGHT OF IRISH POTATOES
OTTO BROVJN, Superintendent
FRANK E. GARRIETT, Part-Time Assistant
COYT WILSON, Assistant Plant Pathologist
Late blight has caused damage to Irish potatoes in certain parts of
the world for more.than a century, but was first reported in Alabama in
1940. Since that time it has occurred each year in the Gulf Coast areas
of the State. The extent of damage has varied with weather conditions
during the growing season. The disease is., and probably will continue to
be, one of the most serious threats to profitable potato production in
southwestern Alabama.
In 1945 an experiment was started at the Gulf Coast Substation,
Fairhope, .to determine if it is possible to control late blight and what
treatments or practices if any would give the greatest net neturn. Since
the results are from one year's test, no conclusions can be drawn or recom-
mendations made. The results can be taken only as indications of what
might be expected from the treatments tested.
Methods and Materials
Triumph was the variety used in the experiment. The materials
included were selected because they had shown some promise of blight con-
trol in other potato-growing regions. Also, all of the materials were
commercially available.
The experiment involved 10 sprays and dusts, and one check plot for
measuring possible yield increases over no treatment. The treatments
(Table 1) included different amounts of Dithane in combination with zinc
sulfate and lime; yellow cuprocide dust, copper-hydro dust, yellow cupro-
cide spray, copper-hydro spray, tribasic copper dust, copper-lime dust,
and Bordeaux spray.
With one exception, both spray and dust materials were first applied
when the plants were 6 inches high. These treatments were repeated every
7 to 10 days until the potatoes were dug. Five applications were made.
The one exception was the plot on which only one spray was used; it was
applied five days after blight appeared in the experiment.
I_/ One-year progress report, 1945.
2/ Gulf Coast Substation,
-2-t
Table- 1. Effect of Various D~usts and Sprays on the
Late Blight as Shown by Potato Yields,
Gulf Coast Substation, 1945
Control of.
Inroximate
Treatment due to : cost per qore/
tretment for materials
Bags Dollars
Dithane, l qt. per 100 gale plus 3/4 lb.
zinc sulfate and 3/8 lb. lime 23.3 
6.75
Dithane, 2 qt. per 100 al. plus 1 lb.
zinc sulfate and 1/2 lb. lime22.3 9.00
Yellow cuprocide dust 20.3 10.35
Copper-hydro dust 13.0 10.50
Yellow cuprocide spray, 11 lb. per 100 gal., 13.0 4.00
Copper-hydro spry, 4-_ lb. per 100 gal.17.0 400
Tribasic copper dustJ14.3 7.00 (Est.)
Copper-lime dust: 30-70 10.3 7.00
Bordeaux spray: 3-6-100 5.7 3.75
None
Treatment 11 0.0 1,.3
1/ Labor aost and depreciation on machinery 
are not included. Insoctim
cides cain be' addd to any of the materials at a-small additional cost.
/ Proprietary compound proparedby the Tennessee Copper Company.
3/ Trecatment 11 received only one spray 
of Dithane 2 quarts per 100 gal-
lons of wai-ter; zinc sulfate, 1 pound; and lime, 1/2 pound in 100 gal-
lons. This spray was applied five days after blight appeared in the
experiment.
The dusts were applied with a ha--1nd gun equipped with two nozzles.
The sprays were FApplied with power sprayer, which developed 300 
to 400
pounds pressure. Good covera~ge WUS obtained with a boo.ra hcail-ring three
nozzles per row. The ratets per !a1c1.re' -if-or Oeach ,applicaition were 2100 to 125
gallons of spray, or 25 to 35 pounds of dust.
Rosults of Experimrents
Blight wa-,s f irst f ound (Pr il 14)- on the rows thait had received no
trea-ltment. At that time .four of the fCive applica4t ions hA already been
mctde on the other plots. The potaoes w-.ere dug MaDy 4, wqhich was 90 -days
a-, fter planting anid 20 da-,ys tafte-r blight app)-e,,ared. A1t tha time aill plants
on the unltrea-,ted plot wvere kdea,-d from ltate blight. In contrast, Call treated
plots still cont .nod sor l1ive foliagi te.
- 3.-
The highest yields were 
obtained on the plots sprayed 
five times with
Dithane but the differences 
were slight. Except for Bordeaux 
and copper-
lime dust the materials were equally 
effective in this experiment. The
most striking fact revealed 
by these results is 
that, in spite of the late
appearence of blight and 
the unfavorable weather 
for blight development,
yields were increased 
by 15 to 20 per cent when 
blight was controlled.
This was more than enough 
to pay all costs of application.
Results obtained in treatment 
11 and observations made in commercial
fields indicate, that materials and 
labor are largely wasted unless the first
application is made when the plants 
are around 8 inches high and repeated
at least every 10 days.
Since the data represent only one year's 
results, the yield increases
might not be the sanme over a period of years. 
Weather conditions during
the spring of 1945 were slightly better 
then average for the control of
blight. Under more adverse weather conditions, the 
disease would be more
severe. Thus, it would be expected that the yield from the untreated plot
would be lower than that of the same plot in 1945. 
Or, in yecrs of severe
blight, treatments would be expected to give greater 
increases over no
treatment than in seasons less favorable to spread of the disease.
For spraying or dusting to be effective, the plants 
must be entirely
covered with material at each application. This requires around 100 gal-
lons of spray per acre applied at 300 to 400 pounds pressure, 
or 30 pounds
dust under sufficient pressure to be broken into 
a complete fog and applied
under a hood.
Satisfactory coverage with spray cannot be obtained with less than
three nozzles per row. One nozzle is placed at each side and the third
nozzle above the row. They aro so adjusted that the three cones of spray
completely envelop the plants from the ground line up. As the plants grow
larger, the nozzles must be set farther away.
Most spray pumps now in use in Alabama do not have the capacity 
to
deliver enough spray at the required pressure when a 6- or 8-row boom 
with
three nozzles per row is used. The number of rows should be reduced or
the size of the pump should be increased.
Both spraying and dusting have advantages and disadvantages. The
advantages of spraying are:
(1) Better coverage can be obtained with a spray than with dust.
(2) Sprays stick to the plant better 
than dusts and remain longer.
(3) It may be done when too windy for 
dusting.
The advantages of dusting are:
(1) Dusters are usually less expensive 
than sprayers.
(2) Labor costs are less.
~
(3) Dusting equipment is lighter end 
may be used on wetter ground.
Practices that may help in producing potatoes under blight condi-
tions include:
(1) Plant blight-free seed insofar 
as possible. If blighted seed
are bought, all potatoes showing any sign of rot should be discarded and
destroyed at the time of cutting seed pieces.
(2) Plant varieties that are less subject to blight. The Sebago
is probably the most resistant.variety in commercial production in Alabana.
The Bliss Triumph and Katahdin are moderately resistant, while the Chippewa
and Whito Roso have no resistance to the disease.
(3) Make all plantings 
within each community at as 
nearly the same
dato as possible. Blight usually starts in the earliest 
potatoes and
spreads to later plantings, which may be killed before the crop is made.
(I4) Avoid planting low, poorly drained areas. Infection usually
begins in such places.-
This experiment is to be continued over a period 
of years. The
results will be publishedo acsthey are obtainod.
I