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Copyright 2005, Board of Trustees, University
Mimeographed Report No. 4 October, 1937
University of Florida
SUB-TROPICAL EXPERIMENT STATION Subtropical Experiment Station
Homestead, Florida Homestead
TREATMENT OF POTATO TUBERS FOR CONTROL OF DISEASES IN SOUTHERN DADE COUNTY
Geo. D. Ruehle
Treating seed potatoes is a common practice in many potato growing sections
and is generally accepted as being valuable for the prevention of seed-piece
decay, for the control of both common scab and Rizoctonia disease, and as an
aid in obtaining increased yields.
In southern Dade County there is a considerable loss each year traceable to
seed-piece decay, Rhizoctonia disease and common scab. The latter was much more
abundant in 1937 than in any past year and on a few farms was the occasion for
Some form of seed treatment has been in use by the majority of the potato
growers of the section but there has been considerable diversity of opinion as
to the most reliable methods to be adopted for local conditions. No careful
experiments had been made whereby the various methods of seed treatment had been
compared under similar conditions. Primarily from the standpoint of scab control
such an experiment was carried out during the 1936-37 season on the East Glade
farm of the Sub-Tropical Experiment Station.
The soil on which the tests were made is a calcareous marl typical of potato
soils in the section. The soil reaction ranged from pH 8.0 at a depth of 3
inches to pH 8.3 at a depth of 9 inches. Potatoes had been grown previously on
this land but the tubers had never been seriously infected by common scab.
Visibly infected seed was selected for the experiment from a single lot of
northern-grown certified Bliss Triumph seed potatoes. The selected tubers gen-
erally were slightly to moderately blemished with a superficial or shallow type
of scab. A few were severely infected and occasionally a deep scab lesion was
observed in the lot.
The potatoes (in sacks) were immersed for two minutes in tap water, allowed
to drain, and thenpiled together and covered with a tarpaulin for 48 hours before
treatment. This pre-soak was given for the purpose of softening Rhizoctonia
sclerotia and scab lesions so they would be more readily permeated by the
chemicals. Eight equal lots were treated individually two weeks before planting
with the following treatments: acid mercury, hot corrosive sublimate, cold
corrosive sublimate made with tap water in o e lot and with distilled water in
another, hot formaldehyde, cold formaldehyde, Seresan-Bel, and control. The
tubers in all lots were allowed to dry thoroughly before storing.
The seed pieces were planted in adequately r eplicated plots, and a very
unifonr and complete stand was obtained with no evidence of delayed emergence
resulting from any treatment. Ordinary cultivation and p raying practices were
applied throughout the season.
At maturity the potatoes were harvested by hand and sized over a standard
Boggs grader. The No. 1 and No. 2 tubers from each plot were carefully graded
for severity of scab infection into three classes: (1) free of scab ( slight
scab, with from 1 to 5 small superficial lesions;and (3) severe scab, with more
than 5 superficial lesions or with one or more lesions of the deep or pitted
type. The amount of black scurf due to Rhizoctoniawas so insignificant in all
lots that the tubers were not graded for this blemish.
The average yields and the average percentage of scab infection obtained
as the result of the various treatments are recorded in the following table.
Total yield Percent of scab infection
Seed t treatment bu. per A, Slight Severe Total
Control (non treated) 176 19.8 13.9 33.7
Acid mercury 166 5.1 0.9 6.0
Hot formaldchyde 162 4.8 0.9 5.7
Cold formaldehyde 166 5.1 0.9 6.0
Hot corrosive sublimate 162 7.0 3.9 10.9
Cold corrosive sublimate (tap water) 167 6.3 3.1 9.4
Cold corrosive sublimate (distilled water) 171 15.7 10.5 26.2
Seaesan-Bel 168 7.4 2.5 9.9
From the results obtained in this experiment, the control of tuber-borne
scab appears to be more readily obtained in the marl soils of Dade County by the
use of either acid mercury of formaldehyde than by the use of the other chemicals
tested. All of the treatments gave some reduction in severity of scab infection.
Since the soil apparently was not infected prior to the planting of the tubers
there is no indication of the effect of the various treatments on soil-borne
infection. The season was unfavorable for the development of Rhizoctonia disease.
which in some years is of greater importance than scab.
All of the treatments caused a slight reduction in yields from those obtained
in the control plots, but differences in yield were not marked between the various
treatments. In a season favorable for Rhizoctonia this situation might be reversed
Usually it is profitable to disinfect all tubers used for seed. The tubers
may be treated any tine before they begin to sprout and at least two weeks before
planting. Stock treated before storing is less subject to decay than untreated
stock. The seed stock to be disinfected shouldbe as free as possible from dirt,
preferably washed clean.
The following treatments may be expected to give good control of scab when
the seed is planted in disease-free soil, and also a reasonably good kill of
tuber-borne Rhizoctonia sclerotia.
Hot formaldehyde dip. This re thod of treating permits of rapid handling f
seed stock but requires facilities for heating and controlling the temperature
such as live steam or taok heat s equipped with the.mostats.. Community operation
of a treating plant where there is considerable acreage to be planted is to be
1. The treating solution is made by adding 2 pints of commercial
formaldehyde to 30 gallons of water. The formaldehyde does not become weaker
with use, so that the volume of the treating solution may be restores by
adding more solution of the original strength.
2. Metal containers may be used to hold the solution since formaldehyde
does not corrode metal.
3. The tubers should be dipped in or sprinkled with water, drained,
and allowed to stand in the moist aacks or covered with sacks for 24 to 48
hours before dipping into the chemical solution.
4. The temperature should be maintained from 124 to 126 degrees F.
during the treatment, but must not rise above 130 degrees F. for the treat-
ment to be effective, and yet not cause injury.
5. Dip the potatoes in the solution 5 to 4 minutes, but no longer.
6. Place the treated tubers in layers 6 to 8 inches deep on the floor
and cover with sacks or canvas for one hour.
7. Dry the tubers thoroughly before storing them in new or disinfected
sacks or in clean boxes. Do not store treated potatoes while they are wet.
Cold formaldehyde soak. Thi s method of treating is slower than the fore-
going but does not require heating equipment.
1. The treating solution is made by adding 1 pint of commercial
formaldehyde to 30 gallons of water.
2. The tubers should be dipped in or sprinkled with water, drained,
and allowed to stand in the moist sacks or covered with sacks for 24 to
48 hours before dipping into the chemical solution.
3. The standard time of treatment formerly was 1 hours. If tubers
are kept moist before treating as described above, the time of treating
can be shortened to 30 minutes and effectiveness increased.
4. Handle the tubers after treating as described above for the hot
Acid-mercury dip. Thi s method of treating requireslass time and labor
than any other method. Directions must be carefully followed to avoid poss i-
bility of injury to the tubers.
1, Add 6 ounces of corrosive sublimate to 1 quart of commercial
hydrochloric acid. (Stock solution.)
2. Pour the quart of acid containing the mercuric chloride into
25 gallons of water in a wood barrel, tub, or concrete tank. The solution
will corrode metal.
3, A wooden crate or a wire basket thoroughly painted with'asphaltuir
paint should be used for dipping the potatoes.
44 Dip the potatoes in the solution and allow them to remain at least
5 minutes. If there is much scurf on the tubers, they ma be soaked longer.
Soaking 40 minutes will not injure then, although 10 minutes is usually
5. It is better to treat the tubers before cutting, although they may
be treated after cutting if desired. In this case, they should be planted
the same day they are treated.
6. Twenty-five gallons of the solution is enough to treat 40 to 50
bushels of potatoes, after which the solution is likely to be too weak. It
can be brought back to approximately oni ginal strength by adding J pint of
the stock solution and enough water to restore volume to 25 gallons. After
45 to 50 more bushels have been treated the solution should be discarded.
7. Unless the tubers are planted immediate ely they must be spread out
to dry. Do not store treated potatoes while they are wet.
8. Remember that mercuric chloride is a poison. The dilute treating
solution is not strong enough to injure the hands in ordinary practice.
The strong acid, however, should be handled with care.
GDR 200 10-1-37