POTATO INVESTIGATIONS LABORATORY
Mimeo Report 56-1 September 20, 1955
CONTROLLING COMMON SCAB OF POTATOES AND BREAKING DORMANCY IN SEED STOCK
A. H. Eddins
Common scab, caused by Streptomyces scabies (Thax.) Wakd, and Henrici, is
one of the best known potato diseases because of the characteristic pustules
which it produces on the skin of tubers and its almost universal occurrence in
potato-growing areas of the world.
The scab organism grows en the skin of the potato and is capable of living
in the soil from one season to the next and also on tubers in storage. The
disease attacks potatoes grown in soils infested with the organism or in non-
infested soils planted with scabby seed. It has also been demonstrated that
tubers of the first crop of potatoes grown in new land may be attacked by scab
even when the seed are disinfected with formaldehyde before planting, thus in-
dicating that the scab-organism is sometimes present in virgin soil
In seasons favorable for its development scab is very severe in Florida in
infested marl soil with reactions of pH 7.5 to 8.0, Potatoes may be severely
seabbed when grown in muck and peat soils'with acid reactions and in some sandy
soils testing pH 5.0 and higher, However, potatoes planted in sandy soils of
the Hastings section are seldom affected with scab, although successive crops
have been grown with non-treated seed for over 50 years, The reactions of soils
used for potato production in the Hastings area range from pH 5.0 to 6.5 and are
approximately the same as those of similar soils which produce very scabby crops
in other sections of the State. Thus, the reaction, if above pH 5.0, is not a
reliable criterion for determining the incidence of scab in Florida.
The disease may be very severe in infested soil one season and very mild
the next. Experimentally, it has been shown'that temperature, moisture and'
aeration of the soih, as well as pH reaction, affect the occurrence of scab, but
at present it is impossible to determine by soil analyses whether or not pota-
toes will be scabbed when grown in soils with reactions of pH 5.0 or higher.
Date of planting experiments on peat soils in the Everglades indicate that
temperature is a very important factor in determining whether scab will be se-
vere or mild. With the fall crop, the disease is more prevalent in early plant-
ings. The percentage of scabby tubers is correlated with the occurrence of high
temperatures when the tubers are very young.
Similarly, observations on the spring crop in the Everglades indicate that
the late-planted potatoes suffer the heaviest losses from scab. Potatoes planted
before February 15 usually set their tubers before soil temperatures rise to the
range most favorably for scab development. Probably these observations explain
the relative mildness and infrequency of scab on potatoes grown during the winter
months in the Hastings section.
Symptoms.-- Scab is primarily a disease of the tubers but may be present
on all underground parts of the potato plant.
The scab bacterium attacks the skin of the young tuber and causes the cells
to multiply or divide in an irregular manner, thus forming the scab or pustule.
The disease first appears as small brown spots on the skin. Spots may originate
at any point but are frequently found at the lenticels. They increase in size
-as the tuber develops and may become more than 1/2-inch in diameter, They also
vary in number and may grow together and cover the entire surface of the tuber.
Scab lesions cOnsist of corky tissue which is formed under the irritating action
of the invading organism. Several different types of lesions are produced, all
of which may be present on 1 tuber. They consist of a shallow roughening of the
skin, warty or swollen areas and corky pits which may be 1/4 inch in depth. In
some cases wireworms, mites and millipeds invade the lesions and extend and
deepen them. The narcissus bulb mite is the most important agent causing deep
scab in the marl soil of Dade County.
Methods of Controlling Scab
Land which has produced very scabby potatoes should be used for other crops,
as no practical method of eliminating the scab organism from infested soil is
Caution is advised in applying lime, ground limestone or other alkaline
materials to potato soils to adjust pH reaction, as they tend to increase the
amount and severity of scab.
Adjusting pH reactions of soils with sulfur is not recommended for con-
trolling scab in Florida. Tests have shown that the quantity of sulfur required
to control the disease in infested sandy soil makes the soil too acid for potato
growth. Furthermore, when a soil which has been treated with sufficient sulfur
to control scab is readjusted with ground limestone to a pH suitable for potato
growth, scab returns and is as severe in the next potato crop as it was in the
crop grown on the same land before it was treated.
Seed potatoes usually are treated with solutions of mercury compounds and
formaldehyde where such scab control measures are necessary. Seed treatment ex-
periments conducted at Homestead, Florida, from 1937 to 1939 with hot and cold
formaldehyde, hot and cold mercuric chloride, acidulated mercuric chloride,
acidulated mercuric chloride-formaldehyde and semesan-bel showed that the hot
and cold formaldehyde treatments and the acidulated mercuric chloride treatment
were almost equally effective in controlling scab and were superior to other
treatments. However, tests conducted at Belle Glade have shown that acidulated
mercuric chloride and all other types of mercury disinfectants promote seed
piece decay in muck and peat soils, particularly when applied to seed planted
during the early fall; therefore, only hot or cold formaldehyde ar6 recommended
for treating potatoes which are planted in soils of organic origin.
Seed treatment is recommended for controlling scab in Florida except in
areas where experience has shown that losses due to planting non-treated seed are
not sufficient to warrant the expense of treating the potatoes.
Disinfection of the seed is improved if the tubers are prewetted before
treatment with formaldehyde and mercury compounds as the water softens the scab
lesions and thus increases penetration of the chemicals. Prewetting consists of
soaking the seed (in sacks'or other containers) for 2 minutes, after which they
should be allowed to drain, then piled together and covered with a tarpaulin for
Hot Formaldehyde Dip.- Uncut seed potatoes can be treated rapidly by this
method which has been used extensively in Dade and Palm Beach counties. Effi-
cient treating outfits will handle up to 200 hundred-pound sacks of seed potatoes
The equipment required for the hot formaldehyde treatment consists of a
large wooden or metal vat, a 10-horse power steam boiler, and a conveyor system
for moving the sacks of potatoes through the vat. Sometimes oil burners are used
under metal vats to heat the solution but these are difficult to control and are
The formaldehyde solution required consists of 1 part 40 percent formaldehyde
and 120 parts water. A tank 16 feet long, 3 feet wide and 3 feet deep will hold
720 gallons of the solution, the conveyor belt and 8 or 9 sacks of potatoes.
The conveyor belt should be geared to move at such a speed that a sack of potatoes
is removed after being submerged for 3 mi utes. The belt should slope into and
out of the tank to facilitate handling of the sacks.
Steam under pressure can be passed through heating coils in the bottom of
the tank or directly into the solution to heat it to 122-124o F. An accurate
dairy thermometer should be suspended in the solution at all times and checked
frequently to see that the correct temperature is maintained. Efficiency of the
treatment is reduced if the temperature falls below 1180 F. and potatoes iwy be
injured by temperatures above 1260 F. Even when heating coils are used it is de-
sirtbie to have some provision for passing steam into the solution so that the
temperature may be raised quickly if it should fall below the correct value.
Likewise, there should be a provision for adding a cold formaldehyde solution to
the vat to reduce high temperatures. When the vat is in operation usually it is
possible to hold the temperature within narrow limits by regulation of a valve
in the steam line so that more or less steam can be admitted to the coils.
The tank should be cleaned and a fresh solution used each day. A tank of the
size described will hold 720 gallons of a solution containing 6 gallons of 40 per-
cent formaldehyde when 2/3 full.
When live steam is passed into the solution the concentration of the formal-
dehyde becomes lower by evaporation and by dilution with the condensed steam.
If heating coils are used the only factor reducing the concentration of formalde-
hyde is evaporation. In either case, it becomes necessary to replace the formal-
dehyde lost by evaporation and dilution or by removal in solution on wet sacks
and potatoes. It has been found when live steam-is passed through the solution
that the volume of the solution remains constant, and that the addition of
li pints of 40 percent formaldehyde every 15 minutes will maintain the proper
concentration. When the steam is passed through heating coils volume of the
solution diminishes and it is necessary to add both water and formaldehyde. This
can be done by adding 15 gallons of a solution containing li pints of 40 percent
formaldehyde every 15 minutes.
After the sacks of treated potatoes have been removed from the vat they
should be placed on end 1 layer deep and allowed to drain and cool. The seed
may be cut for planting at any time after the sacks have dried.
Cold Formaldehyde Soak.-- When treated by this method the uncut tubers must
be soaked 1 hours in a solution consisting of 1 part 40 percent formaldehyde and
240 parts water at any ordinary temperature. However, the soaking period can be
decreased to 30 minutes if the potatoes have been prewetted in water. The same
equipment used for the heat dip, except the heating units, can be used to treat
by the cold method. Enough formaldehyde solution of proper strength from a re-
serve supply should be poured into the tank when needed'to replace that which
adheres to the potatoes and containers during treatment.
The disadvantage of the cold method is the time required. Potatoes can be
treated by the hot method in 1/10 of the time required to treat them by the cold
method. Consequently, the cold method is seldom used to treat large quantities
of seed potatoes in Florida.
Formaldehyde is injurious to the skin and the vapors irritate the eyes and
respiratory passages. The hands can be protected from injury by wearing rubber
or oiled leather loves. If the treatment is made in a ventilated .lI.ce, opera-
tors can work without inhaling the vapors. Any solution left after trLatment
should be poured down a drain or buried.
Acidulated Mercuric Chloride (Acid-mercury Dip).- This is a rapid method
of treatment. The stock solution is made by dissolving 6 ounces of mercuric
chloride in 1 quart of commercial hydrochloric acid. This should then be added
to 25 gallons of water to make the treating solution.
A wooden barrel, tub or concrete tank should be used and the potatoes can
be dipped into the solution in sacks, wooden crates or wire baskets painted with
asphaltum paint. The solution will corrode metal.
The uncut potatoes should be soaked in the solution for at least 5 minutes.
Soaking for as long as 40 minutes, although not necessary, will not injure the
seed. Twenty-five gallons of the solution is enough to treat 40 to 50 bushels
of potatoes, after which it is likely to be too weak. It can be brought back to
approximately original strength by adding 1/2 pint of the stock solution and
enough water to restore the volume to 25 gallons. After another 50 bushels have
been treated the solution should be discarded. Unless the tubers are planted
immediately they must be spread out to dry. Treated potatoes stored while wet
may be considerably injured, Cut seed should not be treated by this method.
The dilute treating solution is not strong enough to injure the hands while
handling the potatoes. Stran hydrochloric acid is injurious. however, and
should be handled carefully, Mercuric chloride is a deadly poison and anmv of
the solution left after treatment should be buried so tt has n huma be.
ings will not be poisonbd with it. Treated potatoes should not be used for cook-
ing or fed to livestock.
Progress has been made in breeding scab-resistant potatoes at several places
in the United States. *However, none of the resistant varieties developed thus
far are equal'in yield, vigor and commercial qualities to scab susceptible,
Bliss Triumph, Pontiac and Red Pontiac, or moderatly-resistant Sebago which are
connonly planted in Florida. When an adequate breeding and testing program is
initiated, it should be possible to create potato varieties that are highly re-
sistant or immune to scab and suitable for commercial production in different
sections of the State.
Eddins, A. H. Effect of sulfur and limestone soil treatments on potato scab in
a sandy soil, Amer, Potato Jour. 18: 312-316, 1941.
Eddins, A. H., Geo, D. Ruehle and G. R. Townsend. Potato diseases in Florida,
Fla. Agr, Ep. Sta. Bul. 427. 1946,
Goss, R. W. The influence of various soil factors upon potato scab caused by
Actinonrces scabies. Nebr. Agr. Exp. Sta. Bul. 93. 1937.
Lutman, B. F. The spread of potato scab in soil by potato plant humus. Vt. Agr.
Exp. Sta. Bul. 528. 1945.
BREAK=I DOMAONS IN POTATO SEED STOCKS
Chemical methods have been developed for the treatment of potato tubers to
shorten the dormant period in seed stocks. These treatments are necessary where
recently harvested seed stocks are planted for fall and early-winter crops in the
southern districts of Florida. They are not needed for the spring crop at Hastings
or in other areas of the State unless home-grown seed of the fall crop is'planted.
The advantages of the treatments are that sprouting is prompt and uniform, con-
siderable seed-piece decay is avoided, and in sections where early frosts are ex-
pected the crop can be matured earlier. Higher yields and tubers of more uniform
size and quality generally are obtained from stocks treated for shortening the
The grower has a choice of 2 treatments for breaking dormancy in potato seed
stocks. The ethylene chlorhydrin treatment which has been used by Everglades
growers is highly effective when properly handled. But because it frequently is
employed in ways that are not approved it sometimes increases the amount of seed-
piece decay. The ammonium thiocyanate treatment has given results equal to those
obtained with ethylene chlorhydrin, The ammonium thiocyanate treatment is less
subject to misuse and is safer for cut seed pieces under unfavorable conditions,
i.e., high temperature, high humidity and wet soils.
EhyLene Chlorhydrin Treatment.- When a potato seed stock is to be treated
with ethylene chlorhydrin the tubers should be cut into seed pieces the day be-
fore the treatment is to be given. The cut seed should not be exposed to the sun
or a drying wind; nor should it be piled too closely and allowed to heat. Good
results are ordinarily obtained if the cut seed is held for 16 to 24 hours in
loosely stacked boxes or bags in a well-ventilated shed or packinghouse. The
purpose of this pretreatment storage is to allow the cut surfaces to form a callus
which will lessen the danger of overtreatment and subsequent seed-piece decay.
A wise precaution is to have the cutters throw out all tubers showing decay.
Bacterial soft rot and Fusarium decay are spread from affected seed pieces to
clean seed pieces by contaminated cutting knives and by contact in the boxes.
The conditions of temperature and humidity in the fall are likely to be especially
favorable to the rapid development of slimy decay in cut seed pieces if these pre-
cautions are not followed. Seed lots that show some decay before treatment with
ethylene ohlorhydrin are likely to be seriously damaged by decay following the
Cut seed pieces held for 16 to 24 hours have formed a callus and are ready
for dipping in the ethylene chlorhydrin solution. The solution is prepared by
adding 1 gallon of anhydrous ethylene chlorhydrin to 75 gallons of water for
treating potatoes which have been dug less than 6 weeks. The concentration may
be 1 to 150 for potatoes which have been dug more than 6 weeks. If the 40 percent
ethylene chlorhydrin is used the concentrations recommended for treating very dor-
mant and partially dormant seed pieces are 1:30 and 1:60, respectively. The cut
tubers may be placed in boxes and immersed in the solution long enough to wet
thoroughly all of the seed pieces. After the excess solution has drained back
into the container the boxes or bags of treated seed should be loosely stacked
for 8 to 16 hours. Covering is not desirable. It is best to arrange to dip the
seed pieces in the late afternoon and to hold them overnight before planting.
Advantage is thus taken of lower temperatures and there is less likelihood that
the seed will be overtreated or that rot will develop. Some growers place their
treated potatoes in an air-conditioned room where the temperature can be held at
70 F. and humidity can be partially controlled. The treated seed is ready for
planting on the following day.
Ammonium Thiocyanate Treatment.- The ammonium thiocyanate treatment may be
substituted for the ethylene dip. There is less danger of overtreatrent when it
is used and the treatment schedule is more adaptable to growers' practices. Re-
sponse from the ammonium thiocyanate treatment is somewhat slower than for ethy-
lene chlorhydrin, but final stand counts and yields generally are equal to or
higher than those with the latter treatment.
The ammonium thiocyanate treatment as recommended is quite simple. The
boxes of freshly cut seed pieces should be dipped for an instant in a solution
containing 3 pounds of ammonium thiocyanate in 50 gallons of water. After dipping,
the excess solution should be allowed to drain back into the tank and then the
boxes may be stacked overnight. The treated seed should be planted the following
day. As with the ethylene chlorhydrin treatment, it is best to treat late in the
day to obtain the advantage of temperatures below 800 F. and to stack the boxes
of treated seed so that they are protected from the sun, wind or rain but with
some ventilation. An open packing shed platform is a suitable site for the oper-
Fifty gallons of the treating solution is enough for the treatment of 100
boxes of potatoes. When larger quantities of potatoes are to be treated, it
sufficies to add more of the treating solution to the tank as it wastes away and
becomes too low to submerge the boxes. The dirty solution should be disposed of
after each day's run of treatments. Since the solution is corrosive to metal,
wooden tanks and boxes should be used. The chemical and waste solutions bholld
be carefully disposed of since they are slightly poisonous if taken internally
by men or animals.
Eddins, A. H., Geo. D. Ruehle and G. R. Townsend. Potato Diseases in Florida.
Fla. Agr. Exp. Sta. Bul, 427. 1946.
Townsend, G. R. Relation of maturity in Bliss potato seed stocks to effective-
ness of ethylene chlorhydrin and other treatments. Fla. Agr. Exp. Sta. Bul.
Townsend, G. R. The ammonium thiocyanate treatment for hastening the sprouting
of dormant Bliss Triumph potatoes. Amer. Potato Jour. 23: 92-94. 1946.
I I .