POTATO INVESTIGATIONS LABORATORY
Mimeo Report 60-1 September 25, 1959
POTATO VARIETIES RESISTANT TO CORKY RINGSPOT
Corky ringspot was first observed on three farms in the Hastings area in 1946,
The disease has persisted in fields in which it first appeared and is now present in
other fields scattered throughout the Hastings area, The trouble has varied in
severity from year to year. Some fields have been abandoned for potato culture due
to losses caused by the disease, Sebago and Red Pontiac, the leading potato vari-
eties grown at Hastings, are very susceptible to corky ringspot.
Symptoms of the disease consist of brown rings, arcs and irregular-shaped
spots on the surface of tubers; and brown, corky areas in the flesh. A severely
affected tuber may be malformed and show deep cracks and heavy corking on the sur-
face. Tubers with symptoms of corky ringspot should be discarded when the crop is
graded and packed, as buyers discriminate against crops affected with the disease.
Severely affected tubers are not salable.
Plant pathologists at the University of Wisconsin have shown that corky ring-
spot is caused by a soil- and tuber-borne virus which they named the Potato Corky
Ringspot Virus, PCRV. These workers also stated that peppers may develop the
disease when grown in PCRV-infected soil. Corky ringspot may be carried from one
part of the country to another in infected seed potatoes.
Tests have been in progress at the Potato Investigations Laboratory, Hastings,
Florida, for the past nine years to determine the reaction of potato varieties and
USDA seedling selections to corky ringspot. Results have shown that some varieties
and seedlings are resistant and others are susceptible to the disease when grown in
soil infected with the causal virus. The reaction of five varieties to the disease
are given in Table 1.
The corky ringspot-resistant varieties, Merrimack, Plymouth and Pungo, were
recommended to potato growers for trial planting in the Hastings area in the fall
of 1957. The three varieties were planted in PCRV-infected soil on several farms
in 1958 and 1959 and none of the disease appeared in the tubers.
Annual yields of five corky ringspot susceptible and resistant varieties grown
in test plots from 1949 to 1959 varied from year to year, as shown in Table 2. The
average yield of resistant Pungo was about 10 percent greater and that of resistant
Plymouth and Merrimack was about 12 and 15 percent less than the average yield of
susceptible Sebago. Red Pontiac produced the highest average yield.
Maturity, disease resistance and some tuber characteristics of the five vari-
eties are compared in Table 3. Merrimack is not as suitable for production in
PCRV-infected soil at Hastings as Plymouth and Pungo, due to its late maturity and
susceptibility to internal browning. Merrimack tubers have a russet skin, while the
skin of the other varieties is smooth, Pungo and Plymouth are resistant to inter
browning and late blight. Chip color of the resistant varieties is good. Pu EI
produces large tubers which may become oversize and rough under high irrigate nd
overfertilization, but this condition can be corrected by thick planting(sp g
seed pieces about 8 inches apart in the row) and by using standard quantity of
fertilizer(l* tons of 6-8-8 or equivalent per acre). Plymouth and Pungo a I 1(
grow oversize and knobby if digging is delayed longer than 100 days from p ing
in a normal season.
Table 1.- Susceptibility of Five Potato Varieties to Corky Ringspot when Grown in
Test Plots of Infected Soil.
Variety Percentage Tubers with Corky Rinigsppt Symptoms Range in Percentage
1951 1952 1953 1954 1955 1956 1957 1958 1959 Tubers Affected
Merrimack 0 '0 0 0 0 0.3 0 00 0.3
Plymouth 7.0 0 0 0 0 0 0 0 0 0 7.0
Pungo 0 0 0.4 0 0 0 0 0 0.4
Red Pontiac 0.4 0 1.0 69.1 10.3 27,0 0 69.1
Sebago 3._,0 53.2 1.1 11.9 0 0.9 74.0 21.1 33.2 0 74.0
Table 2.- Yields of Five Potato Varieties Grown in Test Plots at Hastings from 1949
Variety 100-pound Sacks US 1A Tubers per Acre ..
______ 1949 1950 1951 1952 1955 1956 1957 _1958 1959 Average
Red Pontiac 254 224 178 201 260 178 222 217
Pungo 262 182 225 205 179 186 156 235 204
Sebago 209 187 235 225 151 151 174 168 173 186
Plymouth 194 172 141 157 142 173 163
Merrimack *- 152 159 204 188 92 159
Table 3.- Comparison of Some Characteristics of Five Potato Varieties.
_.___.. _____ Red Pontiac Sebago...P Plymouth P_ _ung.o Merrimack
Maturity Medium early Medium late Medium late Medium early Late
Corky Ringspot Very Very Highly Highly Highly
susceptible susceptible resistant resistant resistant
Late Blight Very Susceptible Resistant Highly Highly
susceptible resistant resistant
Internal Browning Very Very Moderately Moderately Suscep*
resistant susceptible resistant resistant tible
Scab Very Susceptible Moderately Unknown Unknown
Shape Round, Nearly round Oblong Elliptical Oblong
thick to oblong flattened round to round
Eye Depth Medium deep Shallow Shallow Medium deep Shallow
Chip Color Poor Good Good Good Good_
1 MAine A U
* & *&W.f a --&S W B 4dt J L ^ C V '. y g po* o lo s
Florida. Amer. Potato Jour. 23: 330-333. 1946.
2. Eddins, A.H. Susceptibility of Potato Varieties and Seedling Selections to
Corky Ringspot. Amer. Potato Jour. 36t 187-190, 1959.
E Q Procter and Erd t
C rk Rin s t f P tatoe n