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S-POTATO INVESTIGATIONS LABORATORY
Mimeo Report 63-3 March 15, 1963
CORKY RINGSPOT OF POTATO
A. H. Eddins
Corky ringspot of potato is a soil-and tuber-borne virus disease which
capeared at Hastings in 1946. Symptoms consist of brown corky, concentric rir.'
arc-shaped lesions and small irregular-shaped spots on the surface of tubers.
Cracking of the skin in the lesions usually is evident. The flesh beneath the
3:ots also is brown and corky and the lesions may be shaped like those on the
surface. Usually the flesh shows much more corking and discoloration than
appears on the skin of affected tubers. Severly affected tubers may be mal-
formed and the skin roughened. Diseased tubers are worthless for human con-
sumption. Normal yields are produced by affected plants except those attacked
by the disease in early stages of growth which causes the tubers to become
malformed and undersize. Signs of the disease have not been observed in tops
of potato plants grown in Florida.
The causal virus is perpetuated in seed tubers and in infested soil from
one season to the next. Symptoms of the disease have never been detected in
northern-grown seed potatoes used to plant the Hastings crop. Therefore, the
disease which has appeared in Hastings-grown potatoes is attributed to the
causal virus present in infested soil. There probably are many host plants of
the tobacco rattle virus in Florida. Plants grown in Florida and known to be
hosts of this virus in addition to potato are pepper, tomato, tobacco, corn,
gladiolus and several weeds including pigweed and sow thistle. The stubbyroot
nematode, Trichodorus christiei Allen is the local vector of the tobacco
rattle (corky ringspot) virus, as reported by !Nalkinshaw et al. (Phytopath. 51:
During the past 17 years corky ringspot has appeared in potato fields com-
prising 566 acres scattered throughout the Hastings area. The disease may be
present in other fields but not identified or reported. During this period the
disease was very severe, 1 year; severe, 7 years; mild 5 years; and very mild,
Little of the disease has developed in infested f' the soil re-
mained dry during most of the growing season due to n ^ ll and lack
of irrigation. The disease has been worst in season i>ith normal e excessive
rainfall. For example in the wet season of 1961, 1 total r'fl from
January to June was 18.28 inches, corky ringspot t rinfe i r d from
47.2 to 73.3 percent in three susceptible varieties ~ja in infested, soil at
the Potato Investigations Laboratory. In the dry s~ so of 1962 1wfe rainfall
for the same period totaled 9.66 inches, tuber infec 6 ranged o 1.7 to 5-5
percent in the same varieties grown in the same locate father also
retards development of corky ringspot. Much less tuber i on has appeared
in potatoes grown during cool weather from September to January than in those
grown during warm weather from January to May in the same plots of infested soil
at the Potato Investigations Laboratory.
Walkinshaw et al. (Plytopath. 51: 806-808. 1961) reported that the vector,
T. christiei, must be present in infested soil to transmit the tobacco rattle
virus to tobacco plants and that the percentage of plants infected with the
virus varied according to the numbers of T. christiei introduced into the soil.
These findings may explain why corky ringspot of potato which is caused by
tobacco rattle virus has varied in serverity from year to year. In seasons when
corky ringspot was mild it is probable that there were low populations of T.
christiei present in infested soil and in other seasons when the disease was
severe, the vector was present in much greater numbers to transmit the virus.
During the period 1951 to 1962, 81 potato varieties and seedlings were
tested 1 to 12 years at the Potato Investigations Laboratory to determine their
reaction to corky ringspot when grown in infested soil. Thirty-nine varieties:
and seedlings with tuber infection ranging from 0 to 7 percent were clessif:'.e
as resistant to the disease, 27 with 10 to 49 percent tuber infection were li.:,
as having a medium degree of susceptibility to the disease and 15 with 50 to -'00
percent of their tubers infected were considered very susceptible.
Entries found to be susceptible to corky ringspot have been dropped from
the test except Sebago and other susceptible varieties used for checks. Testing
of varieties with poor tuber qualities and poor yielding ability also has been
discontinued. Some entries listed as resistant to corky ringspot may have
escaped infection due to the absence or small concentrations of the causal virus
or vector in the soil. Soil temperatures, moisture and other conditions which
were not favorable for development of the disease also could have reduced or
prevented tuber infection. However it is believed that varieties and seedlings
which were tested 2 to 3 years were exposed to conditions conducive to corky
ringspot development. In some cases only one year was required to determine
that some entries were susceptible.
Yield and tuber characteristics of several resistant varieties were com-
pared to determine their suitability for commercial production in the Hastings
area. Several local potato growers also cooperated by making trial plantings
of three resistant varieties in infested soil on their farms. Pungo was chosen
for commercial production because it produces good yields of marketable tubers
and has never shown over 0.4 percent tuber infection during the 10 years it has
been tested. Pungo has been grown in severly-infested soil on growers farms at
Hastings for the past four years without losses from corky ringspot.