Fitopatologia

Assessment of Resistance of Tubers of Potato Cultivars
to Phytophthora erythroseptica and Pythium ultimum
Bacilio Salas, Gary A. Secor, R. J. Taylor, and Neil C. Gudmestad, Department of Plant Pathology, North Dakota
State University, Fargo 58105

A BSTRACT
Salas, B., Secor, G. A., Taylor, R. J., and Gudmestad, N. C. 2003. Assessment of resistance of
tubers of potato cultivars toPhytophthora erythroseptica and Pythium ultimum. Plant Dis.
87:91-97.
Tubers of 34 potato cultivars were examined for their susceptibility to infection by zoospores of
Phytophthora erythroseptica and mycelia of Pythium ultimum. Incidence of infected tubers (%)
and penetration of rot (mm) were the parameters used to determine the susceptibility of each
cultivar. Tubers of cv. Atlantic appeared to havesome resistance to infection and colonization
by Phytophthora erythroseptica. Cvs. Russet Norkotah and Snowden were the most susceptible
to infection by P. erythroseptica. Snowden was found to be highly susceptible to P.
erythroseptica, but it was the most resistant to Pythium ultimum. Cvs. FL-1625 and FL-1867
also were less susceptible to P. ultimum than the other cultivars. Cvs. Superior,Itasca, and Dark
Red Norland were the most susceptible to P. ultimum. Cultivar susceptibility should be
considered when making disease management decisions, particularly in fields where these
soilborne diseases are a recurring problem.
Additional keywords: control, epidemiology, leak, pink rot, Solanum tuberosum

Pink rot of potato (Solanum tuberosum
L.) caused by Phytophthora erythrosepticaPethybr. and leak caused by Pythium ultimum Trow are two important soilborne
diseases of potato in the United States (32).
The name pink rot describes the diagnostic
pink color that appears in infected tuber
tissue when cut and exposed to air for 20 to
30 min (12,24). The common disease name
of leak is descriptive of the exudation of
water droplets from the blackish, soft
watery breakdownof tissue of infected
tubers (2,16). These diseases can cause
severe yield losses in fields and in storage
facilities (1,16,32). In the United States,
levels of pink rot infection in fields ranged
from 10 to 75% (1,4,8), and often tubers in
infected areas or the entire field may not be
harvested. Tubers infected with Phytophthora erythroseptica usually are found in
wet, low-lying areasduring harvest, and
symptom development occurs soon after
tubers are placed in storage facilities
(1,3,32). The leak pathogen also can cause
seed-piece decay after planting (16) and
kill seedlings from true potato seed (24).
Corresponding author: N. C. Gudmestad
E-mail: Neil.gudmestad@ndsu.nodak.edu
Current address of B. Salas: Department of Plant
Pathology, University of Minnesota, St.Paul
55108.
Accepted for publication 6 September 2002.

Publication no. D-2002-1118-02R
© 2003 The American Phytopathological Society

Infection of tubers by P. erythroseptica
generally occurs via stolons (10,21,27);
however, tubers also can be infected by
zoospores of P. erythroseptica through
tuber eyes (29) and wounded tubers (31).
Pythium ultimum is incapable of penetrating theundamaged skin of the tuber
(14). Thus, infections by the leak pathogen
always start at cuts and wounds, and only
occasionally through the stem end (1).
Studies on cultivar susceptibility to Phytophthora erythroseptica and Pythium
ultimum are scarce and most are outdated,
involving potato cultivars no longer in
production. Most studies on cultivar susceptibility to pink rot and leak involve onepathogen or the other and do not evaluate
susceptibility to both diseases. In studies
involving pink rot, Cairns and Musket (6)
evaluated 51 cultivars and found that all
were susceptible. Similarly, Jones (17)
found that none of the eight cultivars he
tested were resistant. Other studies have
identified that cultivars exhibited various
levels of susceptibility. Goss (12), reported
that...