Compendium of Potato Diseases - (PDF, 101 mb) - USAID

obovoid, and 14-30 X 5-10 pm. Pycnidia production in culture
is rare except on propylenc oxide sterilized leaf tissue in agar.
Epidemiology
The fungus maiatains itself saprophytically on unthrifty or
senescent plant partsand survives unfavorable periodsas microsclerotia.
Pycnidiospores are relatively short-lived. Tubers are
infected through wounds, eyes, enlarged lenticels, and the
stolon.
Tubers are predisposed to infection at temperatures of 320 C
or higher. Rot development is restricted at low temperatures,
slow at 20-25'C, and most rapid at 36'C and above. No
secondary spread is apparent during storage, but infected tubers
rot in warm storage. Rot stops in refrigerated storage, but when
tubers are returned to warm temperature, the rot continues.
Thus, seed from cold storage should be warmed before being
planted so that infected tubers may be removed.
Most commercial cultivars are equally susceptible. Resistance
exists in certain Solanum chacoense clones, in some of its
hybrids, and in hybrids of the series Commersoniana.
Other Hosts
The fungus has been found on underground parts of an
extremely wide range of plants, both cultivated and wild.
Control
I) Harvest early, before soil temperatures become high.
2) Avoid bruising and wounding of tubers in harvest and
postharvest handling.
3) Field irrigation may be useful to prevent excessive soil
temperature.
4) Do not leave tubers in soil after plants have matured.
5) Do not harvest during periods in which soil temperatures
exceed 28' C.
6) Do not store tubers at high temperatures,
7) Do not use seed originating from areas where the disease is
frequent.
Selected References
IIJARGAVA, S. N. 1965. Studies on charcoal rot of potato.
Phvtopathol. Z. 53:35-44.
GOTII, R. W., and S.A. OSTAZESKI. 1965. Sporulation of Macroplominaiphaseolionpropylene
oxide-sterilized leaf tissues. Phytopathology
55:1156.
IOI.LIDAY, P., and E. PUNITIIAI.INGAM. 1970. ,tfacrophonina
phaseolina. No. 275 in: Descriptions of Pathogenic Fungi and
Bacteria. Cornnionw. Mycol. Inst., Kew, Surrey, England. 2 pp.
I1tSlKARNATIIi. 1976. Potato in Sub-Tropics. Orient L.ongman,
SAIIAI, ).eB. DJI, and K. D. PAIIARIA. 197t). Reaction of
sonic wild and cultivated potato varieties to charcoal rot. Am.
potato J.47:427-429.
TIIIRUMAI.ACIIAR, M.J. 1955. Incidence of charcoal rot of potato
in Bihar (India) in relation to cultural conditions. Phytopathology
45:91-93.
von AMANN. Ni. 1960. Untersuchungen Uber einen sklerotienhildenden
Pil an Kartoffeln. vermutlich Sclerottiuo hataticola
(Taub.), synonyn Macrophotina phaseoli tMaubl.) Ashby. Z.
Plianienkr. Pflanzenschutz 67:655-662.
(Prepared by L. J. Turkensteen and W. J. Hooker)
Gangrene
The pathogen Phoma exigua var.foveata was first described
in 1940 and is now prevalent in most northern European
countries and parts of Australia. P. e'xigua var. exiguaoccurs in
most European countries, Russia, the United States, Canada,
and Australasia.
Symptoms
Small dark depressions develop in the tuber skin, usually at
wounds, eyes, or lenticels, and may enlarge to form "thumbmark"
or larger, irregularly shaped, sharp-edged lesions, the
surface area of which isoften unrelated to rot depth. Internally,
diseased tissue is well defined. Rots caused by Phoma exigua
var..ioveata are usually extensive and dark brown or purplish
(Plate 45), with variously shaped cavities; those caused by P.
evigta var. cxiguaaresmaller, become restricted, and are usually
black with small cavities. Pycnidia may form singly or in clusters
on lesions or in the mycelium that lines cavities. Infrequently,
lesions may be only of skin thickness, becoming extensive, dark,
and irregularly shaped; this condition is termed skin necrosis.
Causal Organism
Either of two varieties of Phoma exigua Desm. may cause
gangrene. rheprincipalcauseis P. t'xiguavar./o'eata(Foister)
Boerema (syn. P. foveata Foister; P. solanicola f. foveata
(Foister) Malcolmson; P. exigua Desm. f. sp.foveata (Foister)
Malcoimson & Gray). The more ubiquitous but weaker parasite
is P. exigua Desm. var. exigua (syn. P. solanicola Prillieux &
Delacroix; P tuherosa Melhus, Rosenbaum, and Schultz; P.
exigua Desm. f. sp. exigua Malcolmson and Gray).
The two fungi have similar morphological characteristics.
Pycnidia are usually globoid (90-200 pm) and dark brown to
black. Initially subepidermal, they become erumpent and
extrude hyaline, nonseptate, cylindrical pycnidiospores (4-5
X 2-3 pm). In culture on 2% malt agar, P. exigua var.foveata
(having nonzonate colonies) is readily distinguished from P.
exigua var. exigua (having zonate colonies) (Fig. 63) by its
production of anthraquinone pigments that turn red within
seconds on exposure to ammonia vapor.
Disease Cycle
Infected orcont;ninated seed tubers produce diseased stems,
in which infection remains latent during the growing season
unless the stems become moribund. Pycnidia appear in sporadic
groups, usually associated with nodes, as stems begin to senesce
either naturally or through chemical desiccation. Raindrops
wash pycnidiospores into the soil and spread inoculum to
neighboring plants. Rots in mother tubers usually continue
active in the soil, produce pycnidia, and constitute another
important source of inoculum for tubers at harvest. Before
harvest, tuber infection may occur through eyes and
proliferated lenticels, usually when soil moisture is high. Most
gangrene, however, develops after harvest through damage to
the tuber skin. Wounding introduces infection from contaminated
soil on the tuber surface or stimulates development of the
fungus already latent in the periderm. Wound infection may
occur at lifting, grading, or at any time during handling.
A B
Fig. 63. Gangrene. Distinguishing characteristics: A, Phoma
exigua var. exigua, zonate culture; B, Phoma exigua var. foveata,
nonzonate culture, on malt agar. (Courtesy C. Logan; photographs
by G. Little)
57