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

TORRES, H.. E. R. FRENCH, and I.. W. NIELSEN. 1970. Potato Control
diseases in Peru, 1965-1968. Plant Dis. Rep. 54:315-318. Foliage sprays used for other leaf pathogens are apparently
effective.
(Prepared by L. J. Turkensteen)
Cercospora Leaf Blotches
The disease is reportcd from cool and temperate climates of
Europe and Russia and from the eastern part of the United
States, where it isnot considered an important disease. It is also
reported from restricted areas in Africa and Asia and from
India, where it occurs with early and late blights.
Symptoms
First symptoms on lower leaves are small yellow to purplish
lesions that increase from 0.2 to I cm in size. On the underside of
the lesions, a dense, plush, gray layer of conidiophores and
conidia is formed. Later, lesions are separated from
surrounding tissues by a dark line. When lesions become
necrotic, tissue may drop out, leaving only holes. Necrotic
lesions are distinguished from those of Ahernariasolani by the
lack ofconcentric rings. The disease becomes apparent at about
the same time as late blight. The leaf may be killed; stem lesions
become dark; and the entire plant may die. Symptoms on tubers
have not been described.
Causal Organism
ifYcoVs'llosiela ((erco,jpora)conlcors (Casp.) Deighton has
dark spores formed on densely branched sporophores that
emerge through stomata. The straight or slightly bent, dark
spores are variable in length (14-57 X 3.5-6 pm) and may have
up to six septa or none (Fig. 51).
An additional Cercospora species can attack potato. This
larger spored form, C. solani-tuherosi Thirumalachar, with
conidia 4I- 120 X 3.3 pm (1-12 septate), is described from India.
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Fig. 51. Cercospora leaf blotch. Conidia and conidiophores of
Mycovellosiella (Cercospora) concors. Bar represents 25 pm.
(Reprinted, by special pirmission, from More Dematiaceous
Hyphomycetes, by M. B. Ellis. 1976 Commonwealth
Mycological Institute, Kew, Surrey, England)
Selected References
ELLIS, M.B.1976. More l)emaiiaccous ltyphomycetes. Commonw.
Mycol. Inst., Kew, Surrey, England. 507 pp.
NAGAICH, B.B., G.S.SHEKHAWAT, S. M. KHURANA,and S.K.
BHATTACHARYA. 1974. Pathological problems of the potato
cultivation in India. J. India Potato Assoc. 1:32-44.
SALZMANN, R.. and E. R. KELLER. 1969. Krankheiten und
Sch'idlinge der Kartoffel. Landwirtschaftliche Lehrmittelzentrale
Zollikofen. 150 pp.
THIRUMAI.ACHAR, M. J. 1953. Cercospora leaf spot and stem
canker disease of potato. Am. Potato J. 30:94-9"'.
(Prepared by L. J. Turkensteen)
Phoma Leaf Spot
Yield losses up to 80% have occurred from Phoma leaf spot in
the Andes of Peru and Boliva at altitudes of 2,000-3,500 m.
Symptoms
Small leaf spots, up to I cm but mostly less than 2.5 mm in
diameter, have concentric rings and are similar to early blight
lesions except that the lesions are not depressed into the leaf
tissue (Plate 35). At first a few lesions form on the lower leaves;
gradually infection spreads to the whole plant. Primary lesions
continue to expand, and secondary infections give rise to many
smaller lesions. which may coalesce. Foliage becomes blackish
and when it dies remains attached to the stem fora time before it
drops. On stems and petioles, elongate lesions develop.
Causal Organism
Ploioa anfl(ila Turkensteen ha:. light-colored pycnidia,
125-200 pm in diameter (on artificial media) containing a
distinct ostiole surrounded by 2-3 rows of brown cells. In leaf
lesions, many submerged pycnidia are present at the upper side
only. From the same pycnidium, two types of spores are formed:
I) hyaline, one-celled infective spores, 14-22 X 5-7 pm, shaped
like two-seeded peanut pods (Fig. 49F), and 2)small sporelike
bodies, 5.8-7.8 X 2.0-2.6 um, which do not germinate on
artificial media nor infect plants. On artificial media, single,
hyaline chlamydospores develop in series, but complex
chlamydospores are also formed. In rare cases, spores in old
cultures may be two-celled and considerably larger.
colony is light colored on artificial media. It is relatively
slow growing, and on acid (pH 4.5) media its growth is strongly
inhibited. On potato-dextrose agar and oatmeal agar the
medium turns yellow-green and yellow, respectively, within two
three weeks. When grown on slightly acid agar for one week,
the medium turns yellow when a drop of IN NaOH is added to
the surface.
Hosts
Cultivated and wild species of potato are as yet the only
known hosts.
Control
I) Applications of fungicides are effective when started early
the season before lesions are abundant.
2) Resistance is known.
Selected References
TORRES, H,, E. R. FRENCH, and L. W. NEILSEN. 1970. Potato
diseases in Peru, 1965-1968. Plant Dis. Rep. 54:315-318.
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