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

fungus have been reported. Sporangial production is most rapid
and prolific at l0o( rh and at 21' C. Sporangia are sensitive to
desiccation and, after dispersal by wind or splashing water,
require free water forgermination. The optimal temperature for
indirect germination via zoospores is 12' C, whereas that for
direct germination of sporangia via gern tubes is 240 C. Both
types of germination occur at overlapping temperatures,
however. Zoospores, although quickly killed by drying. produce
germ tubes and appressoria in the presence of free water.
Penetration occursat temperatures between lOand 29' C. Once
penetration has occurred, infection and subsequent development
of disease is most rapid at 21' C.
Systems for forcasting late blight and for timing fungicide
applications rely on records of temperature and rainfall (tlyre)
or temperature and relative humidity (Wallin) and predict the
probability of late blight development, assuming the presence of
inoculum. .\ forecasting system combining bot hthese systems is
"Blitecast" (Krause et al), which is used in the northeastern
United States for timing fungicide applications. Where rainfall
and relative humidityare closely related, fungicides are applied
after rainfall accumulated to 1.27 cm has theoretically washed
previously applied fungicide from the foliage (Barriga et al).
on several continents are directed toward obtaining cultivars
with high levels of generalized resistance that can be used with
reduced amounts of fungicide oreven without fungicide in drier
areas.
Control
I) Avoid development of early season (primary) inoculum by
the use of blight-free seed and destruction of potential inoculum
sources such as cull piles, volunteer plants, etc.
2) Apply protectant fungicides as recommended by a fore­
casting service or (if such service is not availahle) as early as late
blight is present in the area. Apply fungicides regularly as new
vine growth develops and regularly after vines overgrowing the
rows have caused high relative humidity within the canopy. Be
sure that coverage of vines and leaves is thorough and uniform.
3) Prevent tuber infection by maintaining good soil coverage
of tubers through adequate hilling. (Exceptionally large hills are
commonly made in the Andes, resulting in relatively rare tuber
infection.) Maintain adequate foliage protection to reduce
inocnlum production on leaves. Kill vines two weeks before
harvest so that sporangia on leaves dry out and die and infected
tubers rot, thus permitting identification and removal before tite
Other Hosts
L.ate blight often severely affects tomatoes and occasionally
affects eggplant and many other members of the Solanaceae.
crop is placed in storage.
4) Prevent rot in storage by removing infected tubers before
storage and maintaining adequate air circulation and
temperature as cool as is compatible with other considerations.
Resistance
Two types of resistance to A inbr.taii. in potatoes are recognized:
1)specific resistance (also called race specific, vertical,
oligogenic, or monogenic resistance) and 2)general resistance
(also called field, race nonspecific, horizontal, or polygenic
resistance). Before the discovery of specific resistance, fairly
high levels of gencial resistance were obtained. For several
decades after discovery of specific resistance in Solanum
hentissum, breeders incorporated one or a few S. demissin
genes into each new variety. Because /A.itn'eistans is highly
variable, the pathogen rapidly overcame such resistance; use of
specific resistance has therefore contributed little to controlling
late blight. All potato cultivars and all tuber-bearing Solnutnti
species are susceptible to late blight in the Toluca Valley of
Mexico, where (lie sexual stage of P in~fistans occurs; thtus the
probability of obtaining lasting specific resistance is very low.
No cultivars in Furope or North American allow commercial
cultivation of potatoes without fungicide protection. Some
comenrcial cultivars, such as Sebago, have a tmoderate level
of general resistance and are protected by lower amounts of
5) Use resistant cultivars where possible.
Selected References
IIARRIGA, R., 11. ). IIJRSTON, and .. E. llEIDRICK. 1961.
Ciclos de aspersion para el control de li"gota" de lapapa. Agric.
CROSIER, W. 1934. Studies in the biology of I'hrfopht/ora ifi-sOanx
(Mont.) IC flary. N. Y.Agric. lxp. Stn.. Cornell. Mci. 155. 40 pp.
GAL.L.EGILY, M. E. 1968. Genetics of pathogenicity of lv/ytoplithora
ifi,.tans.. Ann. Rev. Phytopathol. 6:375-396.
GAI.L.EGL.Y, M. E. and .1.S. NIEI)ER'IIAUSER. 1959. Genetic
controls of host-parasite interactions in the Phytophthora late blight
disease. Pages 168-182 in: C. S. Ilolon. (i. W. Fischer. R. W.
Flton. II. Hart. and S. F.A. Mc(allan. eds. Plant Patlhology
Problems and Progress,. 1980-1958. Univ. Wis. Press,.
IIYRE, 588 pp.
Madison.
R. A. 1954. Progress in forecasting late blight of tomato and
potato. Plant Dis. Rep. 38:245-253.
KRAUSE, R.A., I.. B.MASSIE, and R.A. IIYRE. 1975. Blitecast: A
computeri/ed forecast of potato late blight. Plant Dis. Rep.
59:95-98.
L.ARGE, E. C. 1962. The Advance of tile Fungi. t)over IPubl., New
fungicide than are required by other cultivars. Breeding efforts York. 488 pp.
S A
V " ' " ' I
WAI.I.IN. J. R. 1962. Summary of recent progess inpredicting late
blight epidemics in the United States and Canada. Am. Potato .1.
39:306-312.
(Prepared by H. 1). Thurston and 0. Schultz)
,-;P- /Powdery Mildew
, ~ J'~iPowdery mildew can be an important foliage disease in arid
',,, \
\
LTh
SUnited
"',.
7j,
-Washington
or semiarid climates.
Mexico, New
It has
Zealand,
been reported
Europe, and
from
thf:
Chile,
Middle
Peru,
States it is of economic
East.
importance
In the
only in the state of
under row irrigation, although it has been reported
K in Ohio and Utah.
Symptoms
Elongated, light brown stipples, 0.5-2 mm in length, may
appear on stems and petioles of infected plants. These often
coalesce to form larger, water-soaked, blackened areas on the
Fig. 46. Disease cycle of late blight caused by Phytophthora petioles. Infections
infestans.
are initially
(Reprinted,
powdery
by
white
special
(Fig. 47A,
permission,
Plate
from Plant 31) and later
Pathology,
tan. Sporulation
2nd ed., by
on
G.
both
N.Agrios.
leaf surfaces
1 1978 Academic
appears as
Press, New dusty, grayish-brown
York)
deposits that superficially resemble soil or
spray residue. Severe infections may superficially resemble late
42