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

more in the tropics. It requires cool nights and well drained soil
with adequate moisture and does not produce well in low
altitude, warm, tropical environments. Certain types of South
American potato have considerable tolerance to warm temperatures
and to temperatures a few degrees below freezing; clones
tolerant to both extremes are being sought.
The potato is a native of the Andean highlands of South
America, where it has served as a staple of the diet of Andeans
for centuries or millennia. Widely diverse types have been
selected. Dehydrated tubers have been preserved since antiquity
as chuho, a product of various types obtained by freezing
tubers, pressing liquid from them after thawing, and then
freezingand drying them. Today tubers are boiled and dried for
preservation.
The potato was introduced into Spain sometime before 1573,
when it was first mentioned as a food source. Its use in England
was first reported in herbals in 1596. From northern Europe it
was first reported in herbals in 1596. From northern Europe
it was returned to North America in 1719 and grown in the
colonies.
Cultivated Types
Cultivated potatoes, consisting of a number of species or
species hybrids, belong to the Solanaceae, section Tuberarium,
which contains approximately 150 tuber-bearing species. In the
Andean highlands, the distinction between cultivated and wild
varieties has little relevance to indigenous populations,
The most common potato is the tetraploid (2n :=4x = 48
chromosomes), Solanum tu/'erosum I..which may he divided
into the completely cross-fertile subgroups Tuberosum and
Andigena or into subspecies tuerosut and adigena.Survival
of these in the wild occurs only in excep'ional cases. Their
survivai and extensive dispersal have resulted from human
selection. Andigena is the most widely grown in South America.
It has deep eyes, is often pigmented. and produces tubers in
days of short length. The Tuberosum type grown in northern
Europe and North America tends to need a long day for effective
tuberization. These two types are not completely distinct, and
obtaining the Tuberosum type by selection from Andigena is
possible.
Another classification also places certain diploids and
triploids within S. tuherostim. Diploid cultivated potatoes (2n =
2x = 24) belong to two main groups, S. stenotomln, with a
period of tuber dormancy, and S. phure/a, without a welldefined
dormant period. S. stenotonmn is regarded as the
ancestral type, giving rise to Andigena types through
chromosome doubling.
Cultivated triploids (2n = 3x = 36) in group S. X chaucha are
possibly naturally occurring hybrids from crosses between
Andigena and Stenotomum or Phureja. Another triploid
species, S. X.juzepczukii, is highly frost tolerant and may have
originated from natural hybridization between the wild species
S. acaule (a tetraploid) and the diploid S. stenotomtm.
A pentaploid, S. X curtilobun (2n = 5x = 60), believed to
have originated from natural hybridization of S. acaule and S.
and, gena, is grown in the high Andes because of its frost
tolerance. It is bitter tasting but useful for making chuio.
The hexaploid S. demisstm (2n = 6x = 72) has been used as a
source for cultivars resistant to late blight,
Perpetuation ofthe many geneticdifferences is attributable to
asexual propagation through tubers. Variability existing within
the potato groups is believed to have originated through: I)
hybridization between diverse types, 2) chromosome doubling,
3) mutations in germ plasm, and 4) nitatiori in vegetative
tissue and perpetuation as chimaeras. Vegetatively propagated
clone lines, assumed to be genetically stable, as are presently
accepted cultivars, are capable of further variation and change
through somatic mutation,
The potato may be propagated from true seed, which is
2
common in genetic studies and in potato breeding programs. In
certain parts of Ecuador and Colombia true seed is planted for
commercial crop production. The Peoples Republic of China
grows over 10,000 ha from true seed to avc,id virus spread and
long distance transport of seed tubers. At the International
Potato Center, transplants from true seed produce
homogeneous families that yield an average of more than I kg
per plant. These practices permit full use of tubers as food and
avoid diseases carried through seed tubers.
Commercial production of most potatoes is primarily through
vegetative propagation by means of lateral buds formed on the
tuber, a modified stem. Through such vegetative propagation,
many diseases are transmitted from generation to generation.
Although disease affects each of the types listed above, much
of potato pathology has been done wth S. tuero.stim ssp.
tuherostm in northern Europe and North America. Increased
importance of the crop in the tropics and subtropics has
occasioned unprecedented work during the past half century on
tropical diseases and evaluation of wild and cultivated
genotypes as sources of disease resistance.
The Pjant
The potato, S. tuberosun ssp. tuherosutn and ssp. andigena,
is an annual, herbaceous dicotyledonous plant with potential
perennial capacity because of reproduction through tubers.
Flowers
Flowers are five-parted of various colors with single style and
stigma and two-loculed ovary. Pollen is typically windborne.
Self-fertilization is natural; cross-fertilization is relatively rare,
and when it occurs, insects are probably involved. Diploids are
self-incompatible with very few exceptions.
Fruits
Fruits are round to oval (1-3 cm or more in diameter), green,
yellowish green or brown, and red to violet when ripe. Fruits are
two-celled, with up to 200-300seeds. Because of several sterility
factors, seeds may be absent even though fruits are formed.
Vegetative Structures
Plants from true seed are typically seedling plants with
primary tap root, hypocotyl, cotyledons, and epicotyl, from
which a stem and foliage develop. In contrast, the commerical
potato plant contains one or more lateral branches, each arising
from a bud on the "seed tuher," and the roots are adventitious
(Fig. I). The "seed" in commerical production is an asexual
propagative organ and not comparable to the sexually derived
true seed.
Stems
These are usually green, but can be red to purple, angular, and
nonwoody. Late in the season the lower portions may be
relatively woody, however. Leaves are pinnately compound,
although early leaves of seedlings and the first leaves of plants
grown from tubers may be simple. Leaf types differ widely
among the many species and cultivars. Stomata are more
numerous on the lower leaf surfaces, and hairs of various types
are present on aboveground parts. Secondary branches are
common, arising from axillary leaf buds. Leaves on the
underground stem are small and scalelike, and stolons arise
from these axillary buds. Stolons and tubers are modified
adventitious stems.
Roots and stolons develop from the underground stem
between the seed tuber and the soil surface. Thus, the vegetative
propagative unit (theseed tuber ora portion of it, theseed piece)
should be planted sufficiently deep to permit adequate root and
stolon formation.