TABLE 2. Current and potential use of sweet potato in tropics.


 USE RELATIVE POTENTIAL
 FARINACEOUS VEGETABLE FOR
 BOILING, EVERYDAY USE VERY HIGH
 COMMERCIAL SOURCE OF CHIPS AND
 FRENCH FRIES HIGH
 FLOUR TO SUBSTITUTE IN PART FOR
 WHEAT FLOUR HIGH
 FRESH, CANNED OR DRIED PRODUCT
 TO MAKE DESERTS (ICE CREAM, MILK
 SHAKES, PIE) MEDIUM
 SWEET, MOIST ROOT TO MAKE DESERT-
 LIKE VEGETABLE MEDIUM
 ANIMAL FEED MEDIUM
 LEAFY VEGETABLE LOW
 SOURCE OF STARCH LOW
 SOURCE OF ALCOHOL UNKNOWN




 TABLE 3. TARS sweet potato breeding program.


 INTRODUCTION OF VARIETIES AND SEEDS, MAINTENANCE
 OF A COLLECTION

 SCREENING OF SEEDLINGS FOR ACTIVITY

 PRINCIPAL BREEDING ACTIVITIES

FOR HEAVY FOR WEEVIL FOR INDUSTRIAL FOR GROWING FOR CLIMBING
TROPICAL RESISTANCE USE, FROM IN SHADE IN MULTIPLE
SOILS SEEDS1 CROPPING

 BREEDING, IN ORDER, FOR PRINCIPAL SITUATION (ABOVE)
 FOR PESTS AND DISEASES
 FOR POTENTIAL USE TYPE

 SELECTION

 WIDE TESTING
 I
 VARIED VARIETIES

 FOR VARIED USES


TABLE 4. Four types of sweet potatoes for the tropics.


might substitute for it in the diet. Furthermore, the current and
future potentials of the sweet potato far surpass current usage
(Table 2).
 If the sweet potato has the advantages that I have described,
why has it not become a more important crop? I believe that the
sweet potato is not a more important crop because people do not
want to eat it every day. For most of us, a sweet can never be a
staple. For many, sweet potato is a good crop for a special dish or
a dessert, but a tiresome crop for every day. Yet, it is not just
sweetness; not everyone likes the taste of sweet potatoes. Thus, if
sweet potato is going to realize its potential, it will have to be
made over to better fit what people want.
 The sweet potato must also conform to farmers' needs. An easy
crop to grow, the sweet potato has one very serious disadvantage.
It is susceptible to the sweet potato weevil (Cylas formicarius
elegantulus), an insect which tunnels into the storage root,
resulting in off-color and bad taste. If this pest is not controlled
the farmer can say goodbye to the sweet potato, for what he
harvests will not be fit for man or animal.

Improving the Sweet Potato-
The Challenge and the Technique
 The sweet potato poses a special challenge to the person who
would improve it. While a perennial, propagated vegetatively
and thus easy to preserve as a variety, the sweet potato is a dif-
ficult crop to breed. It is self-incompatible by nature, and this
restriction is complicated by sterility, probably due to hybrid
origin. Furthermore, the sweet potato is a hexaploid and thus has
six instead of two sets of chromosomes. Hand pollinations are so
unfruitful that they average less than a tenth of a seed per
pollinated flower. Sweet potato cross-breeding has traditionally
been labor intensive.
 A new technique, mass selection, developed and demonstrated
in South Carolina by Alfred Jones (Jones et al., 1976) has made
sweet potato breeding easier (Table 3). Very effective for weakly
expressed characteristics, the technique nevertheless is very sim-
ple. For any trait that can be measured or rated accurately, plants
are selected that display the highest values seen of the trait. These
are planted in an isolated location during the season of short
days, stimulated to flower by tying the vines to upright poles,
and cross-pollinated by bees. The seeds are planted and seedlings
are selected again. While any seedling can become a variety, the
population itself tends to change with each cycle of selection in two
ways, towards increased flowering and towards an increased fre-
quency of genes determining the desired characteristic. The
chromosomes containing favorable genes are selected first, and
with time unfavorable linkages are broken, and progress continues.

Program and Progress at the Tropical
Agriculture Research Station
 The sweet potato breeding project at the Tropical Agriculture
Research Station, named "The Development of Stress-Resistant
Sweet Potatoes for the Tropics," was officially begun October 1,
1981, but plans and activities were initiated nine months before
that time. Supported by permanently assigned funds from the so-
called Section 406 program, the sweet potato project is aimed at
the tropics in general, yet spin-off is expected to touch the
temperate zone as well. It is staffed by one scientist and three
technicians, each dedicating two-thirds of his time to this effort.
Informally speaking, the objective is to make the sweet potato in-
to the most important farinaceous crop of the tropics. This objec-
tive does not depreciate any other crop, but rests on the belief
that the sweet potato, and no other crop, can fill that role.
However, in a broader sense, the objectives are to develop sweet
potatoes that are so attractive to the palate that people will want
to eat them every day, and so tolerant of stresses, diseases, and


pests that farmers will be able to produce them easily, even on


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