Horticulture Principles and Practices

elow 50°F. Some producers dig the mature roots and leave them in the field to dry before
collecting them. Delaying harvesting after the vines die risks having root decay.
Prior to storage, the harvested root products should be cured at 85°F and 90 percent
RH for about 7–10 days for bruises to the skin to be healed. Thereafter, the roots retain peak
quality for about six months if stored at 55–59°F and 85–90 percent relative humidity.
Sweet potato is intolerant of colder temperatures.
Disease in production include Fusarium wilt (stem rot), Rhizopus (soft rot), and
internal cork.
20.15 CORN (ZEA MAYS)
Sweet corn is Zea mays var. saccharata. The sweetness is caused by mutant genes that
cause the accumulation of sugar instead of starch in the endosperm of the corn kernel.
The standard sweet corn has the su gene. To keep its sweetness, the harvested corn must
be stored at a cool temperature without delay. A new breed of sweet corn cultivars with
a new gene, the sugary enhancer (se) gene, causes a higher accumulation of sugar than
the standard cultivars. Also, the gene slows down the rate of deterioration or conversion
of sugar back to starch, and increases shelf life in terms of sweetness. These cultivars are
described as extra sweet. A third category of sweet corn, the supersweets, carries the
shrunken-2 gene (sh2) that further promotes sugar accumulation. These cultivars retain
sweetness longer in the field and in storage. However, the sh2 is linked with poor seed
germination and low vigor leading to poor crop establishment, and hence reduced
acceptability by producers.
Sweet corn is interfertile with regular or field corn. The starchy condition of field
corn is dominant to the mutant sugar allele. Consequently, when sweet corn is crosspollinated
by field corn, the sweetness is suppressed. It is imperative that sweet corn
farms be isolated from field starchy corn farms. Only about 1 percent of corn acreage in
the United States is devoted to sweet corn production.
Like sweet potato, some corn cultivars are grown for ornamental purposes. Most
cultivated corn in the United States is from hybrid seed. Corn cultivars differ in maturity,
some being early (65–74 days), midseason (81–90 days), and late (over 90 days).
Sweet corn may be produced for the fresh market or for processing. Fresh market production
is led by Florida, followed by California, New York, Georgia, and Pennsylvania,
while processing corn is concentrated in Minnesota, Wisconsin, Washington State, Oregon,
and New York, in this order of decreasing production.
Corn is frost sensitive and intolerant of high soil acidity (below pH 5.5). The soil
should be well drained. Unless furrow irrigated, corn is not planted on raised beds. Corn
is direct seeded at 6–18 inches within rows and 2–4 feet between rows, wider spacing
resulting in larger ears. Corn responds to high fertization, especially, nitrogen. The crop
is ready for harvesting when the kernel attains the “milk” endosperm stage. To keep its
sweetness, corn must be refrigerated immediately after harvesting.
Diseases of corn include, corn smut, bacterial wilt, leaf rust, and maize dwarf mosaic
virus. Major insect pests include the European corn borer, corn root worm, and cutworms.
20.16 ONION (ALLIUM CEPA)
Other members of the onion family of interest are garlic (A. sativum), leek (A. ampeloprasum),
and chive (A. schoenoprasum).
The onion is a biennial crop, the economic part being the bulb. The onion bulb
varies in size, shape, inner leaf thickness, skin color, pungency, and other traits. Skin
color can be tan, yellow, white, brown, purple, or red, while bulb size may be small,
642 Chapter 20 Growing Vegetables Outdoors