Southeastern U.S. Vegetable Crop Handbook - 2021 - 96

European Corn Borer (ECB): Continued treatment for ECB may
significantly increase CPB insecticide resistance. However, for
proper timing of ECB sprays, consult your local county Extension
office for further information.
Potato Aphid and Green Peach Aphid: Insecticide treatments
are recommended when aphid counts exceed two per leaf prior to
bloom, four aphids per leaf during bloom, and 10 aphids per leaf
within two weeks of vine kill.
Potato Tuberworm: Treat when foliage injury is first noted. Potato
tuberworms are primarily a problem with late potatoes, in cull
piles, or potatoes in storage. Sanitation is very important.
Cutworms: See " Cutworms " section in Soil Pests-Their Detection
and Control. Cutworms are especially troublesome to tubers where
soil cracking occurs. Variegated cutworms feed on lower leaves and
petioles.
Wireworms: Wireworms are a generic term used for the larvae of
several species of click beetles, which burrow into potato tubers.
In the Southeast U.S., wireworms attacking potato are typically
the corn wireworm, or one of five species in the genus Conoderus.
Wireworm problems are most prevalent when potatoes follow
corn, any cereal crop, sod, or pasture. Wireworms do not move
quickly from one field to the next, but can remain as larvae in a
field for 2-5 years, depending on the species. Since it is difficult
and laborious to monitor for a pest that lives in the ground, field
history is an important tool for determining the need to treat for
wireworms. Treatment must begin at or before planting to be effective.
Options include preplant broadcast or at-planting furrow
application of an appropriate insecticide. There is no control for
wireworms once they have infested potato tubers.
DISEASE MANAGEMENT
Early Blight: Fungicide applications can slow the spread of early
blight, but cannot eliminate it, and will be effective only when
application begins when airborne spores are first present. Spore
formation is most prevalent during repeated wet/ dry cycles, such
as caused by overhead irrigation or frequent dew. Minimizing plant
stress can reduce damage caused by early blight, especially in
younger plants. Early blight spreads more rapidly in young plants
than mature ones. If a field is infested with early blight, potatoes
may still be harvested if adequate time between vine kill and harvest
allows the skins to set and great care is taken not to bruise
the tubers in the field. Tubers can be infected during the harvest
process, and will decay more rapidly in storage than non- infected
tubers. There are some potato cultivars resistant to early blight.
White Mold: High fertility and frequent rain or overhead irrigation
are conditions conducive to white mold, caused by the fungus Sclerotinia
sclerotiorum. This organism can survive in the soil for three
or more years, and impacts other produce crops such as lettuce,
beans, broccoli, peppers, and others. Fungicides should be applied
as a protectant in fields with a history of white mold. In addition,
eliminate canopy wetness by reducing overhead irrigation or aligning
rows with prevailing winds to promote rapid evaporation of
water in the canopy. Avoid fields with poor air movement or poor
drainage. Rotate with a non-susceptible crop for three yields after a
heavy infestation of white mold.
Common Scab: Common scab is characterized by brown lesions
on tuber skin that may be slightly raised or sunken in relation to
surrounding surface. The causal agent is a bacteria (Streptomyces
spp.), which can be seedborne or soilborne. Scab occurs most
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commonly in warm dry soils with pH 5.5 to 7.5, and does not affect
yield. If lesions are significant, marketability and quality are
affected. Scab is difficult to manage. In addition to seed treatments
at planting, management strategies include rotation out of the field
for 3-4 years, maintaining low soil pH, using resistant varieties,
and maintaining high soil moisture during tuber formation.
Late Blight: Caused by the fungal pathogen Phytophthora infestans,
late blight is the disease responsible for the potato famine and
continues to plague potato crops worldwide. Environmental conditions
that favor late blight include frequent rainfall (or overhead
irrigation), cool weather (50-75°F), and high humidity. Spread
is usually quite rapid and complete defoliation can occur within
2-3 weeks of initial infestation. Infection can occur at any growth
stage, and in any plant part including the tuber. Fungicides should
be applied protectively; after infection, only systemic fungicides
(those that penetrate plant tissue) can inhibit the spread of the disease.
Additional management strategies include reducing periods
of leaf wetness by decreasing overhead irrigation or increasing air
movement. Use resistant cultivars when they are available. A critical
strategy is reducing the initial amount of inoculum available in
the field. Use certified disease-free tubers, and dispose of infected
tubers and volunteer vines in a pit with at least 2 ft soil coverage to
avoid sprouting. Consider fungicide treatment of seed pieces.
In recent years, forecasting and reporting for this disease have become
available, particularly valuable since Phytophthora spores
can travel long distances on air currents. One such model is http://
www.usablight.org. Use late blight modeling to forecast when disease
pressure is most likely to be present and time fungicide sprays
accordingly for most efficient use of chemicals.
HARVESTING AND STORAGE
Harvest indicators: Tuber formation of potatoes ends when soil
temperatures are consistently over 80°F, regardless of whether or
not vine tops have died back. For most " new " potatoes, tubers will
be well developed between 65-75 days. Flowering is not a reliable
indicator of tuber formation, as some varieties may flower little or
not at all. Check for readiness by hand harvesting a few tubers for
evaluation.
Vine Kill: Also known as desiccation, many growers chemically (labeled
herbicide application) or physically (rotobeating or chopping)
defoliate potato vines once optimum marketable size of tubers has
been achieved. This allows the tuber skin to set and mature and
helps minimize skinning prior to digging. This technique provides
benefits including efficiency in harvest, better control over harvest
timing, skin set to reduce harvest injury, and reducing impact of
diseases like late blight. Vine killing halts the translocation of nutrients
and sugar accumulation from the leaves, triggering the conversion
of tuber sugars to starch for storage. Vine killing also weakens
the juncture of the tuber and stolon, making tubers fall from the
plant more easily. If vine killing is used, harvest of tubers should
occur at 2-3 weeks after vines are completely dead. Harvest before
this and tuber skin may not have had adequate time to set, while
harvesting later increases the chance for rotting organisms to attach
the crop in the ground. Care should be taken to monitor this period
and harvest at the optimum time to minimize mechanical damage
and breakdown. See Table 14 for further postharvest information.
2021 Vegetable Crop Handbook for Southeastern United States
http://http:// http://www.usablight.org
Southeastern U.S. Vegetable Crop Handbook - 2021

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