UF UNIVERSITY of
2007 Florida Plant Disease Management Guide: Sweet
Tom Kucharek and Pam Roberts2
Specific Common Diseases
Black Rot (Ceratocystis finbriata)
Symptoms: All underground plant portions are
susceptible to this fungus. Disease spread usually
occurs in the seedbed as the causal fungus grows
from an infected potato into sprouts. Infected sprouts
exhibit a small black lesion near the potato. This
lesion will enlarge, often up to the soil surface,
girdling the sprout and causing leaf yellowing,
stunting and finally sprout death. Infected potatoes
may or may not exhibit lesions at digging. Spots are
blackish in color, slightly sunken and circular. Under
favorable storage conditions, lesions enlarge. The
fungus can be observed as short, dark bristle-like
structures within a 1/2 inch circle in the lesion center.
The potato injury may extend to the potato center as
black flesh. The fungus will cause the potato to
develop a bitter flavor.
Cultural Controls: Plant seedbeds in areas that
have not produced sweet potatoes for at least 2 years.
Where permanent seedbeds are used, fumigate prior
to setting potatoes. Plant only the best, pathogen-free
seed potatoes. Dip potatoes in a seed treatment
fungicide before planting. After sprouting, clip
sprouts about the soil line and reset in new ground or
fumigated ground for rooting prior to setting these
slips into the field.
Chemical Controls: See PPP-6.
Pox or Soil Rot (Streptomyces ipomoea)
Symptoms: Infested plants appear stunted and
chlorotic, often in spots in the field. Roots exhibit
rotted tips with frequent black lesions. These lesions
also occur on the stems. Harvested roots will have
black, crusty lesions that will be sunken. Lesions may
be in rows on the potatoes and often roots may be
misshapen due to the one-sided occurrence of lesions
on a root. These injuries do not enlarge in storage.
Cultural Controls: Select fields without a
previous pox history. Choose disease-free seed
potatoes and plant in a new or fumigated seedbed.
Propagate clean slips as explained under the section
on Black Rot in a second, clean seedbed. Avoid
contamination of clean fields with infested soil
cared by equipment and machinery.
1. This document is PDG-V3-52, one of a series of the Department of Plant Pathology, 2007 Florida Plant Disease Management Guide, Florida Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised December 2005. Reviewed January 2007. Please visit the
EDIS Web site at http://edis.ifas.ufl.edu.
2. T. A. Kucharek, professor emeritus, Plant Pathology Department; P. A. Roberts, associate professor, Plant Pathology Department, Southwest Florida
Research and Education Center--Immokalee, FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of
Florida, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
2007 Florida Plant Disease Management Guide: Sweet Potato 2
Slightly infested fields should not be limed. Aim
for a pH of 5.2 or lower to minimize disease severity.
Consider in-row fumigation to reduce disease
incidence and severity. Severely infested fields
should be rotated out of sweet potatoes for 3-4 years.
Chemical Controls: Same as for Black Rot. See
Rhizopus Soft Rot (Rhizopus spp.)
Symptoms: The fungus causes a soft, spongy
moist decay in storage or transit. An abundant growth
of gray fuzzy mold is usually produced on the
surface. When the rotting process is completed, or
checked, the parts of the affected potato becomes
shrunken, dry and hard.
Cultural Controls: Avoid injuring potatoes at
harvest. Wash or dip with an appropriate fungicide.
Store or cure under warm moist conditions of 850 F
and 90% relative humidity and aeration for 5-7 days.
Store potatoes at 55-600 F under aerated conditions
of 80-85% relative humidity.
Chemical Controls: See PPP-6.
Scurf (Monilochaetes infuscans)
Symptoms: This disease involves only the outer
layer of the underground parts of the potato. It does
not cause a rot or reduce yield, but it causes a
superficial dark discoloration of the skin. The
discoloration may be only a few spots, or may cover
most of the surface of the potato. The casual fungus
can survive in the field or plant bed and usually is
worse during wet seasons.
Cultural Controls: Plant only disease-free seed
potatoes into new seedbeds or those previously
fumigated. Take vine cuttings or cut sprouts above
soil line as described for Black Rot to insure
disease-free slips to set in the field. Treat both
seed-piece and subsequent cuttings with fungicides.
Practice crop rotation for 2-3 years where disease has
Chemical Controls: See PPP-6.
Southern Stem Rot (Sclerotium rolfsii)
Symptoms: Plants in beds wilt suddenly and then
turn yellow to brown and die. Stems of sprouts will
have brown lesions. White fungal mycelia may be
present on the lower stem, and surface of the soil or
mother roots. Sclerotia that are about the size of
cabbage seed form among the mycelia. They are
white initially but later they become brown. This
disease is likely to be the most severe when the
canopy of the crop is dense in the bed.
Cultural Controls: Rotate the sites of the beds.
Do not allow bed covers to remain over the bed after
Surface Rot (Fusarium oxysporum)
Symptoms: The fungus produces circular,
slightly sunken spots that are lighter in color than the
lesions caused by black rot. The lesions are quite
shallow. Infection takes place around harvest time
and is usually worse in years when harvesting follows
a wet period. During storage, moisture escapes
through the spots and results in considerable
shrinkage and numerous, hard, mummified potatoes.
Cultural Controls: Do not harvest when soil is
too wet. Avoid injuries to the potatoes that will
provide entrance points for this soilbore fungus.
Severely infested fields should be rotated out of
sweet potatoes for at least 2-3 years.
Wilt/Stem Rot (Fusarium oxysporum f. sp.
Symptoms: The fungus in the vascular tissue
causes the plant to wilt, yellow, and stunt. If the
attack comes early in the season, the plant may die.
The water-conducting tissues of the potato stem turn
dark in color, often making the stem appear blue from
the outside. Affected stems may crack open. This
disease can be carried in or on the seed potatoes and
is able to live for long periods in the soil once
Cultural Controls: Select disease-free seed
potatoes for slip production. Use new land or
fumigated land for seedbeds. Infested land should be
rotated to other crops for 4-5 years. Use resistant