Soil Rot (Pox) of Sweetpotato
Vegetable Disease Information Note 3 (VDIN-003)
Charles W Averre, Extension Plant Pathologist
Jean B. Ristaino, Research Plant Pathologist
soil rot (pox) is a widespread and destructive disease of sweetpotato in
North Carolina. The disease has caused significant losses in New Jersey,
Maryland, Louisiana, North Carolina, and California for many years. The
yield and quality of sweetpotatoes are greatly reduced, and infested fields
can remain unproductive. Since the pathogen is soilborne, efforts must be
made to prevent introduction and avoid build-up of inoculum in the field.
Economic and practical controls for growers in North Carolina are possible
by following the recommendations described in this information note.
Large "hot spots" (50 X 200 feet or larger) in the field with
groups of infected plants usually indicate initial entry points of the
pathogen. The disease is often worse in light, well-drained soils. These
areas are often grassy.
Above ground symptoms
on sweetpotato plants include stunting and yellowing of the growing
three months in foilage. This condition can be confused with soil infertility,
residual soil herbicide injury, drainage problems, or wilt (Fusarium)
in wilt-susceptible plants. In severe cases plants may die or produce
The tips of fibrous
roots are rotted. Numerous black lesions of varying size occur on fibrous
roots as well as on the underground stem. As the lesions develop, they
turn black and have distinct margins.
Storage roots exhibit
sunken lesions that are black and crusty. One to several pox lesions
can appear in rows and are usually associated with the lateral roots.
The affected sweetpotato will be misshapen and roots can be severely
constricted or indented in one or more places. Pox lesions can be confused
with circular spot caused by Sclerotium rolfsii. Circular spot
lesions are usually not associated with lateral roots and tends to be
light brown, round with distinct borders, and bitter to the taste. With
both diseases the lesions do not enlarge following harvest.
soil rot (or pox) is caused by the pathogen Streptomyces ipomoea,
a prokaryotic actinomycete. The pathogen is difficult to see in infected
tissue under the microscope and is difficult to isolate on culture media
in the laboratory. The organism produces spores in spiral chains which
are believed to be overwintering structures.
is favored by dry soils and increases as the soil pH rises above 5.2.
The pathogen can survive many years in the soil, however, the disease
potential decreases over the years in the absence of sweetpotatoes, especially
in acid soils. The pathogen can infect a number of weeds in the morning
glory family. Infection is thought to occur through small lateral roots
when soil temperature is above 68F (68 to 108F). Mature storage roots
are resistant to direct infection.
is spread by movement of contaminated transplants, storage roots, soil,
boxes, vehicles, etc. Spread by wind and water erosion is possible. Disease
spread does not normally occur on stored sweetpotato roots. However, the
pathogen can survive in infected storage roots and infest new fields if
these roots are used for seed.
manage Streptomyces soil rot if integrated control measures are used.
These measures are directed at preventing spread to new fields and reducing
disease potential in fields where the disease has occurred.
FUMIGATION MAY NOT GIVE SATISFACTORY CONTROL IN SEVERELY INFESTED FIELDS.
Select fields where the disease has never occurred. Never plant sweetpotatoes
in fields where the disease has recently caused serious losses.
Select fields with
heavier soils that are not especially subject to drought. Early season
irrigation may prevent infection of the fibrous roots, reduce disease
incidence, and increase yields.
of the pathogen into new fields by using only certified disease-free
storage roots to produce disease-free plants. In this operation, use
only non-contaminated equipment, boxes and vehicles. Avoid moving equipment,
etc., that may carry contaminated soil from infested fields to new fields.
a resistant cultivar. Cultivars 'Jasper' and 'Beauregard' are resistant
to the streptomyces soil rot. 'Beauregard' is susceptible to root-knot
Do not lime infested
fields until you have sent a soil sample for fertilizer and lime recommendations
to: Agronomic Division, N. C. Dept. of Agriculture, Raleigh, NC 27611.
State on the form: "for sweetpotato in streptomyces soil rot pox
problem field.; Select fields with a pH of 5.2 or lower to reduce disease.
Develop a long-range,
crop-rotation, soil-management program in infested fields to assure
a low disease potential. For example: year 1 - check soil pH, lime if
needed, and plant soybean; year 2 & 3 - plant corn; year 4 - plant
tobacco, and year 5 - check soil pH and if at pH 5.2 or under, fumigate
soil and plant sweetpotatoes (see 6c). When developing a rotation program,
sweetpotato should not follow a crop requiring a high soil pH.
Treat the soil
with a broad spectrum soil fumigant containing chloropicrin 2-4 weeks
prior to planting. Fumigants must be used as stated on the label. Proper
soil preparation, soil moisture, temperature and depth of application
are essential. The materials are injected 6 to 10 inches deep with one
chisel per row, and the bed is prepared simultaneously. At planting,
cleanly remove two inches from top of the bed and then plant. Do not
plant if fumigant residues remain in the soil. (See Plant Pathology
Information Note 160 - How to check for residual fumigant). Do not rework
the beds prior to planting.
Below are some label rates for fumigants recommended for control of pox.
36-42 inch rows
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of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data. All recommendations for pesticide
use were legal at the time of publication, but the status of registration
and use patterns are subject to change by actions of state and federal
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30,
Employment and program opportunities are offered to all people regardless
race, color, national origin, sex, age, or disability. North Carolina
University at Raleigh, North Carolina A&T State University, U.S. Department
Agriculture, and local governments cooperating.
Oct'99 by G. J. Holmes