Streptomyces Soil Rot (Pox) of Sweetpotato
Vegetable Disease Information Note 3 (VDIN-003)
Charles W Averre, Extension Plant Pathologist
Jean B. Ristaino, Research Plant Pathologist


Streptomyces 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.

Symptoms

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 no yield.
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.

Cause and Disease Development

Streptomyces 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.

Disease development 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.

The pathogen 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.

Management

Growers can 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.

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.
Prevent spread 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.
Consider growing a resistant cultivar. Cultivars 'Jasper' and 'Beauregard' are resistant to the streptomyces soil rot. 'Beauregard' is susceptible to root-knot nematode.
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.

SOIL FUMIGATION MAY NOT GIVE SATISFACTORY CONTROL IN SEVERELY INFESTED FIELDS.
Below are some label rates for fumigants recommended for control of pox.

Product
Rate per acre
36-42 inch rows
Telon C-17
10.5 gallons
Chloropicrin
1.8 gallons

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Recommendations 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 regulatory agencies.

Published by
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.
Employment and program opportunities are offered to all people regardless of
race, color, national origin, sex, age, or disability. North Carolina State
University at Raleigh, North Carolina A&T State University, U.S. Department of
Agriculture, and local governments cooperating.
Revised Oct'99 by G. J. Holmes