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How to Manage Pests
UC Pest Management Guidelines
Potato
Powdery Scab
Pathogen: Spongospora subterranea
(Reviewed 8/07,
updated 8/07)
In this Guideline:
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Tubers infected with
powdery scab develop small purplish brown pustules about 0.06 inch (1.5 mm) in
diameter. The pustules typically become raised, brown, and wartlike as they
enlarge to about 0.37 inch (9 mm) in diameter and rupture the tuber periderm.
Powdery scab lesions on tubers may be confused with lesions of common
scab, and laboratory confirmation
of powdery scab is advised. Microscopic observation of mature powdery scab
lesions typically reveals diagnostic dark brown spore balls of the causal fungus.
In addition to tuber symptoms, S. subterranea causes galls on roots and
stolons. Foliage symptoms have not been observed in California.
The inoculum that
initiates powdery scab can originate from infected seed tubers or infested
soil. Once Spongospora subterranea is introduced into a soil, its resting spores
survive indefinitely (3 to 10 years). The pathogen can also persist in noncomposted
manure from animals that have fed on infested tubers. When done properly,
however, composting can apparently destroy infective inoculum of S.
subterranea. In the presence of susceptible potato roots,
resting spores can germinate and release swimming zoospores that infect the
host. Infection and disease development are favored by cool, wet conditions.
Powdery scab can be
minimized by using good cultural practices. Avoid planting seed tubers affected
by powdery scab. Avoid planting potatoes on sites with a history of the disease.
Russet varieties show less tuber damage than smooth, red- and white-skinned varieties;
russet-skinned varieties rarely show tuber symptoms although root galls are
common. Susceptibility varies among red and white varieties. Rotations out of
potatoes for 3 or more years may be beneficial on some infested sites. Do not
use noncomposted manure from animals that have fed on tubers infested with the
pathogen. Plant in well-drained soils, and avoid overirrigation. In fields with
a history of powdery scab, preplant chemigation with metam sodium can reduce
disease in a subsequent crop of potatoes.
| Common name |
Amount/Acre |
R.E.I.+ |
P.H.I.+ |
| (trade name) |
|
(hours) |
(days) |
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| When choosing a
pesticide, consider information relating to the impact on environmental
quality.
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| A. |
METAM SODIUM* |
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(Vapam, Sectagon) |
37.5–75 gal |
48 |
0 |
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(Metam Sodium) |
50–100 gal |
48 |
0 |
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COMMENTS: Follow
manufacturer recommendations on waiting interval between treatment and planting.
Fumigants such as metam sodium
are a source of volatile organic compounds (VOCs), which are a major
air quality issue. Fumigate only as a last resort when other management strategies
have not been successful or are not available. |
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UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Diseases
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis
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