Symptoms and Signs
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.
Comments on the Disease
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 un-composted 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 and proper environment, resting spores can germinate and release swimming zoospores that infect the host. Infection and disease development are favored by cool, wet conditions.
Management
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 three or more years may be beneficial on some infested sites. Do not use un-composted manure from animals that have fed on tubers infested with the pathogen. Plant in well-drained soils, and avoid overirrigation. The addition of zinc sulfate (ZnSO4) may reduce incidence of the disease. 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 per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | METAM SODIUM*§ | |||
| (Vapam, Sectagon) | 37.5–75 gal | See label | NA | |
| 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. | ||||
| B. | METAM POTASSIUM*§ | |||
| (K-Pam) | Label rates | Label rates | NA | |
| 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. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| § | Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates. |
| * | Permit required from county agricultural commissioner for purchase or use. |