Symptoms and Signs
Affected stems on plants with sclerotium stem rot first show a moist decay at or slightly below the soil surface where infection is initiated. Stem lesions expand up and down the stem, and all plant parts can be infected. Stem infection leads to wilting and yellowing of the foliage. Tubers are typically infected by way of stolons. The fungus quickly grows over the tuber surface and invades, resulting in a moist cheesy decay. Portions of infected plant parts and nearby soil often are covered with the white, radiating mycelium of S. rolfsii. The mycelium generates small spherical sclerotia (about 1–2 mm in diameter) that are white when young and brown when mature.
Comments on the Disease
S. rolfsii attacks many field and vegetable crops in warm regions. The fungus persists in soil between crops. Germination and infection by the sclerotia are favored by hot temperatures (80° to 90°F) and moist soil surfaces. Sclerotium stem rot is considered to be a problem only in hot climate growing areas. Losses typically occur at the end of the season. The fungus can invade dead vines as well as living ones. Extensive tuber losses can be initiated within a few days of harvest if the fungus is present. Potato cultivars vary in their degree of susceptibility but current cultivars have not been well classified.
Management
Relatively early planting minimizes stem and tuber rot by avoiding the late season high temperatures that favor disease. Plant fields infested with S. rolfsii before planting fields without infestation. Do not store tubers in the ground unnecessarily long before harvest; this allows more time for infection at favorable warm temperatures. Rotate to crops that are less susceptible (e.g., nonfleshy, root or tuber crops).
Preplant chemigation with metam sodium is recommended for fields known to be infested with S. rolfsii; the treatment has afforded good control for at least one season. Applications of the soil amendment ammonium bicarbonate made just before harvest will kill mycelium, but not the sclerotia, of the fungus, thus preventing tuber infection for about 3 to 5 days.
| 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 | |
| ‡ | 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. |
| NA | Not applicable |