How to Manage Pests
UC Pest Management Guidelines
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.
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, and rot can continue in transit. Potato cultivars vary in their degree of susceptibility but current cultivars have not been well classified.
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 noninfested fields. Do not store tubers in the ground unnecessarily 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.
UC IPM Pest Management Guidelines: Potato