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White mold


© Iowa State University

Pathogen(s) causing disease:

Symptoms:

White mold first becomes apparent when single plants within a generally healthy canopy wilt and die rapidly in mid-summer. Leaves remain on the stem but turn brown, and the entire plant dies. Close inspection of the lower stem reveals a bleached area, often originating from a leaf axil and extending 2 to 6 inches in each direction along the stem. When a lesion girdles the stem, the tissue above it dies — not all the stem tissue will die, but all leaf tissue generally dies. Under moist conditions the bleached area may be covered with the fungus’ fluffy white mycelium. Eventually, black, oblong structures, from 1/8-inch to 3/4-inch long, may be visible in the center of the bleached area.

Conditions:

Cool, wet conditions during early reproductive stages favor disease development when the pathogen is present. The disease is often most severe in varieties that have a denser, faster-closing canopy when rain, cool temps and high humidity is present.

Management:

If a field with white mold is harvested, clean the combine before moving to fields with no history of the disease. If white mold is restricted to a portion of the field, that restricted area should be harvested last and independently from the rest of the field. If the disease is already present in a field, keep sclerotia out of the upper layer of the soil, and prevent the sclerotia from distributing over a wider area. Avoid planting 200,000 plants per acre regardless of row width. In fields with a history of white mold, 125,000 to 150,000 plants per acre are recommended. Conventional wisdom suggests that burying sclerotia deeply by plowing will reduce white mold. Rotating with nonhost crops can potentially reduce white mold incidence. Introducing a small grain somewhere in rotation with soybean will eventually result in a lower incidence of white mold. Partial resistance to white mold has been identified, and seed dealers provide ratings on the resistance levels of their varieties. Several fungicides can provide some level of disease suppression, but proper timing and good canopy penetration are essential. Fungicide application may be more effective when disease pressure is no more than moderate. Seed treatments are also a viable option.

Sources:

Purdue
ISU

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