White mold severity in dry beans relative to planting date in 2024
Warm weather experienced in the second half of July into the first few days of August was a significant mitigating factor for white mold development in North Dakota in 2024.
That warm weather in the second half of July coincided with the period when most dry bean crops entered the period of maximum dry bean susceptibility to white mold: full bloom and initial pod development. Soil moisture was high in most fields as dry beans entered bloom, facilitating development of the mushroom-like apothecia that produce spores of the Sclerotinia fungus. The recurrent rains and heavy morning dews experienced during much of the summer were highly favorable for infection by the Sclerotinia spores and white mold disease development when daytime highs were in the 70s to low 80s but were less favorable for white mold when daytime highs were in the mid to upper 80s.
Those dry bean crops that entered full bloom in the second half of July, when daytime highs regularly reached the mid to upper 80s, were spared significant white mold by the warm weather.
In the crop disease management program in Carrington, we conducted a little over 10 acres of dry bean white mold management studies in 2024. One block of studies was planted on May 22, one block was planted June 6, and one block was planted June 7. The dry beans planted May 22 entered bloom when daytime highs were often in the 70s and developed severe white mold with very little supplemental irrigation (total 0.65 inches all season). The dry beans planted June 6-7 entered full bloom shortly after warm weather hit, and developed much more moderate white mold pressure even with aggressive supplemental irrigation. Our experience in Carrington parallels the observations shared with me by crop advisors: In 2024, the dry bean fields planted after Memorial Day, which generally entered full bloom during the period of warm weather in the second half of July, had much lower white mold pressure than early-planted dry beans.
The differences in white mold pressure relative to dry bean planting date observed in 2024 are a reminder of the importance of daytime high temperatures in determining white mold risk. When daytime highs are in the mid-70s, heavy dew is sufficient for white mold disease development. As daytime highs reach into the mid to upper 80s, white mold risk drops sharply, with severe white mold typically only observed with very high, recurrent rainfall.
Michael Wunsch, Ph. D.
Michael.Wunsch@ndsu.edu
Plant Pathologist
