By Dr. Anne Dorrance and Dr. Pierce Paul, OSU Extension Plant Pathology, adapted from C.O.R.N. 2020-21
Frogeye leaf spot and white mold on susceptible varieties when the environment is favorable for disease easily pay the cost of application plus save yield losses. Both of these diseases are caused by fungi but frogeye leaf spot is a polycyclic disease, meaning that multiple infections occur on new leaves through the season while white mold is monocyclic and the plant is really only susceptible during the flowering stage. Both of these diseases are also limited geographically in the state. White mold is favored in North East Ohio and down through the central region where fields are smaller and air flow can be an issue. Frogeye has been found on highly susceptible varieties south of 70, but it is moving a bit north so it is one that I am watching.
White mold is also favored by closed canopy, cool nights and high relative humidity. So farmers in these areas should double check their variety ratings first. If it is moderate to low score for resistance (read the fine print) then this year a spray may be warranted. We have gotten consistent control of white mold with Endura at R1. Herbicides that are labeled for white mold suppression have also knocked back this disease, but if a drought occurs or no disease develops, losses of 10% or greater can occur due to the spray alone. For these purposes R1 is a flower on the bottom of 1/3 of the plants in the field.
Frogeye leaf spot –There also must be some inoculum or low level of disease present in the field for this disease to cause substantial and measurable yield losses. This disease will only move in the canopy when there is regular rainfall, and only on susceptible varieties. With dry weather, this will sit and hold. Time to scout for this will be at the end of flowering if it can be found in the field. With drought conditions, the disease will not impact the crop.
Scout fields to see what is out there and at what level before investing in fungicide application.