By Anne Dorrance and Pierce Paul, CORN Newsletter
Several calls this past week for fungicide applications on corn and soybean at all different growth stages. So let’s review what might be at stake here.
Soybeans. 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. Let’s dig a bit deeper. Both of these diseases are caused by fungi but the 
frogeye leaf spot is a polycyclic disease, meaning that multiple infections occur on new leaves through the season while the 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 airflow 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 a 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 again 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.
The story is very similar to a corn pathology standpoint. Most of our major diseases (gray leaf spot, northern corn leaf blight, eyespot) are driven by wet, humid conditions, consequently, the dry weather we have experienced over the last several days will keep most diseases in check. Fungicides are not warranted under these conditions; it just does not pay. Although some product labels may mention yield responses under drought-like conditions, our data do not support such a benefit. We see the highest yield responses when fungicides are applied to susceptible hybrids at VT-R1 under disease-favorable conditions. These conditions would include extended periods of dew and high relative humidity, especially during the early- to mid-morning hours.
For a disease like southern rust that usually blows up from the south, and tar spot, an emerging disease of increasing concern in the state, fields should be scouted before making an application. Both diseases develop well under warm conditions, but they also need moisture and high relative humidity to spread. In the case of tar spot, based on what we have seen in 2018 and 2019, it usually develops well into grain fill (R4-R5), and as such, may have little effect on grain yield. Data from some states in the western half of the corn belt show that when tar spot develops early, yield loss may be substantial. The same is true for early southern rust development. So, scout fields to see what is out there and at what level before investing in fungicide application.