Dr. Kiersten A. Wise, University of Kentucky Extension Plant Pathologist for Corn, confirmed southern corn leaf rust in Union County last Monday and prepared the following comments on this important disease.
Movement of the disease can be tracked on the Corn ipmPIPE website at https://corn.ipmpipe.org/southerncornrust/. This website reports only university-confirmed observations on southern rust, meaning that counties are only turned red after samples are confirmed by a University of Kentucky diagnostic laboratory. If you suspect southern corn leaf rust, call me so it can be confirmed.
Mid-July is pretty typical timing for when we see southern rust in Kentucky. Farmers may need to spray corn to protect yield from southern rust. Based on several years of Kentucky experience, fungicides could be needed between VT-R3 (tasseling to milk stage) to protect yield.
Any corn that is already at the milk stage will likely not need a fungicide application or an additional fungicide application.
Corn that had a fungicide application pre-tassel will need to be monitored for southern rust development to determine if a second spray is needed. Corn that had a fungicide application at VT or later will likely not need a second fungicide application if it has reached blister (R2) when southern rust is confirmed in a field
Farmers with corn still in vegetative growth stages should wait to spray until disease is detected in a field and try to get as close to VT as possible to apply a fungicide.
Extension Plant Pathologists in the south advocate that it is better to wait and protect plants during the VT-R3 stage. This will require scouting late-planted fields to determine disease onset.
There are many options of fungicides that have efficacy against southern rust. If farmers are concerned about input costs they can choose less expensive products and expect to see similar levels of protection. A list of the current ratings for fungicides for southern rust efficacy can be found on my website at http://daviess.ca.uky.edu/ANR.
Foliar Fungicide Considerations for SoybeanFungicide application in soybeans has been an ongoing activity over the past few weeks. When fields approach the R3 (beginning pod) developmental stage, it generally is a time to consider an application of a foliar fungicide to protect against foliar diseases.
Dr. Carl Bradley, University of Kentucky Extension Plant Pathologist for Soybeans, compiled the following thoughts on fungicide consideration this year. In areas of the state that have received frequent rainfall, the risk of foliar diseases has increased. Besides rainfall, the risk of foliar diseases is also affected by other factors, such as the soybean variety planted and the cropping history in a field.
The primary foliar diseases of concern that have shown the ability to cause economic yield losses in Kentucky recently are frogeye leaf spot and target spot. Both of these diseases are influenced greatly by the soybean variety being grown. Some varieties are highly resistant to frogeye leaf spot, while others may be susceptible.
Therefore, it is important to be aware of the disease ratings of the varieties planted in your fields. Target spot is a relatively new disease to Kentucky. In general, observations of target spot causing severe disease in Kentucky have been limited, and in most cases, symptoms developed too late to be an issue.
However, there were a few fields in western Kentucky in 2019 that had severe target spot, likely due to planting a very susceptible variety.
If the decision is made to apply a foliar fungicide, it is important to choose a product that has efficacy against the spectrum of diseases that might affect your field. It is also important to choose a product that contains multiple modes of action to help manage the potential of fungicide resistance.
Isolates of the frogeye leaf spot pathogen and the Septoria brown spot pathogen that are resistant to strobilurin (QoI) fungicides are present in Kentucky and isolates of the Cercospora leaf blight and target spot pathogens that are resistant to strobilurin fungicides have been confirmed elsewhere in the U.S., so fungicide resistance is an important consideration.
To help make a decision on which fungicide products might work best for the diseases you intend to manage, the Fungicide Efficacy for Control of Soybean Foliar Disease publication on the Crop Protection Network website can provide information that will help with that decision.
Clint Hardy is the agricultural extension agent for the Daviess County Extension Office. He can be reached at 270-685-8480.