It is important to continue to scout for diseases in both corn and soybeans. Recent rain events have increased favorable environmental conditions for the development of foliar diseases in both crops.
In our scouting rounds this week we are starting to see gray leaf spot, northern corn leaf blight, and Physoderma in corn (Figure 1), and frogeye leaf spot, downy mildew and Septoria brown spot in soybean (Figure 2). In addition, we continue to add counties with active tar spot and southern rust in Indiana.
The most frequent question I have received is, “Should we make a fungicide application?” My response – What diseases are you finding in your field? What is your hybrid/variety susceptibility and field history? What growth stage? Are you irrigating?
A fungicide application can be effective at reducing disease and protecting yield, but there are a number of factors that need to consider: the field history/previous crop, the amount of disease present in the field, hybrid/variety susceptibility, weather conditions, the value of the crop, and cost of fungicide application.
Southern rust was officially confirmed in our sentinel plots in Randolph County late last week, and since we have also confirmed in Whitley, Daviess, Jennings and Lawrence. (Figure 3). I suspect southern rust can be found across the state wherever spores settled after moving on weather systems from the south.
We need your help – if you are out scouting field in the surrounding counties please let us know if you find any suspect samples please send to the Purdue Plant Pest Diagnostic Lab.
Southern rust pustules generally tend to occur on the upper surface of the leaf, and produce chlorotic symptoms on the underside of the leaf (Figure 3). These pustules rupture the leaf surface and are orange to tan in color. They are circular to oval in shape. We are seeing a lot of common rust as well and both diseases could be present on a leaf.
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There are a few characteristics to use to try to distinguish southern rust from common rust. Common rust will form pustules on both sides of the leaf. In addition, common rust pustules tend to be spread out across the leaf, and less densely clustered. Common rust pustules have a brick red to brown coloration and may be more elongated than southern rust pustules.
Check out the southern rust publication for more images of southern rust and other diseases that might mimic it. This publication also has good information on determining when a fungicide application will be beneficial. The publication is available here.
Each year the rust spores (urediniospores) travel on air currents from tropical regions to fields in Indiana. Short periods of leaf wetness are required for infection by both rust fungi. Morning dews in Indiana can provide the six hours of moisture required for infection and disease development.
Generally, southern rust prefers warmer temperatures — with infection occurring between 77-82°F. Southern rust is usually detected in Indiana late August and September and generally not something to worry about. Now that we have found it late-July it will be very important to keep eye out for southern rust in your field.
Favorable weather can cause the infection to repeat in a disease cycle as short as seven days, resulting in secondary infections and new pustules. Each pustule can produce thousands of spores that can infect corn leaves and produce additional pustules.
Disease intensity can reach epidemic levels very quickly as these cycles continue. The speed at which corn rust can reach damaging levels is why it is necessary to pay careful attention to the level and timing of initial disease infection in susceptible hybrids.
Young leaves are more susceptible to rust infection than mature leaves. Our late-planted corn may be at greater risk for infection since the rust spores are now here in Indiana. Recent weather conditions continue to favor disease development therefore I cannot stress enough how important it will be to scout your corn fields and be on the lookout.
Tar spot continues to be on everyone’s mind. The first confirmations in Indiana for 2020 occurred in Porter and LaPorte Counties on July 6. Tar spot was identified at an extremely low incidence in this field (Figure 4), this past week we have begun to see an increase in tar spot severity as it has begun to move up in the canopy (Figure 3).
In addition, we have confirmed tar spot in St. Joseph and Cass counties, and suspect a site in Jasper County. Tar spot had previously been found in 68 counties in Indiana, with the northern part of the state most at risk.
These early tar spot detections are like finding a needle in the haystack and required intensive scouting, but as the disease progresses it will be easier to find as the number of spots increase and it moves up the canopy. We will continue to monitor and update as the season continues.
We are working hard to try to understand this new disease to minimize losses, but have limited data on optimum fungicide timing based on disease threshold. The good news is that we found a number of fungicides are highly efficacious against tar spot.
I would recommend picking a product with multiple modes of action. The national Corn Disease Working Group has developed a very useful fungicide efficacy table for corn diseases.
We will continue keeping a close eye on these diseases. I am interested in adding more locations in surrounding counties in northern Indiana; please contact me if you are interested in helping. If you suspect a field has tar spot please contact us and send a sample to the Purdue PPDL for confirmation.
Both gray leaf spot and northern corn leaf blight are also active in the lower canopy of corn across the state. It is going to be extremely important to be out scouting, especially if you are trying to make a decision on a fungicide application.
Gray leaf spot
This disease is also active in the lower to mid canopy at multiple sites across the state. The lesions are light tan in color and generally narrow and rectangular, and can be as long as 2 inches. As the lesions age they turn grey in color and are delimited by leaf veins (Fig. 1).
This annual disease has become one of the most important foliar diseases in Indiana. Hybrid susceptibility and weather will have the greatest impact on the severity in a field. Fungicide options that are available for gray leaf spot would be a cost effective application in fields that have a history of disease and planted to susceptible hybrids in no-till or reduced-till system.
As a reminder, fungicide applications add an additional cost to corn production. Therefore, economic factors and other disease issues need to be considered before deciding to apply a fungicide to manage gray leaf spot.
Previous research has determined the best time to apply fungicides in preventing yield loss with the most economic return occurs when fungicides are applied in response to disease at tasseling (VT) through early silking (R1).
As a reminder the field history, disease activity, hybrid susceptibility, weather conditions, the value of corn and soybean, and cost of fungicide application are factors that should be considered in making a decision to apply a foliar fungicide.
Several fungicides are available to help manage these foliar diseases with a recommended application occurring at late vegetative stages through R1 in corn, and R1- in soybean for white mold and R3 in soybean for frogeye leaf spot.