Time to Scout Field Crops: What to Look for in Corn

Yes, we have found tar spot in Indiana.

Early planted corn in Indiana is reaching late vegetative stages and tasseling in the south. Therefore, it is time to start monitoring for diseases to make an informed decision if a fungicide is necessary. As a reminder for disease to occur three things need to be present; 1. Virulent Pathogen, 2. Susceptible Host, and 3. Favorable Environment. Hot, dry weather is generally not conducive for many of the foliar diseases in corn that we monitor in Indiana such as, gray leaf spot, northern corn leaf blight, northern corn leaf spot, and tar spot (figure 1 and 2). This week we have found a low incidence of tar spot, gray leaf spot, northern corn leaf spot, Anthracnose, and common rust in the lower canopy.

Figure 1. Examples of A- gray leaf spot, B-northern corn leaf blight, and C-northern corn leaf spot lesions on a corn leaf. Photo Credits: Darcy Telenko

Figure 1. Examples of A- gray leaf spot, B-northern corn leaf blight, and C-northern corn leaf spot lesions on a corn leaf. Photo Credits: Darcy Telenko

A few questions to think about when scouting and looking for disease:

  1. What is the disease history in the field? How much residue is still present? (What happened in 2018, if you have a 2-year rotation?)
  2. What growth stage is the field? Early planting vs. late
  3. Is irrigation being applied? How much and how often? If water is being applied, it can change the environmental conditions and disease risk in a field.

Tar spot of corn continues to be a concern this season after the localized epidemics we experienced last year in Indiana. In our scouting rounds this week we did find tar spot. We suspect the infection happened early June as it was located on the lowest leaves of the plant in both fields. The disease has not progressed and the current hot and dry condition DO NOT favor tar spot. The fields were located in Porter and LaPorte Counties in Indiana (see map Figure 3). Michigan has also had a positive confirmation in Gratoit County. In the Indiana fields, the corn was between V12 and V14, had a history of tar spot in 2018 or 2019, and was under irrigation. The disease was found in some of the lowest leaves in the canopy. We will continue to monitor the disease progression in these fields and will provide updates on any significant spread in the field or increases in disease severity. Again, the hot and dry conditions are not favorable for tar spot. See the forecast from the Tarspotter App under development (Figure 4). Early June was when there was favorable temperature and moisture conditions, and most likely when these first few leaves were infected. The risk as of today, July 7, 2020, is low (blue) at all our sites in Indiana (Figure 4).

 

Figure 2. Corn leaves infected by tar spot. Infection can range from mild to severe on a leaf. The spots will be raised (bumpy to the touch) and will not rub off. In addition, they may be surround by a tan or brown halo, and high severity can lead to a rapid blighting of the leaf. Photo Credits: Darcy Telenko

Figure 2. Corn leaves infected by tar spot. Infection can range from mild to severe on a leaf. The spots will be raised (bumpy to the touch) and will not rub off. In addition, they may be surround by a tan or brown halo, and high severity can lead to a rapid blighting of the leaf. Photo Credits: Darcy Telenko

 

Figure 3. Tar spot map for 2020. Source: EddMaps at https://corndev.ipmpipe.org/tarspot-2/

Figure 3. Tar spot map for 2020. Source: EddMaps at https://corndev.ipmpipe.org/tarspot-2/

 

Figure 4. Tarspotter App forecast from June 5, 2020 and July 7, 2020. Red color indicates favorable environmental conditions for tar spot if corn at V5 or larger, blue color indicates unfavorable environmental conditions for tar spot. Source: Tarspotter App v. 0.47 Smith, D., et al. ©2020 Board of Regents of the University of Wisconsin System.

Figure 4. Tarspotter App forecast from June 5, 2020 and July 7, 2020. Red color indicates favorable environmental conditions for tar spot if corn at V5 or larger, blue color indicates unfavorable environmental conditions for tar spot. Source: Tarspotter App v. 0.47 Smith, D., et al. ©2020 Board of Regents of the University of Wisconsin System.

 

After hearing this news we know the next question – should I be putting out a fungicide?

Research has shown the best return on investment in making a fungicide application in corn occurs when the fungal diseases are active in the corn canopy. Most of our corn sites across the state are quite clean and disease pressure is minimal, so far this season. It is important to keep scouting.

Based on our 2019 research, to minimize the impact of tar spot on crop yield we need to be protecting the ear leaf and above until the corn reaches black layer. In our fungicide timing trials applications made at VT/R1 (tassel /silk) did a good job controlling tar spot, but we did see that once the fungicide ran out of steam (3-week window) tar spot began to pick up. A well-timed, informed fungicide application will be important to reduced disease severity when it is needed, and we recommend holding off until the diseases become active and corn is nearing VT/R1 (tassel/silk) or even R2 (blister). This is especially important if the hot and dry conditions continue.

If you suspect tar spot in your fields, please consider submitting samples for confirmation.  We are interested in documenting the disease in Indiana, similar to last year. Research funding from the Indiana Corn Marketing Council is supporting sample processing, therefore there will be no charge for corn tar spot samples submitted to the clinic.

What to look for: Small, black, raised spots (circular or oval) develop on infected plants, and may appear on one or both sides of the leaves, leaf sheaths, and husks. Spots may be found on both healthy (green) and dying (brown) tissue. Sometime, the black spots may be surrounded by a tan or brown halo; this is especially obvious on healthy leaves (see Figure 2).

I want to ask before you submit a sample you do a quick and dirty “scratch test” to see if you can rub the spot off the leaf, especially if you have leaves with just a few small spots. I have been successful in detecting these false spots by using my nail to scratch as the suspect lesion. This is a quick way to check, but as always if you are unsure send an image or the sample to the Purdue Plant Pest Diagnostic Lab. Please collect several leaves showing the symptoms and send them with a PPDL form https://ag.purdue.edu/btny/ppdl/Documents/Forms/PPDL-Form_13MAY15FILLABLE.pdf.

Please wrap the leaves in newspaper, ship in a large envelope, and ship early in the week. If you are sending samples from multiple locations please label them and provide the date collected, variety of corn, field zip code or county, and previous crop.

Mail to: Plant and Pest Diagnostic Laboratory, LSPS-Room 116, Purdue University, 915 W. State Street, West Lafayette, Indiana 47907-2054.

The lab is operating and the building is open, but the lab door is remaining locked. If dropping off a sample is more convenient than shipping, please call or email the lab prior to stopping by: Phone – 765-494-7071; Email – ppdl-samples@purdue.edu.

In addition, the 2020 tar spot and southern rust maps are live that will be updated when a positive county confirmation is detected. If you are interested in up-to-date information on the current detection of these diseases, the maps are available on the front page of our Extension website https://extension.purdue.edu/fieldcroppathology/ or at https://corn.ipmpipe.org/

If you have any question please contact Darcy Telenko (dtelenko@purdue.edu/764-496-5168) or PPDL (ppdl-samples@purdue.edu/765-494-7071)

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