Itches and tasseling corn

July 10, 2024 8:51 AM
Blog Post

As corn approaches tasseling, I hear more and more rumblings about fungicide application.  The good to abundant moisture we have had this growing season, has resulted in some itchy trigger fingers. In addition, the early reports of tar spot haven’t helped with the itch.  Do we need to scratch that itch and get a fungicide application on a.s.a.p? Here are some of my thoughts:

  1. Disease incidence (number of plants with disease) and severity (percentage of leaf area with disease) is still very low across Iowa. Disease usually develops exponentially – it starts off at very low levels and then sharply increases (Figure 1).
  2. Fungicides are very effective at protecting corn against disease.  Every year corn pathologists across the U.S. update and publish “Fungicide Efficacy for the Control of Corn Foliar Diseases ” to help with decisions regarding which product to use.
  3. The effective period of a fungicide ranges from 21 to 35 days.  Grain fill (VT through black layer (R6)) ranges from 55 to 65 days. Figure 1 illustrates how applying a fungicide at VT contrasts with applying a fungicide at R2 (blister) on disease development. Applying a fungicide at R2, protects the corn later into the grain fill period and consequently disease development is slowed and has minimal impact on yield.


Three exponential curves illustrating disease development with and without a fungicide.

Figure 1. Graphical representation of disease development and the effect of a fungicide at two application timings on disease development in corn. A fungicide application delays the exponential increase in disease development. The period disease development is delayed is known as the fungicide effective period and it will depend on the fungicide product. Generally, effective periods range from 21 to 35 days. 


Tar spot observed before tasseling

We have had numerous reports of tar spot across the southern half of Iowa. A lot of folks are concerned and itching to get in and spray as soon as those tassels appear. Based on Figure 1, it might be worth scheduling an application a little later.  Thus far, tar spot has been found at very low levels, one or two spots on a leaf and not on every plant. The hot conditions we had in mid-June likely slowed tar spot development.  While moisture is important for tar spot development, temperature is more important (Webster et al. 2024). Extended periods (30 days) of mild (64-73F) temperatures are very favorable for disease development.  At temperatures greater than 73F, tar spot development slows down.  Although the last week of June was cooler, warmer conditions are forecasted for July. When temperatures finally cool down in August/September, tar spot may develop quickly. Consequently, waiting to apply a fungicide may be prudent to protect the crop later into grain fill. If a fungicide is applied too early, a second application at R2 to R3 may be required to protect plants through grain fill.


Other diseases observed

I and many of the ISU Field agronomists have observed common rust but at very low incidence and severity.  Common rust is not usually of concern since most hybrids have very good resistance, and summer temperatures are usually too warm to favor disease development.

Rebecca Vittetoe shared with me some photos of bacterial leaf streak (BLS; Figure 2) from a field in in southeastern Iowa. This disease is not too surprising to see considering the precipitation we have had, and there are some hybrids that tend to be more susceptible to the disease. Note that BLS can look very like gray leaf spot (GLS).  Probably the easiest way to distinguish the two diseases is GLS will first occur on the lowest leaves of the canopy while BLS will be observed in the mid- to upper canopy. Other distinguishing features of BLS are water soaking (Figure 3)  and the edges of the lesions are wavy (Figures 2 and 3.) Remember, BLS cannot be managed with a fungicide.

Corn leaf with symptoms of bacterial leaf streak.

Figure 2. Typical symptoms of bacterial leaf streak (BLS) on corn. Note the wavy margins that distinguish BLS from gray leaf spot.

Corn leaf with symptoms of bacterial leaf streak and watersoaking

Figure 3. Water soaking associated with bacterial leaf streak lesions on corn.


Webster et al. 2023. Tar spot prediction: The weather matters.



Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...