Scouting for disease

By Jason Hartschuh, CCA, Agricultural and Natural Resources Educator, Ohio State University, Extension Crawford County

Q: We had tar spot bad in our area last year should we be planning to spray all of our corn acres this year?

A: Tar spot treatment, like with all other diseases, should rely on a strong scouting program. The risk is higher this year, especially in continuous corn, but we also have to have favorable environmental conditions. In fields where corn is following soybeans or wheat, the risk is slightly lower but if favorable conditions develop, spores may move in from other areas. With all diseases, scouting is critical to determining if a fungicide needs to be applied. Lesions will be small, black, raised spots appearing on both sides of the leaves along with leaf sheaths and husks. Spots may be on green or brown, dying tissue. Spots on green tissue may have tan or brown halos. Once tar spot is identified, fields should be monitored every 7 to 10 days for incidence levels to increase, even if a fungicide is applied.

Q: What is the best time to apply fungicide to corn? 

A: While yield increases have been seen from V6 through R3 the most consistent response to fungicide application is when the application is made between Tasseling (VT) and Silking (R1). This is when we see the best disease control and yield response. While corn prices are high this year it is still important to scout for disease pressure. We consistently see positive returns to the fungicide application when diseases are present. Disease pressure can vary by hybrid and planting date. The disease you are targeting can also affect timing. Gibberella ear rot control is most effective when silks are still wet with disease control dropping off greatly at brown silk. While tar spot trials have seen good control at R1 if disease is found at Kernel Blister (R2) or Kernel Milk (R3) and no fungicide has been applied a fungicide application may have a positive return. Most trials have shown that about 3 weeks after the R1 fungicide application tar spot spread increased. 

Q: I feed my own corn and have concerns with vomitoxin. Will any fungicide help control vomitoxin?

A: Certain fungicides have been shown to help lower Vomitoxin levels. The vomitoxin DON is cause by Fusarium Graminearum which also causes head scab so in a similar fashion only a few products control this ear rot. Proline 480SC and Miravis Neo have shown the most consistent ability to reduce DON levels but other products that contain a group 3 triazole active ingredient may also have some control. Control has been variable though and maybe improved with better fungicide coverage of the silks.

Q: What weather conditions favor disease development in corn? 

A: Each disease has slightly different environmental conditions that they thrive in, but these conditions can overlap or happen within days of each other. Gray leaf spot is favored by warm temperatures between 70 and 90 degrees F and high relative humidity. Northern corn leaf blight also favors wet conditions, but prefers cooler weather of 64 degrees to 80 degrees F. Tar spot is a cool weather disease favoring temperatures from 59 degrees to 70 degrees F during humid conditions of 85% relative humidity or more keeping leaves wet for greater than 7 hours. Another cool weather disease is common rust with optimal disease conditions being temperatures of 61 degrees to 77 degrees F and 6 hours of leaf wetness. Southern rust is more of a late season disease, preferring the warmer temperatures of 77 degrees and 88 degrees F. 

Q: How do I distinguish between gray leaf spot and northern corn leaf blight?

A: While fields often have both northern corn leaf blight and gray leaf spot these two diseases have very different appearances. Gray leaf spot has gray to tan lesions developing between the veins and are distinctly rectangular with smooth, linear margins along the leaf veins. Lesions are slow to develop, needing 14 to 21 days, and begin in the lower leaves. Northern corn leaf blight lesions typically have a tan color and are elliptical or cigar shaped with smooth rounded ends. Knowing which disease, you have can help you pick the most effective fungicide to apply.

Q: How can common rust and southern rust be told apart?

A: Common rust is what we usually have in Ohio but on occasion southern rust may be present. Common rust is rarely of economic concern but the develop of southern rust can have economic yield impacts. The colors are different between the two. Common rust is brownish to a cinnamon-brown while southern rust has a reddish orange appearance. Southern rust mostly develops on the top of the leaf and maybe on the stems and husks while common rust is on both sides of the leaves and generally only on the leaves. The last difference is in shape and distribution common rust pustules are large and oval to elongated with a scatted appearance over the leaf. Southern rust is small, circular, and evenly distributed over the leaf.