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Issue No. 6, Article 4/May 10, 2012

Does Early-Stage Corn Need a Fungicide Application?

Applications of foliar fungicides to early-stage corn (V5–V6) are being marketed again this year. Some of the common questions I have received in recent years about applying fungicides to early-stage corn, along with answers, are provided here.

What diseases would a fungicide applied at V5 protect against?

Anthracnose leaf blight generally is the only foliar disease that might be present around the V5 growth stage, but management may not be necessary. Once temperatures warm up and corn begins to grow rapidly, new leaves often are not affected by this disease. In some years, common rust also could be present when corn is at this stage. This may be especially true in years when corn planting is delayed. The common rust pathogen is negatively affected by hot temperatures, so fungicides generally are not needed to control common rust in a typical growing season in Illinois. In addition, most dent corn hybrids have a relatively high level of resistance to common rust.

Anthracnose leaf blight symptoms on corn.

Common rust pustules on a corn leaf.

If I protect against anthracnose leaf blight early in the season, will I also protect against anthracnose stalk rot?

Although anthracnose leaf blight and stalk rot are caused by the same fungal pathogen (Colletotrichum graminicola), their disease cycles differ. Results from research has indicated that anthracnose stalk rot is more likely to be caused by root infection of the pathogen, rather than by spores from leaf blight lesions, so control of anthracnose leaf blight will not necessarily mean than anthracnose stalk rot also will be controlled.

What effect will a V5 fungicide application have on the disease cycle of "later-season" foliar diseases like gray leaf spot and northern leaf blight?

Some foliar diseases do progress vertically on the corn plant (the disease begins on lower leaves, spores are splashed to leaves above, lesions from those leaves produce spores that are splashed to the next set of leaves, etc.). It would be possible, then, that if a fungicide were still providing protection to a lower leaf when a spore landed on it, the fungicide could slow the progression of a disease like northern leaf blight. This is unlikely, however, because a fungicide's window of protection is only up to about three weeks, so these initial infections on the lower leaves would have to occur within that window. Under conditions favorable for disease, millions of spores may be landing on plants, some of them on unprotected corn tissue--likely enough to cause infections and continue normal disease progression.

Since some of the fungicide may come into contact with corn residue on the soil surface, will this reduce the amount of disease inoculum (spores) that may come from the residue?

Fungicides differ in how they inhibit fungi, and not all have antisporulation activity. In addition, since corn residue is not actively growing, spread of the fungicide within that residue is very limited (there is no active xylem for movement, etc.). For these reasons, it is highly unlikely that the amount of inoculum borne from these residues would be affected by V5 fungicide applications.

I only want to apply once, and it is much easier to spray at the V5 growth stage compared to the VT-R1 growth stages because I can use a ground rig and apply with my postemergence herbicides. Which is more effective, a V5 or VT-R1 application?

The best data that I have on this came from the University of Illinois research farm near Monmouth in 2010 and 2011, when there were contrasting levels of disease pressure (high in 2010, low in 2011). The results (Figure 7) show that a VT or R1 application provided the best disease control and yield benefit in 2010 and that no application provided a yield benefit in 2011 (because of the low disease pressure).

Figure 7. Results from corn fungicide trials conducted near Monmouth, Illinois, in 2010 and 2011.

--Carl A. Bradley

Carl A. Bradley

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