Recently, there has been interest in making early season fungicide applications to corn – at the V5 to V7 stages. This concept has been promoted by some fungicide manufacturers. There has been some research on this by universities in the Corn Belt, but not much.
There is little question that fungicide applications made at the traditional time, VT – R1(tasseling through brown silk), will usually have a much greater effect on corn yield than an application at V5 to V7 – assuming disease pressure is heavy enough to justify a fungicide application at all!
But early fungicide applications are made using low rates and can be done at comparatively little cost, especially if they are combined with a postemergence herbicide application. So the question is whether producers would get enough of a yield increase from a low-rate, early application of fungicide to justify the low cost and potential disease resistance risk. Corn producers looking at this option would probably be thinking about using it as a supplement to a VT/R1 application, not a replacement.
To address the question of early fungicide applications to corn, we have to start by looking at what is going on with corn plants at that stage of growth, and what diseases might be expected to be present early in the season in Kansas.
Kernel row number determination of the uppermost ear begins shortly after the ear shoot is initiated (V5 to V6) and continues through at least V8. Anything from about V5 to V12 that severely limits photosynthesis, such as loss of leaf tissue, can result in fewer kernel rows or, more likely, fewer kernels per row. Although such early stress can be important, it will have much less potential to reduce yield than the same level of stress that occurs shortly before to shortly after pollination.
From this perspective, it might seem to make some sense to apply a low rate of fungicide to help protect the plants from disease stress at V5 to V7, and for the period of time following application consistent with the level of residual activity from the fungicide being used.
But what diseases might be present at these stages of growth? The most common early season diseases on corn in Kansas would be anthracnose leaf blight and possibly common rust.
Anthracnose leaf blight is most commonly found in continuous corn fields under no-till or strip-till residue management systems. The disease infects corn most often in the seedling stage, prior to V5, so a fungicide application made at V5 will usually be too late to help control anthracnose leaf blight. Contrary to popular thinking, controlling anthracnose leaf blight will not help reduce the incidence of anthracnose stalk rot since the pathogen overwinters in corn residue. Anthracnose stalk rot typically occurs later in the season, since the stalk rot pathogen is caused by an infection through the roots, not the leaves.
Common rust has not been shown to cause any yield loss in field corn, and like anthracnose stalk rot, generally infects the plants after the V5 stage of development.
The disease that is responsible for most of the yield loss to corn in Kansas – gray leaf spot – typically occurs later in the season. Fungicides do have some residual activity (about 21 days for strobilurins and 14 days for triazoles), and would protect the leaves on which they were applied against any early infections. But since fungicides are not translocated from one leaf to another, an application made at V5 to V7 would not protect leaves emerging later, when protection against gray leaf spot may be most needed.
The bottom line is that an early season application of fungicide to corn would primarily be used to help control anthracnose leaf blight, but would have limited effect on other diseases.
There has been some university research on early fungicide applications on corn, including a 2010 K-State trial at Garden City. Headline has most often been used as the fungicide of choice in these studies. A compilation of test results from Iowa, Illinois, Nebraska, and Wisconsin was recently conducted by Carl Bradley at the University of Illinois. For information on his findings, see: http://bulletin.ipm.illinois.edu/article.php?id=1284
In short, Bradley found there was an average yield response of 1.5 bu/acre to a V6 application of fungicide (Headline at either 3 or 6 oz/acre) in tests so far, compared to an average yield response of 8 bu/acre when applied at the VT to R1 stages of growth. There was essentially no yield difference when comparing a combination of V6+VT applications to a single VT application. In Kansas, there was only a 0.1 bushel difference in yield between a V5 + R1 and an R1-only application.
There is considerable variability of corn yields in tests like this when conducted in small, replicated plots. It takes many tests to begin to get a consistent picture of the kind of results to expect. We are not there yet in the study of early fungicide applications. There have not been enough test results to be entirely confident in predicting the kind of yield response producers might expect.
The research results to date would suggest that it is unlikely that a V5 to V7 application of a low rate of fungicide would pay, despite the low cost of such an application.
Another and probably more important factor to consider is the potential for overuse of strobiluron and triazole fungicides to result in the development of resistant strains of disease pathogens. When half-rates of a product are used, there is typically not a 100% kill of the pathogen. The survivors are likely to have a higher tolerance to the fungicide, and after several years of this type of selection pressure, a population that is totally resistant to the fungicide may develop.
Gray leaf spot is caused by the fungus Cercospora zeae-maydis. Several members of the Cercospora genus have shown the ability to develop fungicide resistance, including C. sojina, the cause of frogeye leaf spot in soybeans.
Currently, K-State Research and Extension does not recommend the use of half-rates of fungicide at any application timing due to the increased risk of fungicide resistance development. Keep in mind also that the product labels may not support the use of reduced rates. The Headline label, for example, specifies product use rates of 6-12 oz/acre, but states nothing about rates lower than 6 oz/acre. The label does not specify when the product should be applied, except that it should be applied prior to disease development.