Issue No. 14, Article 10/June 29, 2007
Can Foliar Fungicide Raise Corn Yield When There's Little Disease?
Disclaimer: This article is not meant to challenge anything that Carl Bradley and Matt Montgomery wrote in their June 8, 2007, Bulletin article concerning the use of foliar fungicides on corn.
I am weighing in on the topic of using foliar fungicides on corn only because there has been some publicity and a lot of questions about the possibility that strobilurin fungicides, especially Headline (a BASF product), might boost corn yield in the absence of foliar disease. With dry weather over much of the state until last week, there has been very little foliar disease development so far. So as tasseling gets underway in the earlier fields this week, the question about using foliar fungicide on corn even when there is little or no disease is on the mind of many, perhaps most, Illinois corn producers.
While BASF is careful to say that the primary activity of Headline is to control disease, there are also suggestions that the fungicide can affect plant growth directly, through such mechanisms as reducing respiration. They have also mentioned such effects as increasing tolerance to cold and drought stress. In general, a decrease in foliar disease will increase photosynthetic rates, which could contribute to all of these beneficial effects. But will there be such effects if no disease develops?
Respiration is the process by which plants (and animals) break down sugars to produce carbon dioxide, water, and energy. It's an essential process, but in plants, we think that high night temperatures might result in excessive rates of respiration, with the loss of sugar but little benefit to the plant otherwise. We refer to "net" photosynthetic rate as the rate of sugar formation by photosynthesis minus the rate of sugar loss through respiration. Reducing respiration, which may be one way that a fungicide acts to stop fungal growth, could thereby be an advantage in plants, as long as the reduction weren't too severe. This could help explain how such a fungicide could benefit plant growth. As an example, the chemical dinitrophenol affects respiration rate, and it was used as a herbicide, a fungicide, an insecticide, and a growth regulator to increase corn yield before being banned due to toxicological problems in the 1980s. Such different uses require different rates, and regulating how much of the chemical gets to the right place in the plant is never easy.
In any case, a small reduction in respiration rate, especially if it came at a time when the plant tends to be low in energy (sugar), might be beneficial. The plant usually experiences its lowest sugar levels at the time of pollination, when demands of continuing vegetative growth overlap with demands due to pollen production and ear and silk growth, thus stretching the supply of photosynthate (sugar) in the plant. Cloudy days or water deficits contribute to this problem on the supply side by reducing the photosynthetic rate. Thus there are several paths by which net photosynthetic rates and plant sugar levels might be increased, and any of these would often be beneficial to the plant at pollination time.
In the absence of foliar disease, does a possible physiological effect justify application of foliar fungicide at tasseling? The results of several of our recent studies involving Headline fungicide on corn are shown in Table 1. Some of these trials included nitrogen rate as a variable, in which case results are averaged over N rates. All applications were made at tasseling (VT stage), using the labeled rate of 6.1 oz of Headline per acre. Foliar disease pressure was low in all of these trials, and at sites where disease could be rated, fungicide reduced the incidence of disease from low to very low.
Averaged over all 10 sites, fungicide increased yield by 6.2 bushels per acre. Neither yield level nor previous crop had a consistent effect on yield response to fungicide. At the reported cost of $20 to $25 per acre for the fungicide applied by air, the average response of 6.2 bushels would barely cover costs at $4 per bushel of corn, and it would not cover cost at today's corn price.
While the average response in these trials might not cover the cost of fungicide, three of the 10 trials showed a response above 11 bushels, which would cover costs and return a profit. Unfortunately, we don't know how to predict when such a response will happen. All three of these responses occurred in corn following soybean, so the general idea that corn following corn should benefit more than corn following soybean does not always hold true. If this response is primarily a physiological one rather than one related to disease, having more response under higher-yielding conditions might make sense: yields under such conditions are more often limited by the ability of the plant to supply sugar during pollination than by other stress factors such as shortage of water, root problems, and so on. In that case, even a slight increase in the sugar supply in the plant should increase yield when other factors are not limiting.
Because we are talking here about the decision to apply fungicide for something other than disease control, hybrid traits such as foliar disease resistance might not help us make a better decision. Previous crop might also be of little help, since the effect of previous crop on current crop stress level might be more important than its effect on disease. The fact that we saw some yields above 240 bushels per acre and no effect of fungicide indicates that high yields are not always associated with a positive effect of fungicide on yield, either. We simply do not have a way to predict when there might be a yield benefit from the use of a foliar fungicide when there is not a disease present. We have clear evidence that such an effect can take place, but because it's likely to be a product of complex interactions between crop stress level, weather, and previous growing conditions, we do not know in advance, based on current knowledge, when it will happen.
The BASF company provided some of the funding to do the work I reported here, and we are grateful for the support. We are continuing to work on this in several different studies and locations this year, but results to date do not inspire confidence that surefire guidelines will result from this work.--Emerson Nafziger