This is the time of year when agronomists talk non-stop about watching for disease development. Their message usually centers around determining if there is enough disease pressure showing up to warrant a fungicide application. It’s also the time of year when retail dealers scramble to make sure they can meet customer needs if lots of acres need to be sprayed with fungicide.
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What people seldom talk about is whether or not a small amount of disease that destroys leaf tissue can impact yield potential, even if conditions change and the disease stops.
Based on the Purdue University Corn & Soybean Field Guide, created by the Purdue Crop Diagnostic Training and Research Center, the answer may be "yes."
The impact may be small, but easily could be on the order of what you can lose to unevenly spaced stands, for example. When budgets are tight and every bushel counts, leaf damage could be a factor. It may be another reason for digging deeper when researching hybrids before each new season to find those with the best foliar disease resistance packages.
Estimate leaf loss
Leaf tissue turned brown by disease is lost leaf tissue — just as lost as if it were knocked out by hail. According to the Purdue guide, a smattering of lesions can make up 5% leaf damage. It takes a number of larger lesions, with some growing together, to reach 25%, and more to reach 50%.
Dave Nanda, a crops consultant, recommends checking several parts of a field, not just one spot, to better estimate percentage of leaf loss that has already occurred due to disease. Disease severity won’t necessarily be uniform across a field.
Here’s another key point. Determine leaf loss per plant based on all leaves, not just one or two of the most affected leaves. If one leaf has 25% damage but another has none, average the losses together.
Once you’ve estimated how much leaf tissue has been lost to disease lesions, you can refer to a chart in the Purdue guide to determine how much, if any, yield potential may have already been sacrificed by the simple loss of leaf tissue. When leaf tissue is lost, the photosynthetic factory shrinks, Nanda says. That can mean less sugars produced, and if it is severe enough, some yield loss.
Check yield loss chart
At the 12-leaf stage of corn, with 12 leaves with collars exposed, it takes 25% leaf loss to get 2% expected loss in yield potential. However, as you move toward tasseling, the percent yield loss could increase. At tasseling, if 25% of the leaf is destroyed — by disease or hail, either one — you can expect a 9% yield loss based on averages over time. If the potential was 200 bushels per acre, the new potential is just over 180 bushels per acre.
If leaves show an average of 25% damage, you would likely have the field sprayed. If damage has already reached 50% by tasseling, or if it reaches 50% soon after because the field isn’t sprayed, yield loss can escalate to more than 30%.
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The message is simple. Pay attention to leaf loss due to disease lesions whenever you see it. Then refer to guidelines that are available to make decisions about either fungicide applications yet this year, or hybrid choices for next year.