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  • Writer's pictureRachel

A Foray into Foliar Fungicides

As we begin to tassel and silk across the area, we’ve been getting a lot of questions about foliar fungicide in corn. We recently published an infographic outlining some high points of foliar fungicides but wanted to go a little more in depth.

Factors Favoring Infection

Several factors should be taken into consideration when looking into foliar fungicides in corn. Having more of the following issues present increases the likelihood of an economic return on investment of fungicide. The first factor is susceptibility of hybrids. While all hybrids can benefit form application, those with particularly low disease scores should be taken into account as more at risk. Pioneer studies show those hybrids with a 4 or below rating can result in a 23 bushel per acre yield increase. Environmental conditions are another important factor to consider. Humid and wet weather favor development of multiple diseases. These conditions, in combination with slightly cooler weather than what we are currently experiencing, can result in potential for a large infection. Dense fog and prolonged leaf wetness produce ideal conditions for disease development. Field history is another major factor in the decision-making process. In locations with corn after corn or heavy amounts of residue, the chance for disease infection is higher. Inoculum can overwinter and maintain populations in heavy residue. Additionally, sites that historically have heavy pressure are more likely to be infected again. Locations with irrigation or that were planted later have an increased risk. As always, if disease pressure is found early in the season, it is likely that treatment will be warranted.

The Decision Making Process

So how do we decide how much pressure is too much? When should we trigger treatment? Our objective is to keep disease below the ear and ear leaf. Seventy percent of the carbohydrates for grain fill come from the top 8 to 10 leaves on the plant. If these leaves are compromised by lesions, the photosynthetic rate is reduced and can impact grain fill. One source suggests initiating treatment when 50 % of the plants show lesions 3 leaves below the ear leaf. Another source suggests once disease has reached one leaf below the ear leaf to begin treatment. Yet other sources suggest waiting until the silk has started to brown. Multiple options are available for timing; however the goal is the same, preserve green leaves at and above the ear leaf.

The time from silking to maturity is approximately 60 days. In those 60 days, about 50 percent of the grain dry matter will be set in the last 35 days. It is important to provide the plant full protection through that grain fill window. Since applied fungicides last somewhere between 14 and 21 days, it is imperative that we time our application to match the most critical part of that window. If we apply too soon, the protection that the fungicide gives could be gone before the potential for yield damage is over. Late season development of GLS or southern rust is possible as conditions conducive to infection improve. With our current conditions, we need to place a large emphasis on the potential for disease development, plant stress, and weather patterns. The amount of disease we see at this point is coming from infections that occurred from 4-14 days ago. With relatively dry and hot weather the last few weeks, the amount of inoculum present should be fairly low. However, a stretch of three cooler, wetter days from June 19-22 is contributing to the disease we are seeing now. A rain of 0.40” and temperatures that did not exceed 83 degrees favored infection of disease. Our current conditions would not lead to large amounts of infection that we would see manifested in the next 4 days to two weeks. Rainfall over the weekend was variable, with rainfall estimates from 0.2” to 1.5”. Depending on where you fell in that range, the potential for disease development may be present, particularly with the cool and damp nights and early mornings on Sunday and Monday. Be looking for development of disease any where from the end of this week, through the 14th and 15th of July. Additionally, we do need to take into account cooling of temperatures heading into August. While that seems a long way off, that does fall within our 60 days from silking to maturity. With cooler evening, development of diseases like grey leaf spot could be favored.

The Plant Health Component

There is a final component of fungicide that is crucial to consider and much harder to place an economic threshold on. Improved crop health is often a byproduct of foliar fungicide application. We see improved stalk strength, standability and ear retention with fungicide application, particularly in fields that we leave to dry for a moderate to substantial amount of time in the fall. Its important to consider the retention of harvestable yield as a component of fungicide application. This factor is much more difficult to put a yield breakeven or economic return on, but nevertheless should not be overlooked.

Case Studies

Over the past three years, Pederson Seed and Services has seen economic returns from 2-22 bushels per acre with a foliar fungicide. Across 12 sites over a span of 3 years the average was 9.6 bushels per acre. Disease levels in two years could be described as moderate, and one year as low. A study from Pioneer shows yield return around 7 bushels per acre on hybrids that are very resistant to disease, 12 on moderately resistant hybrids and 23 bushels per acre on susceptible hybrids. This gives us somewhat of a framework for possible return for various levels of pressure. With our current market, our breakeven will be around 6-7 bushel per acre.

We understand that there are a lot of factors wrapped up in this decision-making process. We’d be glad to help you in any way we can. Contact one of the Pederson Seed Team with any questions that you might have.


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