Posts Tagged ‘strobilurins’

Spraying Soybeans with Fungicides After Hurricane Irene

Friday, September 2nd, 2011

Arvydas (Arv) Grybauskas, Extension Plant Pathologist, University of Maryland;

The question has come up after the tremendous rainfall and damage caused by hurricane Irene whether or not one should spray their soybeans with fungicides. There was also a question just before the hurricane as to whether or not it was advisable to throw in a fungicide since many fields were to be sprayed for insects like podworm and stinkbugs. The answer to both those questions is highly dependent on each individual situation and a blanket recommendation cannot be made, especially because soybean rust is not going to be an issue this season.

Before we discuss the factors that play a part in determining fungicide need let me briefly explain why we rely on statistics and statistically valid research to answer these questions. Most of you are familiar with yield monitors and their output either because you have equipped your combine with one, or the custom combining operation that does the harvesting has one, or you have seen sample maps in a trade publication. If you haven’t then I’m sure you are still aware of the issue of variability in a field because of all the hoopla associated with any precision Ag equipment article or ad. These have been invaluable tools as they have really shown that an apparently uniform 40 acres is really quite variable. The obvious reasons for the variability are variations in soil type, depth, moisture and fertility just to name a few. But there are also many other less obvious reasons associated with pests since they are not uniformly distributed across any field as well. Some fields are more variable than others but every field is to some degree not uniform. The fact is if you took any production field and arbitrarily split it in half the average yield in each half would not be identical. In fact I would suspect that the vast majority in our area would easily have about 3-5 bu/A yield difference between each half. Again some might have greater uniformity and a smaller difference and some might have more. It is this variability that illustrates the need for statistics. Think of it this way. If the same field that we split arbitrarily in half produces yields that are different by 3-5 bu/A without a treatment then how do we know if we put a fungicide (or anything else) on half that it did anything? We use statistical tools and design experiments following certain statistical rules to answer that question. Two of the most important rules are replication and randomization. We will replicate so that we test a treatment more than once and get a measure of the variability in the response. We will randomize where each replicate of each treatment goes in the field to make sure they all have an equal chance of being in a high yield spot or low yield spot in a field. By using these and other tools we can then use the variability that is there to say whether or not a treatment has a certain probability of having an effect. The point is a bigger number doesn’t always mean that the treatment has a real or significant effect for those conditions. Sometimes a 3-5 bu yield difference could just be chalked up to being part of the background variation.

Ok, getting back to should we spray fungicides on soybeans after this heavy rainfall and wind damage. Let’s start with the damage. There are have been no independent scientifically sound experiments that show a consistent and reliable yield advantage to spraying fungicides in the absence of diseases. Physical damage by itself is not going to be corrected by a fungicide. Some fungicides reportedly have a positive effect on plants by counteracting components of plant stress responses. However, there is no proof from independently conducted field trials that this affect is large enough to affect yields in the field in non-diseased plants.

Stands that are lodged, leaning or matted down because of high winds tend to favor the development of certain diseases because the canopy remains closed and therefore remains wet for longer periods. Also, the distance a fungus needs to move either from leaf to leaf or soil to leaf in lodged or leaning stands is generally less. Heavy rains play a more important role than the stand damage in driving disease development. The rain from Irene provided two main avenues for greater disease development: spread of disease-causing agents if they were present; and a wet or humid environment that favors pathogen growth and infection of plants. A key that cannot be ignored is there has to be a pathogen around and in a form capable of being moved.

There are soybean diseases that will not respond to foliar fungicides. Bacterial blight and Bacterial pustule are diseases that are favored by hard driving rains. They look very similar to several fungal diseases, but because bacteria cause these fungicides cannot control them. Don’t get talked into spraying just because there are some leaf spots. There are also some fungal diseases that will not respond to foliar applied fungicides. The stem and root diseases in particular, Charcoal rot, Fusarium wilt, SDS and Phytopthora all are fungal diseases that cannot be controlled either preventatively or curatively at this time of year. One on-line source of brief disease descriptions with pictures is .

The soybean diseases that can occur in Maryland that can have an impact on yield, and can be managed with foliar applied fungicides are primarily: soybean rust, frogeye leaf spot and brown spot. Soybean rust due to the earlier drought in the South is not likely to be a problem this year. Its reestablishment and increase on kudzu has been severely delayed and is at the lowest level of infestation in August since monitoring for this disease began in 2005. Hurricane Irene therefore was highly unlikely to pick up enough spores from Florida and deposit them in Maryland to be an issue. Of the remaining two diseases Frogeye appears to have more potential for damage but is fortunately less common than Brown spot. Brown spot is the most common foliar disease but tends to be a lower canopy problem. Both are much more likely to develop if the previous crop was soybeans. I cannot find any data that would indicate there is much of a chance of getting a yield response from a fungicide by controlling frogeye or brown spot when the fungicide is applied after the R3 growth stage. The reproductive growth stages are: R1 initial flowering, R2 full flower, R3 small pods are formed at upper nodes, R4 full pod, R5 beginning seed, and R6 full seed. One on-line source for soybean growth stages can be found at: . There are other common diseases that tend to form late in the season and tend to be considered seed quality issues. Another way to understand these late season diseases is that fungicides reduce them but without a significant change in yield. Only seed quality is improved. Although seed size is often improved, only rarely does that translate to significant yield increases. The most common of these diseases are purple seed stain, pod and stem blight, and anthracnose. There is a third category of generally late appearing diseases that are reduced by fungicides but we have little evidence that they respond to the fungicide with a yield increase, for example Phyllosticta leaf spot. In short as a general rule it takes a fair amount of foliar disease that starts early in the reproductive period, if not before, to cause enough damage to reduce yields that can be countered with a fungicide.

With regard then to our current post-Irene situation, here are my thoughts on fungicides in soybeans.

● Scout first. If you know there is a specific problem that is starting then you can much more easily decide on the need for a fungicide.

● If you are a seed producer and the soybeans are between R1-R3 then consider spraying. You will protect some seed quality and have the potential for managing brown spot or frogeye if present and in that case may see a yield benefit. If the beans are between R4 and R5 then consider spraying if you haven’t sprayed earlier. However, expect only a seed quality improvement.

● If you are growing soybeans for general production and are between R1-R3. Scout first! If you have symptoms of frogeye leaf spot or of brown spot in the middle of the canopy then a fungicide application may be warranted. This is much more likely to occur if the soybeans were planted no-till into soybean stubble. If the field is at R4 and symptoms of frogeye or brown spot are evident then spraying is much less likely to improve yields. If the soybeans are at R5 then there is little evidence that you will get your money back from a fungicide application especially if it is just brown spot. If there is frogeye, it’s R5, and it continues to be wet the disease will get worse but there is no evidence that you will get a return on the fungicide. If you are not comfortable with not spraying then consider spraying in strips. In essence run your own test. It will cost you less and you will have a better basis for making the decision next time.

● Knowing the field history and rotation history are helpful. If you have soybean planted no-till into soybean stubble (it could be wheat-bean or barley-bean stubble) then you have the best chance of having diseases that can be reduced with fungicides and therefore get a yield response with a fungicide. The next level of risk based on crop rotation is when soybeans are just a year apart in the rotation, e.g., full-season beans followed by corn followed by wheat or barley and then double-crop beans. The lowest level of risk is there is a two-year or more rotation to soybeans. The most likely thing to happen with a fungicide application at R3 to R4 is improvement of seed quality (for example reduced purple seed stain or improved test weight) but not necessarily improve yields. The higher the risk based on rotation then the higher the chance a yield improvement will be attained. In my opinion, you have a 30-50% chance of getting a yield boost (3-5 bu/A) in soybeans with the higher chance coming from the higher risk situation and spraying at R3.

● Last point. Spraying for yield protection without knowledge of what is out there has already led to the development of a strain of frogeye that is resistant to strobilurins (Headline, Quadris, etc.). This has been confirmed in IL, KY and TN. It’s not a very good insurance program to spray against diseases when you just spray for yield boost because you are destroying one of the best tools you might need later on.

Resistance Management Strategies for Strobilurin Fungicides (FRAC Code 11)

Thursday, April 29th, 2010

Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University;

The strobilurin, or QoI, fungicides (FRAC code 11) are extremely useful in controlling a broad spectrum of common vegetable pathogens. You may know some of strobilurins as azoxystrobin (Quadris), pyraclostrobin (Cabrio), or Pristine (pyraclostrobin + boscalid, 11 + 7). All strobilurin fungicides inhibit fungal respiration by binding to the cytochrome b complex III at the Q0 site in mitochondrial respiration. Simply said, the fungicide works by inhibiting the fungi’s ability to undergo normal respiration. The strobilurin chemistries have a very specific target site, or mode-of-action (MOA). Although highly effective, fungicide chemistries like those in FRAC code 11, with a very specific MOA, are susceptible to fungicide resistance development by some fungi. For us, knowing the specifics on the technical jargon isn’t so important, its understanding what is at stake. So, if you read or hear someone speak about G143A resistance development to the strobilurin fungicides (where resistance is known in cucurbit powdery mildew and downy mildew, for example), you know what they are talking about and how important it is. So much so, if cucurbit powdery mildew develops resistance to one strobilurin fungicide it may develop what is known as cross resistance and become resistant to all other chemistries in FRAC code 11 — even if only one chemistry has been used!

How do we avoid the chances for fungicide resistance like this to develop? It’s simple, don’t let the fungus ‘figure out’ what it is being sprayed with and do this by rotating different fungicide chemistries (i.e. FRAC codes). Proper fungicides rotations are necessary when fungicides with specific MOAs are used in fungicide programs for controlling important diseases. That’s why it is important to follow a fungicides label precisely and be certain that some fungicide chemistries aren’t overused. All strobilurin fungicides should be tank mixed with a protectant fungicide, when possible. Remember tankmixing high-risk fungicides (i.e. FRAC code 11) with low-risk, protectant fungicides (FRAC codes M1-M9) helps reduce (and/or delay) the chances for fungicide resistance development. Never tank mix strobilurins together and never apply any strobilurin fungicide (either the same chemistry or different chemistry) in consecutive applications if stated by the label. Remember, azoxystrobin acts against the fungus the same way as pyraclostrobin does and so on. Even though you are spraying two different fungicides, each has the similar MOA and is acting against the fungus in the same exact way.

The publication”Fungicide Resistance Management Guidelines for Vegetable Crops in the Mid-Atlantic Region-2010” is available from the county extension offices or online at

Corn and Corn Fungicides

Thursday, July 2nd, 2009

Bob Mulrooney, Extension Plant Pathologist;

The following was written by Dr. Arv Graubaskas, Extension Field Crops Plant Pathologist at the University of Maryland in his Maryland Field Crop Disease Notes. The issue of applying fungicides to corn when part of the field is replanted is addressed. This question has been asked and this is the best answer we know.

In some parts of the state the earliest planted corn is close to developing a tassel. There has also been a lot of corn that had to be replanted due to drowning. In many other cases corn was only recently planted because the soil finally dried out enough to get equipment onto the field. There is thus a tremendous range of growth stages of corn out there and in many cases in the same field. Considering the weather pattern is still pretty wet the legitimate question of whether or not a fungicide program is warranted is raised.

First don’t get sold on the plant health or plant physiology response to strobilurin fungicides (Headline, Quadris, and to a lesser extent Quilt and Stratego) alone. Key word here is alone. The usual sales pitch involves stating that millions of acres have been treated and the average response has been 8, 10 or even 15 bu/A. These averages include cases that had significant foliar diseases as well as cases with little or no disease. These fungicides are great disease management tools and when diseases like gray leaf spot are significant will outperform other classes of fungicides. The performance of these fungicides when diseases are clearly yield limiting is so outstanding that it skews the average response number.

Let me illustrate with my data from last season. I had 16 replicated “trials” across the state across a range of crop histories, planting dates and hybrids that compared untreated corn vs. a strobilurin fungicide. The average response across these trials was 7.3 bu/A, but in only five of these trials was the positive (yield beneficial) response statistically significant. More importantly in those five trials gray leaf spot was a serious problem and the actual responses to the fungicide in those five cases ranged from 24 to 38 bu/A. In other words, where I did not have enough gray leaf spot to cause losses and thus the fungicide could only provide a yield advantage through alteration of plant physiology the average response was -0.7 bu/A. The overall average looks good but it is skewed by the cases that really benefited which were those cases where the fungicide primarily worked as a disease control agent. The plant physiology or plant health type effect occurs in concert with disease control to often outperform other classes of fungicides when diseases are a problem. Where there is no disease the plant physiology side benefits have little or no effect on yield or stand on a consistent basis. The bottom line is an insurance program where there is no need for insurance will only cost you money. Use a fungicide when you have a real risk of a foliar disease. The highest risk of getting gray leaf spot, the primary foliar disease of corn, involves three factors: 1) a susceptible corn hybrid, 2) no-tilling corn into corn stubble, and 3) a relatively wet season.

There is one additional factor that needs to be considered regarding fungicides in corn, especially this season. It gets us back to the comment that there is quite a range of growth stages out there and in many cases in the same field. Fungicides applied by air in a fullgrown corn crop generally perform better if surfactants are used. The surfactants help the fungicide to penetrate the canopy and be better distributed throughout the canopy. Arrested ear syndrome, where the development to corn ears is damaged resulting in a percentage of small malformed ears, has been associated with fungicides applied with a non-ionic surfactant (NIS) or sometimes other products (certain formulations of tankmixed products) especially if the corn is in the late vegetative stages of development just before tassel. Most of the damage seems to be associated with NIS surfactants but the fungicide formulation and tank-mixed products are not completely exonerated. It is therefore important that if one decides on using a fungicide that it is applied at or after tassel formation is completed. If it is close or there are parts of the stand that are not yet in tassel then NIS surfactants should not be used and be wary of any tank-mix products. One final comment on fungicides in corn, these products have not been shown to directly reduce stalk rots. Fungicides affect stalk rots and therefore improve stand, when there is a foliar disease. There is no direct action of the fungicides applied at or near tassel on the stalk rot pathogens that develop much later in the season. However, significant loss of effective leaf area from foliar diseases predisposes plants to stalk rots. Therefore, when there is a foliar disease problem and you manage it with a fungicide you get the additional benefit of reducing stalk rots.

The bottom line is fungicides have been proven to be excellent disease management tools for foliar diseases in corn and have an indirect effect on lodging due to stalk rots of the crop. The additional benefits of strobilurin fungicides are the reason they often outperform other fungicide classes with regard to yield when diseases are an issue. They should be considered when the risk of a foliar disease is high. Otherwise they are expensive insurance with a relatively low chance of a return if significant foliar diseases do not develop.

Supplemental Label for Headline Fungicide for “Plant Health”

Friday, March 6th, 2009

Bob Mulrooney, Extension Plant Pathologist;

Headline and other related fungicides called strobilurins including Quilt, Quadris and Stratego have been applied extensively in the US in last few years to enhance yields of corn, soybeans and wheat. Locally, the majority of the use has been on field corn in irrigated high yield environments. The “Plant Health” claims in this new supplemental Headline® label initiated a vigorous discussion among Extension Plant Pathologists throughout the East and Mid-West concerning the claims on this new label. The following article addresses those concerns from a scientific, unbiased perspective of the authors. This is a thoughtful and well articulated article and I am sure this is not going to be the last word on this topic but it explains the issue and the concerns pretty well.

Supplemental Label for Headline® Fungicde for “Plant Health”: Will It Improve Corn, Soybean and Small Grain Health?
By Paul Vincelli, Don Hershman, and Chad Lee*
Departments of Plant Pathology and *Plant and Soil Sciences
Kentucky Pest News, Number 1187, February 24, 2009, online at:

A couple of weeks ago, we learned of a supplemental label for Headline® fungicide for use on several crops for “disease control and plant health.” The impacted crops grown in Kentucky are corn, small grains (barley, rye and wheat), and soybean, as well as other edible legumes. Headline® and related strobilurin fungicides (Quadris®, Quilt®, and Stratego®) provide excellent control of certain fungal diseases of the above crops. In Kentucky, for example, use of these products to control gray leaf spot and/or northern leaf blight in corn, frogeye leaf spot and brown spot of soybean, and tan spot and leaf rust of wheat makes sense when the risk of disease is high. However, this new supplemental label makes claims that go way beyond disease control.

Claims Made on the New Supplemental Label
The supplemental label indicates that, through preventive applications of Headline® to crops, the plant health benefits may include improved host plant tolerance to yield-robbing environmental stresses, such as drought, heat, cold temperatures, and ozone damage. The supplemental label also claims that Headline can improve plant utilization of nitrogen and can increase tolerance to bacterial and viral infections. These benefits often translate to healthier plants producing greater yields at harvest, especially under stressful conditions. The supplemental label also claims that additional specific benefits can occur, including:

● Improved stalk or straw strength and better harvestability (barley, corn, rye, wheat)
● Induced tolerance to stalk diseases (corn)
● Better tolerance to hail (corn)
● More uniform seed size (corn, soybean, and edible legumes)
● Better seed quality (soybean and edible legumes)

Will “Plant Health” Be Improved?
Based on publicly available research reports, we see very little evidence that Headline® or other strobilurin fungicides should be applied to any of the above crops for any reason other than disease control. To date, no data have been circulated in either the scientific or farm communities which suggest that any strobilurin product, including Headline®, can reliably live up to the claims made for stress tolerance under field conditions.

Claims of stress tolerance sound exciting but, based on the data we have seen, deserve to be viewed with cautious skepticism. There are certainly studies in the laboratory, the greenhouse, and occasionally in the field that show beneficial physiological changes in crops treated with strobilurin fungicides. But don’t assume that the beneficial changes observed in those studies result in increased yield under field conditions. When a greening effect and/or yield improvement is observed in a treated crop (in the absence of significant disease pressure), it is assumed that stress tolerance and/or improved plant health (apart from disease control) is at work. This isn’t necessarily true. In order for any real-world stress tolerance claims to pass muster, scientifically, it is necessary to conduct replicated field studies where the appropriate environment, plant, and crop measurements are made, and appropriate experimental controls are in place. We do not believe these data exist in sufficient quantity to support the above stress tolerance claims. Certainly, it is inappropriate to draw conclusions about stress tolerance based solely on crop appearance and yield. For example, we have observed the greening effect in field crops, but it often does not translate to higher yields. We have also observed occasional yield increases in crops (mostly soybean) following a fungicide application, when no obvious disease symptoms were present. But there are a large number of potential reasons why yields are improved in treated crops. Tolerance to one or more stresses is a possibility, but it is also possible that some soil-borne disease or disease complex is being controlled, but we cannot easily observe it. There are many other possible reasons and the only way to know for certain is to conduct the appropriate replicated, controlled field studies.

Let us look at an example from soybean from two replicated studies conducted at the Research and Education Center where disease pressure was minimal and late season moisture stress was significant (especially in 2007). If Headline® application improves tolerance to drought stress (as per the supplemental label), then the application should improve yield in treated crops. But as can be seen in Table 1, soybean yields were not improved by Headline® in either year. Table 2 shows the results of a similar field trial for corn conducted on a Kentucky farm under drought conditions. You can see that Headline® provided no yield bump.

Table 1. Results of Headline® application (6 fl oz + Induce at R3 stage) in soybean where disease pressure was insignificant, under late season moisture stress (UKREC, Princeton, KY, 2007-2008)


Yield (bu/A)

Yield (bu/A)

Check 24.5 51.5
Headline 23.8 53.0
Statistical result (LSD, P=.05) No statistical difference No statistical difference

Table 2. Results of Headline® application in corn where disease pressure was insignificant, under late-season moisture stress (Logan County, KY, 2007)


% Gray Leaf Spot*

Yield (bu/A)

Check 1.3 160
Headline 2.0 155
Statistical result (LSD, P=.05) No statistical difference No statistical difference

*Disease assessed on ear leaf at half milk line.

It is important to emphasize that the data in Tables 1 and 2 are merely examples. The above data are typical of what has been seen over and over in a large number of university-conducted trials conducted over the past several years in corn, soybean, and small grains. If Headline® regularly improves yields by imparting stress tolerance to crops in the absence of disease, then more complete and convincing proof needs to be made public. And in the world of science, claims based on evidence that has not been made public are treated with suspicion.

The claims about improved stalk health in corn are not unreasonable. Occasionally (and we stress the word occasionally), applications of strobilurin fungicides have been shown to improve stalk strength and/or reduce stalk rots in university-conducted field trials. However, in our experience, that improvement in stalk health relates to control of foliar diseases (gray leaf spot, for example). You see, if foliar diseases are aggressively attacking the plant during grain fill, then the corn plant will attempt to fill the grain by cannibalizing the reserves in its own stalk. That weakens the stalk and can result in more aggressive stalk rots as well as reduced stalk strength. So, if foliar diseases are killing the upper and middle foliage during grain-fill, then it makes sense that a fungicide like Headline® might sometimes improve stalk health, which it sometimes does. But note carefully: this benefit still relates to control of foliar diseases. And like we said above, strobilurin fungicides are very good for controlling foliar diseases like gray leaf spot and northern leaf blight of corn if these diseases are present.

What about a fungicide enhancing tolerance to hail? Actually, conducting a study that tests for this type of benefit is more complex than you may realize. You must have the right kind of experimental design or you could be misled by the results. The only study we are aware of that tests this claim with a valid experimental design is one conducted in 2008 by Dr. Carl Bradley and colleagues at the University of Illinois. In that study, researchers used a weed-eater to simulate hail damage. In that study, they found absolutely no yield benefit from Headline®, Quadris® or Quilt® when applied following simulated hail damage.

Is There a Downside?
Producers should be aware that sometimes the late-season “greening” effect observed with strobilurin fungicides can result in higher grain moisture and therefore additional drying costs and a slower (more expensive) harvest. Conversely, if crop harvest is delayed until the desired harvest moisture content is reached, there can be a yield and/or quality penalty, depending on the crop. For example, delaying wheat harvest will result in delayed planting of doublecrop soybean, which can lead to lower yields in soybean. In soybean, if harvest is delayed, pod and stem blight levels may increase, which can reduce the quality of grain destined for seed use. This may necessitate additional grain clean-out and/or the use of seed-treatment fungicides prior to planting next season. (Strobilurins, in general, do not do a good job in controlling soybean pod and stem blight). The bottom line is that fungicides applied to corn, soybean, and wheat will sometimes increase production costs.

Another concern specifically relating to the plant health issue is that the use of a fungicide when disease activity is too low to affect yield increases the risk of fungicide resistance. It is because anytime you expose a fungus to the fungicide, even when fungal activity is low, you increase the selection pressure on the fungus towards resistance. Resistance to strobilurin fungicides is an important concern worldwide, and the use of any strobilurin fungicide for plant health reasons increases the risk of developing strobilurin-resistant gray leaf spot. Use of strobilurins may also incite flares in certain insect and mite populations under field conditions, because fungicides can sometimes suppress fungi that kill these arthropod pests.

Bottom Line
The strobilurin fungicides are very good for control of specific crop diseases (see product labels for a list), if they are present at high enough levels (or the risk is high enough) to reduce yields. However, applying a strobilurin fungicide for plant health or stress tolerance reasons alone – with little or no threat from foliar diseases – doesn’t make sense to us, based on our extensive study of the best available information. Land-Grant University trials, thus far, generally do not support claims of reliable improvement in crop yield under stress conditions from an application of Headline®, or any other strobilurin fungicide. Nor have fungicide manufacturers provided sufficient field evidence in support of these claims. In fact, the vast majority of industry data show yield impacts (usually in side by side comparisons) associated with specific fungicide treatments, but provide no measurements of diseases or stresses. The upshot of this is that there is absolutely no way to know what the cause of apparent yield improvement is in the vast majority of industry studies. Thus, at this time, we do not feel there is a scientifically defensible basis for assertions of improved plant health/stress tolerance in the absence of the diseases the fungicide was originally developed to control.