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Are We Still Shooting Ourselves in the Foot?

Fact. Dependence on glyphosate as the major or sole herbicide for weed management or control in US soybean production has resulted in the selection for glyphosate resistance in several problem weed species. This poses a real threat to the continued sustainability of production; i.e., the soybean sector is left with the daunting task of choosing ever-dwindling options for control of problem or troublesome weeds.

Fact. The continued use of the strobilurin [quinone outside inhibitor or QoI] class of foliar-applied fungicides whether needed or not [these fungicides have long been touted for enhancing plant health] over the last decade and a half has resulted in the selection for strobilurin resistance in the pathogen that causes frogeye leaf spot [FLS] in soybean. This will be an increasing threat to the continued sustainability of high-yield soybean production.

Furthermore, there is now evidence of resistance in other pathogenic fungi such as Corynespora cassicola [causal agent of target spot] to this fungicide class. This is likely an inadvertent happening that resulted from the continued exposure of this pathogen to this fungicide class during this past extended period of its widespread application. Obviously, any fungi that are susceptible to the strobilurins would have selected for resistant types during this time. It is likely, then, that these other foliar pathogens, which previously had not been problematic, will become so in the near future.

Fortunately, there are other fungicide classes that can be applied to control FLS in soybean to avoid yield reduction. Regrettably, this may not be the case for target spot. Hopefully, it is not too late to start rotating fungicides that have different modes of action or apply fungicide mixes that have more than one mode of action to prevent further resistance development within fungal populations.

Drs. Tom Allen, Ext. Plant Pathologist at MSU-DREC, and Trent Irby, MSU Ext. Soybean Specialist, address this issue in a June 3, 2017 article titled "Considering the R3/R4 Automatic Application in Soybean: Time to Apply Two Modes of Action Instead of One" on the MCS blog site. In this article, they state that for the better part of a decade farmers have applied foliar fungicides as a necessary management practice in high-yield environments. They further state that "data from more than 10 years of trials suggested that in approximately 50-60% of the situations, a modest 3-5 bu/acre return could be expected" (notice the word modest) when a QoI fungicide product was applied at either the R3 or R4 growth stage. These fungicide applications "were made based on growth stage timing generally in the absence of disease and independent of the long-range forecast". In studies they conducted in 2015 and 2016, a stand-alone QoI product provided a -0.4 bu/acre response, whereas pre-mix fungicides (more than one mode of action) provided a +3 bu/acre response. This leads to their recommendation "that individuals wanting to continue to receive the same benefit from the R3/R4 automatic fungicide application should apply either a pre-mix or tank mix that contains at least two modes of action." This recommendation still relies on the automatic fungicide application concept, which may lead to further selection for resistance among fungal populations.

Fact. The over-dependence on or over-application of a valuable herbicide [glyphosate] was done at the exclusion of other classes of herbicides. The over-application of a valuable class of fungicide [strobilurin] was done even when there was no perceptible presence of a fungus or fungi that warranted a fungicide application [again, done for promoting “plant health”]. Both scenarios have or will lead to the continuing appearance of resistant weed and fungal types.

Fact. For over twenty years, soybean producers have been planting soybean seed that were treated with a fungicide seed treatment. One of the fungicide components of all seed-applied products was and still is metalaxyl or mefenoxam, both FRAC Code 4 fungicides. These materials are very effective against Pythium spp., a pathogen that is capable of severely reducing soybean emergence and stand establishment. There is no present or forthcoming varietal resistance to Pythium (could be in the distant future), so the current soybean production system is totally dependent on this fungicide to prevent damage from this pathogen. Fortunately, I have not heard or read of stand failures where the present seed treatments were used to control Pythium. It remains to be seen if this long-term use of this very effective fungicide class will lead to resistant types within the Pythium complex.

There are several classes of fungicides in present seed treatments that are effective against Fusarium, Rhizoctonia, and Phomopsis fungi, all of which can contribute to poor emergence and stand loss. Therefore, it is likely a good practice to choose seed treatment products that at least rotate the classes of fungicides that are effective against the non-Pythium pathogens. Unfortunately, most seed suppliers/dealers pretreat their soybean seed with a proprietary product that the recipient has little or no choice about, or may not even know the components of. Thus, it behooves soybean producers to become familiar with what protectant materials are applied to their seed, and even insist that these products be rotated so as not to use the same FRAC class of fungicide(s) year after year.

Fact. The use of insecticide seed treatments has become widespread in Midsouth soybean production as a risk management tool; i.e., they are being used without knowledge of whether or not they are needed because it is virtually impossible to sample for soil insects. Also, the majority of research results has shown small but consistent and profitable soybean yield increases from their use. However, the insecticide seed treatment products that are being used are all in the neonicotinoid sub-group 4A [IRAC MOA Group 4]. Will the continued widespread use of this one class of insecticides in seed treatments lead to resistance development among the targeted insect species down the road? That remains to be seen, but experience with the above-cited herbicide- and fungicide-resistance issues leaves little doubt that it can happen.

With insecticide seed treatments, there may be a window of opportunity to thwart potential insect resistance development to these insecticides by considering the following questions. 1) Is the small yield increase from their use worth the risk of resistance development in targeted species? In other words, is the short-term small gain worth the risk of the potential long-term negative of overuse? 2) Shouldn’t their use be reserved for those situations where the highest risk of early-season insect damage is likely to occur; e.g., soybeans planted behind cover crops–click here and here? 3) Since development of insect resistance to foliar-applied insecticides is well-documented, isn’t it logical to assume that the same can happen when insecticide seed treatments are applied on a continued widespread basis?

Opinion. The potential for the development of pathogen resistance to a pesticide should be given priority consideration whenever the blanket use of any pesticide is considered or promoted.

Opinion. The development and release of soybean varieties that are resistant to pests, both fungal and insect, is the true long-term solution for dependable pest management. Until that happens [may or may not for some pests], the use of pesticides should be reserved for situations where there is a documented case for significant yield loss if they are not used.

Fact/Opinion/Metaphor. If we continue to apply pesticides just because they are perceived to provide only slight or even unknown benefit, “we are still shooting ourselves in the foot”.

Composed by Larry G. Heatherly, June 2017,