- Fungicides can provide yield protection from fungal foliar diseases when applied at the appropriate timing and when disease risk is high.
- Disease impact for foliar disease is driven by the amount of time the points of the disease triangle are cooccurring, and when the disease occurs.
- Protection of the ear leaf and above, and upper 1/3 of the soybean canopy is key to yield protection.
- Evaluating your potential disease risk can help determine which acres may benefit the most from fungicide applications.
We have heard time and time again about the disease triangle. Disease only occurs when a susceptible plant host is present, a pathogen that is capable of infecting that host is present, and environmental conditions favor the growth, reproduction, and disease inciting capacity of the pathogen. This simplistic model is great for teaching plant disease basics, but it is limited in practical application. This is because diseases that can continue to develop and reproduce over time (e.g. most foliar fungal diseases of field crops) the ultimate amount of disease that occurs in the crop and yield impact is significantly impacted by the amount of time that those three points of the disease triangle cooccur. For example, if we have cool-moderate, humid conditions for a week, followed by 3-4 weeks of hot dry weather, we may see tar spot or northern corn leaf blight start up, but the ultimate impact will be reduced as the heat and dry conditions would prevent additional infections and generation of infective spores. Contrast this with a situation where we have storm fronts pushing through every week or so for a month. In this situation, we would expect these diseases to be able to continue to develop, produce spores, and those spores can infect and spread as a result of the multiple events where the points of the triangle are cooccurring. This is why thresholds for these sorts of diseases are highly unreliable, and a combination of proper scouting and protecting the vital components of the crop at critical timepoints is essential in maintaining optimal yields in achieving positive ROI.
With late planted fields in many regions, and a majority of acres hot and dry for much of the season, many producers are investigating if a foliar fungicide application is needed to maximize potential yield. Not every situation requires a foliar fungicide; however, productive fields at elevated risk for yield-limiting diseases may benefit from added disease protection. The greater the disease risk, the more likely fungicide applications will pay for themselves. Research indicates that the best timings for applying fungicides to soybean and corn coincide with critical points in crop development. When applied at these timings yield losses are most likely to be reduced, as essential tissues (ear leaf and above in corn, upper 50-30% of canopy in soybeans) is protected and able to fill grain at an optimal level during these critical periods. In corn, this timing is VT/R1 and in soybean R3 has been shown to provide optimal protection. Altough it is true that on occasion there will be specific situations where applications made earlier or later may provide slightly better results, when we look at the overall picture, those timings are most likely to win out across years and environments.
What is going to determine your approximate risk level for most fungal diseases? Below is a risk table that can be used to approximate the likelihood that a fungicide may be needed to control commonly occurring residue-borne corn (grey leaf spot, northern corn leaf blight, tar spot) and soybean (frogeye leaf spot, Cercospora leaf blight, brown spot) diseases. Keep in mind that this is a tool to help guide general risk for each crop across the different diseases. Each disease has other environmental factors, such as temperature, leaf wetness duration, and light quality, that may impact disease development. A side note- avoid spraying a solo QoI (strobilurin-FRAC group 11) fungicide product to soybean fields with elevated risk for Frogeye leaf spot. The frogeye leaf spot pathogen has developed widespread resistance to this mode of action over the past 10 years. Instead, use a mixed mode of action fungicide, of which there are numerous to choose from.
Table 1. Relative foliar fungal disease risk decision aid. Use this as a guide to assist in foliar fungicide decisions. This aid is meant for residue borne foliar diseases in corn and soybeans but can also be used in other crops where residue-borne foliar diseases are of concern.
- History of the disease in the field, confirmed via reliable diagnostic techniques
The disease needs to be present to cause disease. Residue-borne diseases can overwinter in fields, providing local inoculum sources that do not need to travel large distances to inoculate and infect the crop.
If yes add 4 points __________
- Non-rotated crop production systems
Major residue-borne fungal diseases feed on and infect either corn OR soybean. The more you produce the same crop in the same field, the more potential inoculum is present and able to produce inoculum to infect the crop in that field.
If yes add 4 points __________
- Minimal or no-till production systems
Residue provides sustenance for residue-borne foliar plant pathogens, and the longer that residue is present on the soil surface, the more likely fungal pathogens will build up and be able to sporulate under the correct environmental conditions.
If yes add 3 points ___________
- Hybrid/cultivar is susceptible to commonly encountered foliar fungal diseases
Susceptible hybrids/cultivars are more easily infected by foliar fungal pathogens. The use of resistant cultivars slows lesion development and sporulation, which frequently reduces disease development to a point that economically significant losses do not occur. Remember, resistance is specific for each disease.
If a susceptible cultivar is used based on company ratings, add 3 points.
If a moderately resistant cultivar is used (GLS, NCLB) based on company ratings, add 1 point.
If a highly resistant cultivar is used based on company ratings, no points added.
_________
- The disease is present on plants at/before critical stages of plant growth (VT-R3 in corn, R1-R4 in soybeans).
If yes add 3 points _________
- Forecast weather over the next several weeks contains a likelihood for significant rain events, OR fields are irrigated.
If yes add 2 points _________
Total score: _________
High risk: 15-19
Medium risk 10-15
Low risk 9 or less
In cooler climates Sclerotinia white mold can be problematic in fields with a history of the disease. White mold is favored by wet conditions after canopy closure and moderate temperatures. Practices that favor rapid canopy closure allow for humidity to be trapped within the canopy, which in addition to altering light quality, can result in the pathogen germinating from overwintering structures and dispersing infective spores onto susceptible tissues such as senescing flower petals. Applications of fungicides labeled for white mold suppression can be beneficial under elevated risk/pressure situations when applied between R1-R3. Although there are several products with white mold suppression on their labels, university research trials indicate that Endura and Aproach programs reduce disease with the greatest efficacy.
References:
Wise KA, et al. 2019. Meta-analysis of yield response of foliar fungicide-treated hybrid corn in the United States and Ontario, Canada. PLOS ONE 14(6): e0217510. https://doi.org/10.1371/journal.pone.0217510
Kandel, YR., et al. 2021. Meta-analysis of soybean yield response to foliar fungicides evaluated from 2005 to 2018 in the United States and Canada. Plant Disease: PDIS-07.
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