Corn and Soybean School: Q and A on Corn Disease Management with Fungicides

On Feb 11, 2021, I gave a talk entitled “Management of Gibberella ear rot and Vomitoxin in Corn with Fungicides: Lessons Learned from Head Scab” as part of the 2021 Virtual Corn and Soybean School. I summarized years of fungicide efficacy research on head scab, a disease of wheat caused by the same fungus (Fusarium graminearum [Gibberella zeae]) that causes Gibberella ear rot (GER) in corn. Head scab and vomitoxin in wheat have been more widely studied than GER and vomitoxin in corn, as a result, a lot more is known about fungicide efficacy against scab/vomitoxin than against GER/vomitoxin. I therefore used lessons learned from head scab research, coupled with data from a limited number of GER fungicide efficacy studies to provide guideline on GER and vomitoxin management in corn. More than 220 people attended the 40-min-long program, asking questions covering various aspects of corn pathology. Below are more complete responses to several of these questions:

Q: How do you explain high vomitoxin levels in grain with no apparent ear rot observed?  Can drought stress alone be a culprit?

A: Infection of the ear, development of visual symptoms (ear rot), and contamination of grain with vomitoxin all depend on weather conditions during the weeks after silk emergence. Once the fungus enters the ear via the silks (infection) and begins to colonize the developing grain, it produces vomitoxin, even if subsequent weather conditions are not favorable for mold and ear rot to develop on the outside of the ear. This is particularly true if infections occur late and conditions become relatively dry and unfavorable for visual symptoms to develop.

Q: It looks like the triazoles are doing the work on VOM, more than strobies, is this correct?

A: Pulling from my years of experience with head scab and a limited number of fungicide efficacy studies on Gibberella ear rot and vomitoxin in corn, I would be more inclined to recommend a triazole than a strobilurin fungicide for Gibberella ear rot and vomitoxin control in corn. Miravis Neo (a combination fungicide of a triazole, an SDHI, and a strobilurin) also looks promising.

Q: Is there any relationship between using a strobilurin for vomitoxins in corn compared to what is found in wheat?

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Corn College and Soybean School

The Agronomic Crops Team will host a virtual Corn College and Soybean School on February 11, 2021. Corn College is in the morning, from 9:00 – 12:00pm, with Soybean School in the afternoon from 1:00-4:00pm. Each program will feature updates from OSU Specialists. CCA CEUs are available. The schedule for the day is as follows:

 

Corn College, 9:00am-12:00pm

  • Corn Management for 2021, Peter Thomison, 1.0 CM CCA CEUs
  • Meeting Nutrient Needs in Corn, Steve Culman, 1.0 NM CCA CEUs
  • Disease Management, Pierce Paul, 1.0 PM CCA CEUs
  • Insect Management, Andy Michel, 1.0 PM CCA CEUs

Soybean School, 1:00-4:00pm

  • Soybean Management for 2021, Laura Lindsey, 1.0 CM CCA CEUs
  • Weed Management, Mark Loux, 1.0 PM CCA CEUs
  • Disease Management, Anne Dorrance, 1.0 PM CCA CEUs
  • Insect Management, Kelley Tilmon, 1.0 PM CCA CEUs

This program is free to attend. Register at www.go.osu.edu/agronomyschools.

Gibberella Ear Rot and Vomitoxin in Corn

Source: Dr. Pierce Paul, OSU

If your grain was harvested from a field with Gibberella ear rot (GER), it is more than likely contaminated with mycotoxins. Deoxynivalenol, also known as vomitoxin, is one of the mycotoxins most commonly produced by the fungus Fusarium graminearum that causes GER. Another name for this fungus is Gibberella zeae, hence the name of the disease. Before storing grain harvested from GER-affected fields or areas where conditions were favorable for the disease, pull a sample and test for the presence and level of contamination with vomitoxin. Mycotoxin tests are either qualitative, semi-quantitative, and quantitative. Qualitative tests provide a yes/no answer for the presence of the toxin and are useful for initial screening. Semi-quantitative tests estimate whether the toxin is at or above certain levels (>5 ppm) or within a given range, whereas quantitative tests provide more precise estimates of contamination. There is a trade-off between precision, price, and speed. Quantitative tests tend to be the most precise but are also more expensive and take longer to complete than the qualitative or semi-quantitative tests. Semi-quantitative quick-test kits are very common and relatively easy to use and inexpensive. They are often very specific for one particular toxin. A test developed specifically for Aflatoxin or Fumonisins will NOT work for vomitoxin.

Unfortunately, there are no commercially available treatments to reduce vomitoxin levels in stored grain.  Poor storage may cause toxin levels to increase. Warm, moist pockets in the grain promote mold development, causing the grain quality to deteriorate and toxin levels to increase. Aeration is important to keep the grain dry and cool. However, it should be noted that while cool temperatures, air circulation, and low moisture levels will minimize fungal growth and toxin production, these will not decrease the level of toxin that was already present in grain at the time of storage.

  • Dry and store harvested grain to below 15% moisture of lower to minimize further mold development and toxin contamination in storage.
  • Store dried grain at cool temperatures (36 to 44°F) in clean, dry bins. Moderate to high temperatures are favorable for fungal growth and toxin production.
  • Periodically check grain for mold, insects, and temperature.
  • If mold is found, send a grain sample for mold identification and analysis to determine if toxins are present and at what level.
  • Clean bins and storage units between grain lots to reduce cross-contamination.

Several companies sell test strips for mycotoxin analysis, including Romer Labs (http://www.romerlabs.com) and Neogen (http://www.neogen.com/). These tests are fairly easy to use once you read and follow the manufacturer’s guidelines carefully.

More information on sampling, testing, and storage can the found in factsheet # PLPATH-CER-04 (http://ohioline.osu.edu/factsheet/plpath-cer-04).

Drought Projections Do Not Go Well With Fungicide Applications

Source: Anne Dorrance, Pierce Paul, OSU

Several calls this past week for fungicide applications on corn and soybean at all different growth stages.  So let’s review what might be at stake here.

Soybeans.  Frogeye leaf spot and white mold on susceptible varieties when the environment is favorable for disease easily pay the cost of application plus save yield losses.  Let’s dig a bit deeper.  Both of these diseases are caused by fungi but frogeye leaf spot is a polycyclic disease, meaning that multiple infections occur on new leaves through the season while white mold is monocyclic and the plant is really only susceptible during the flowering stage.  Both of these diseases are also limited geographically in the state.  White mold is favored in North East Ohio and down through the central region where fields are smaller and air flow can be an issue.  Frogeye has been found on highly susceptible varieties south of 70, but it is moving a bit north so it is one that I am watching.

White mold is also favored by closed canopy, cool nights and high relative humidity.  So farmers in these areas should double check their variety ratings first.  If it is moderate to low score for resistance (read the fine print) then this year a spray may be warranted.  We have gotten consistent control of white mold with Endura at R1.  Herbicides that are labeled for white mold suppression have also knocked back this disease, but if a drought occurs or no disease develops, losses of 10% or greater can occur due to the spray alone.  For these purposes R1 is a flower on the bottom of 1/3 of the plants in the field.

Frogeye leaf spot –There also must be some inoculum or low level of disease present in the field for this disease to cause substantial and measurable yield losses.   This disease will only move in the canopy when there is regular rainfall.  And again only on susceptible varieties. With dry weather, this will sit and hold. Time to scout for this will be at the end of flowering if it can be found in the field.  With drought conditions, the disease will not impact the crop.

The story is very similar from a corn pathology standpoint. Most of our major diseases (gray leaf spot, northern corn leaf blight, eye spot) are driven by wet, humid conditions, consequently, the dry weather we have experienced over the last several days will keep most diseases in check. Fungicides are not warranted under these conditions; it just does not pay. Although some product labels may mention yield responses under drought-like condition, our data do not support such a benefit. We see the highest yield responses when fungicides are applied to susceptible hybrids at VT-R1 under disease-favorable conditions. These conditions would include extended periods of dew and high relative humidity, especially during the early- to mid-morning hours.

For a disease like southern rust that usually blows up from the south, and tar spot, an emerging disease of increasing concern in the state, fields should be scouted before making an application. Both diseases develop well under warm conditions, but they also need moisture and high relative humidity to spread. In the case of tar spot, based on what we have seen in 2018 and 2019, it usually develops well into grain fill (R4-R5), and as such, may have little effect on grain yield. Data from some states in the western half of the corn belt show that when tar spot develops early, yield loss may be substantial. The same is true for early southern rust development. So, scout fields to see what is out there and at what level before investing in fungicide application.

Corn and Soybean Seedling Blights

Source: Dr. Anne Dorrance, OSU

Low stands or poor development of plants is, unfortunately, a common occurrence for fields that were planted in many regions of Ohio with heavy soil or are poorly drained soil.  Symptoms include skips, missing plants, or dried up and brown seedlings.  There may also be, wilting plants with and rotten, brown, decaying spots or lesions on the roots. Now is an excellent time to scout stands and check to be sure that the fields are not just crusted over – and that the seeds and seedlings that are there are still healthy.

While there, dig up a few of the affected plants, if the roots are brown and soft, the seedling will die eventually or be very weak.  So don’t count them as part of your total stand.  On soybeans check to see if there are nodules, the corky looking knobs on the roots that help legumes fix nitrogen.  The cold, wet weather does not favor nodulation, so this may take a bit longer, for now, native Rhizobium spp. to get a foothold in the plants.  Once the plants have nodules, they will recover and grow.  On corn, the root (mesocotyl) between the young seedling and the seed, should be white.  If it is dark brown or soft, this will also be a weakened plant.  Some pathogens, if the environment is right, will continue to multiply and grow to kill the seedling.

For management, improving soil drainage, and having at least two ingredients in the seed treatment mixture targeting water molds (Pythium and Phytophthora) are necessary for the challenging areas in Ohio that have a history of replanting.  If you do have to replant, take a look at what the seed treatment package is and note what is in the mix.  The one caution, though, is if the field was submerged for more than 24-48 hours (Ponding), this is flood injury, and there are no seed treatments for this.

Overwintering of Pathogens and Insects – What do Winter Temperatures Tell Us About Next Season?

Source: Anne Dorrance, Kelley Tilmon, Andy Michel, OSU Extension

Over the years we have developed databases of winter temperatures followed by scouting to indicate starting pathogen populations for Ohio.

Frogeye leaf spot – We have documented early infections and overwintering ability of the fungus, Cercospora sojina, that causes frogeye leaf spot. It appears that when there are less than 10 days during the months of December, January and February of less than 17 F, we have had reports of outbreaks of frogeye leaf spot.  This occurred in fields where there was a high level of inoculum at the end of the season the same or similar moderately to highly susceptible cultivar was planted into the same field again which then initiated the epidemic that much sooner.  Losses of greater than 35% in yield or very early fungicide applications were necessary.

Expecting continued warmer winter temperatures, for fields with a history of frogeye leaf spot, and no-till production systems, the first thing for farmers is to do now to mitigate losses in 2020:

  1. Rotate fields with high levels of frogeye leaf spot into corn or another crop.
  2. If it is still targeted for soybean, look at their soybean varieties frogeye leaf spot resistance scores.  Your seed dealer will have more information.  Plan now for what fields they will go into.
  3. Scout the susceptible cultivars much earlier than what we have called for in the past and monitor levels.

Another pathogen that may be more prevalent after a warm winter is Stewart’s bacterial wilt.  This disease is transmitted to corn by corn flea beetle which survives in greater numbers in warm winters. This is a greater problem in popcorn and sweet corn as most field corn has high levels of resistance to the bacterium.

Most other field crop insect pests in Ohio are not highly influenced by winter conditions as they are well-adapted to withstand cold overwintering conditions.  Once exception is Mexican bean beetle, an occasional pest of soybean (especially in central Ohio).  Warm winter conditions may cause higher populations of this insect the following field season.

Tar Spot of Corn in Ohio Again this 2019

Source:  Pierce Paul, Felipe Dalla Lana da Silva, OSU Extension

Tar Spot, a new disease of corn caused by the fungus Phyllachora maydis, was reported for the first time in Ohio at the end of the 2018 growing season. At that time, it was found mostly in counties close to the Indiana border, as the disease continued to spread from the middle of country where it was first confirmed in 2015. Over the last few weeks, there have been several new, confirmed report of Tar Spot in Ohio, this time not only in the northwestern corner of the state, but also from a few fields in central and south-central Ohio. As was the case last year, disease onset was late again this year, with the first reports coming in well after R4. However, some of the regions affected last year had more fields affected this year, with much higher levels of disease severity. It could be that Tar Spot is becoming established in some areas of the state due to the fungus overwintering in crop residue from one growing season to another. This is very consistent with the pattern observed in parts of Indiana and Illinois where the disease was first reported. We will continue to keep our eyes out for Tar Spot, as we learn more about it and develop management strategies. You can help by looking for Tar Spot as you walk fields this fall, and please send us samples.

What does it look like? Even though corn is drying down, if Tar Spot is present, you can still detect it on dry, senescent leaves almost as easily as you can on healthy leaves. So, please check your fields to see if this disease is present. “Symptoms of tar spot first appear as oval to irregular bleached to brown lesions on leaves in which raised, black spore-producing structures call stroma are formed… giving the symptomatic areas of the leaf a rough or bumpy feel to the touch… resembling pustules on leaves with rust. Lesions … may coalesce to cause large areas of blighted leaf tissue. Symptoms may also be present on leaf sheaths and husks.” As the name of the disease suggests, symptoms look like the splatter of “tar” on the leaves. In some cases, each black tar-like spot may be surrounded by a necrotic halo, forming what is referred to as “fish-eye” lesions.

What causes Tar Spot and how damaging is it? In the past, the greatest impact of this disease in terms of yield loss were observed when P. maydis-infected plants were co-infected with a second fungus called Monographella maydis. In other words, the damage tended to be much more severe when the two fungi worked together to affect the plant. So far, only the first fungus, P. maydis, has been reported in the US, but based on work done in Illinois, this pathology alone is capable of causing substantial yield reduction on highly susceptible hybrids when conditions are favorable and infections occur early.  

Where did it come from and will it survive and become established? At this point it is still unclear as to how Tar Spot got to the US in the first place and how it continues to spread. The fungus is not known to be seed-borne or infect other plant species, so corn seeds and weeds are unlikely to be the sources of inoculum. However, the fungus can survive and be moved around on fresh and dry plant materials such as leaves and husks. In addition, since spores of the fungus can be carried be wind, it could be blowing in from neighboring states/counties/fields. Although not yet confirmed through survival studies, it appears that the fungus could be overwintering in infected crop stubble between growing seasons.

What should I do if I find Tar Spot? If you see anything that fits the description of, or resembles (Picture) Tar Spot, please inform your state specialist, field specialist, or county extension educator, but most importantly, please send samples to my lab (1680 Madison Ave, Wooster, OH) for confirmation. We will also be using your samples to study the fungus in order to develop effective management strategies.

Read more about Tar Spot of Corn at:

https://cropprotectionnetwork.org/resources/articles/diseases/tar-spot-of-corn

https://www.extension.purdue.edu/extmedia/BP/BP-90-W.pdf

 

Corn Grain Test Weight

Source: R.L. Nielsen, Purdue Univ. (edited)

Among the top 10 most discussed (and cussed) topics at the Chat ‘n Chew Cafe during corn harvest season is the grain test weight being reported from corn fields in the neighborhood. Test weight is measured in the U.S. in terms of pounds of grain per volumetric “Winchester” bushel. In practice, test weight measurements are based on the weight of grain that fills a quart container (37.24 qts to a bushel) that meets the specifications of the USDA-FGIS (GIPSA) for official inspection (Fig. 1). Certain electronic moisture meters, like the Dickey-John GAC, estimate test weight based on a smaller-volume cup. These test weight estimates are reasonably accurate but are not accepted for official grain trading purposes.

The official minimum allowable test weight in the U.S. for No. 1 yellow corn is 56 lbs/bu and for No. 2 yellow corn is 54 lbs/bu (USDA-GIPSA, 1996). Corn grain in the U.S. is marketed on the basis of a 56-lb “bushel” regardless of test weight. Even though grain moisture is not part of the U.S. standards for corn, grain buyers pay on the basis of “dry” bushels (15 to 15.5% grain moisture content) or discount the market price to account for the drying expenses they expect to incur handling wetter corn grain.

Growers worry about low test weight because local grain buyers often discount their market bids for low test weight grain. In addition, growers are naturally disappointed when they deliver a 1000 bushel (volumetric bushels, that is) semi-load of grain that averages 52-lb test weight because they only get paid for 929 56-lb “market” bushels (52,000 lbs ÷ 56 lbs/bu) PLUS they receive a discounted price for the low test weight grain. On the other hand, high test weight grain makes growers feel good when they deliver a 1000 bushel semi-load of grain that averages 60 lb test weight because they will get paid for 1071 56-lb “market” bushels (60,000 lbs ÷ 56 lbs/bu).

These emotions encourage the belief that high test weight grain (lbs of dry matter per volumetric bushel) is associated with high grain yields (lbs. of dry matter per acre) and vice versa. However, there is little evidence in the research literature that grain test weight is strongly related to grain yield.

Hybrid variability exists for grain test weight, but does not automatically correspond to differences in genetic yield potential. Grain test weight for a given hybrid often varies from field to field or year to year, but does not automatically correspond to the overall yield level of an environment.

Similarly, grain from high yielding fields does not necessarily have higher test weight than that from lower yielding fields. In fact, test weight of grain harvested from severely stressed fields is occasionally higher than that of grain from non-stressed fields, as evidenced in Fig. 2 for 27 corn hybrids grown at 3 locations with widely varying yield levels in Kansas in 2011. Another example from Ohio with 22 hybrids grown in common in the drought year of 2012 and the much better yielding year of 2013 also indicated no relationship between yield level and grain test weight (Fig. 3).

Conventional dogma suggests that low test weight corn grain decreases the processing efficiency and quality of processed end-use products like corn starch (U.S. Grains Council, 2018), although the research literature does not consistently support this belief. Similarly, low test corn grain is often thought to be inferior for animal feed quality, although again the research literature does not support this belief (Rusche, 2012Simpson, 2000Wiechenthal Pas et al., 1998). Whether or not low test weight grain is inferior to higher test weight grain may depend on the cause of the low test weight in the first place.

Common Causes of Low Grain Test Weight

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