Plant Health

Corn Ear Rots: Identification, Quantification and Testing for Mycotoxins

By Pierce Paul OSU Extension

Ear rots differ from each other in terms of the damage they cause (their symptoms), the toxins they produce, and the specific conditions under which they develop. So, a good way to determine whether you do have a major ear rot problem this year is to quantify the disease in your field and get suspect samples tested for mycotoxins. And the best way to tell the difference among the ear rots is to know the types of symptoms they produce.

TRICHODERMA EAR ROT – Abundant thick greenish mold growing on and between the kernels make Trichoderma ear rot very easy to distinguish from Diplodia, Fusarium, and Gibberella ear rots. However, other greenish ear rots such as Cladosporium, Penicillium and Aspergillus may sometimes be mistaken for Trichoderma ear rot. Like several of the other ear rots, diseased ears are commonly associated with bird, insect, or other types of damage. Another very characteristic feature of Trichoderma ear rots is sprouting (premature germination of the grain on the ear in the field). Although some species of Trichoderma may produce mycotoxins, these toxins are usually not found in Trichoderma-affected ears under our growing conditions.

DIPLODIA EAR ROT – Diplodia causes a thick white mass of mold to grow on the ear, usually initiating from the base of the ear and growing toward the tip. Eventually the white mold changes to a grayish-brown growth and infected kernels appear glued to the husk. Infected ears are usually lightweight and of poor nutritional value. When infections occur early, the entire ear may become moldy. When infections occur late, only a fine web of fungal growth appears on and between the kernels.

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Frogeye Leaf Spot – Is It Worth Spraying in 2019?

By:  Anne Dorrance, Ohio State University

Frogeye leaf spot

Several reports over the last two weeks of heavy frogeye leaf spot pressure in some fields as well as low to moderate pressure in others.  This disease will continue to increase and infect new foliage as it develops on these late planted soybeans. Based on our previous research, only once (2018) in 14 years of studies did applications at the soybean growth stage R5 contribute to preserved yield.  At the R5, the leaf at the terminal is fully developed and the pods at any one of the top four nodes is fully expanded, but the seeds are just beginning to expand.

Soybeans that have frogeye and have just begun to flower, are at full flower, or have just reached the R3 growth stage, these decisions are going to be challenging.  In full disclosure, we don’t have data or examples to rely on here.  This late planting and late development is all new territory for all of us.  But there are some sound principles to rely on.

For soybeans that are in the R3 growth stage, pods are tiny, 3/16 of an inch at one of the four uppermost nodes of the plant. This is the time if frogeye leaf spot can easily be found in the canopy, a lesion on one plant every 40’ has in our studies, preserved yield in a normal growing season.  This growth stage in Ohio typically occurs in mid to late July on May planted soybeans.

So here are the questions to address for 2019 and in the order of importance.

  1. The value of the crop – are these soybeans grown for seed, then yes error on the side of caution and apply the fungicide and make a second application 14 days later.
  2. Are these soybeans under contract, and will you actually be able to sell them? If the answer is no, then adding more inputs into the crop may not be a sound investment.
  3. Will the soybean finish making grain before harvest? This question will most likely affect soybeans that are just now in full flower, we are hoping for a very long fall, but this will impact the return on applying the fungicide.
  4. How susceptible is the variety? For some resistant varieties, the frogeye leaf spots are small and only a few will form on each leaf.  So double check with your seed supplier to look at the ratings. In any event your seed dealer will want to watch this variety and work with their breeders.

If you do decide to spray, please leave unsprayed check strips – at least 3 separate locations in the field and collect the yield off each of these separately, the same is true for the fungicides sprayed strips, collect the data from these as well.  The yield maps will be especially important this year.  Secondly, choose the cheapest triazole fungicide that you can find.  This is going to be very important for the economic viability of this year’s crop. Also remember, we have detected QoI resistance in Ohio, and it is not advisable to spray these types of fungicides at these late dates on crops that are further behind in development.

If you don’t spray, and it is a highly susceptible variety, the disease will continue to increase on the plants, but only if periodic rains and heavy dews or fogs continue through the remainder of this crazy field season. Mark this field and this variety. These are important considerations for 2020 field season as this disease does now overwinter in Ohio. Replanting in the same field with the same variety or one that is susceptible to this disease is a recipe for further yield loss in the future.

Frogeye leaf spot

Keep an Eye Out for Tar Spot

Tar spot of corn

Tar spot on corn. Photo: Martin Chilvers, MSU.

By:  Stephanie Karhoff OSU Extension

As the season progresses, keep an eye for tar spot, a new corn disease caused by the fungus Phyllachora maydis. The pathogen originates from Mexico and Central America, but has made its way to the Buckeye State.

Tar spot of corn was first detected in 2015 in Indiana and Illinois, and was later observed in Florida, Iowa, Michigan, Ohio (2018), and Wisconsin. It thrives in wet, cool conditions. Since this is a newly emerging disease, it is important to be vigilant and scout your fields to track its spread and impact on corn yield.

Last season (2018) was the first year yield losses due to tar spot were evaluated in hybrid trials in Illinois, Indiana, Michigan, and Wisconsin.1 Across all trials and hybrid maturities, a 0.32-1.36 bushel per acre loss occurred per 1% increase in tar spot. Based on this, we would predict a field with 40-50% tar spot severity on the ear leaf by R5/6 would have a loss of 16.8-38.7 bushels per acre. In these trials, all hybrids experienced some level of tar spot.

When scouting, look for small, raised black spots called ascomata on leaves, leaf sheaths, and husks. These spots are often surrounded by a tan halo (see picture). It is easy to confuse these ascomata with saprophytes or insect frass. The key difference is that ascomata cannot be rubbed off the leaf surface with your fingers. If you suspect it is tar spot, call the Williams County OSU Extension Office at 419-636-5608.

You can find real-time tracking of tar spot occurrence in 2019 here  https://corn.ipmpipe.org/tarspot/

Sources:

1Telenko, DEP, MI Chilvers, N Kleczewski, DL Smith, AM Bryne, P Devillez, and et al. 2019. How Tar Spot of Corn Impacted Hybrid Trials During the 2018 Midwest Epidemic. Crop Protection Network. doi.org/10.31274/cpn-20190729-002

MI Chilvers. 2019. Corn Tar Spot Outlook for 2019. Michigan State University Extension. https://www.canr.msu.edu/news/corn-tar-spot-outlook-for-2019

Fusarium Head Blight (Scab) of Wheat: Things to Consider When Harvesting

BY: DARCY TELENKO (Purdue University Extension)

Wheat harvest has begun in Southern Indiana. Fusarium head blight (FHB) or scab is one of the most important diseases of wheat and most challenging to prevent. In addition, FHB infection can cause the production of a mycotoxin called deoxynivalenol (DON or vomitoxin). The environmental conditions have been extremely conducive to FHB development and it is not surprising that I have started to receive reports about issues with FHB and DON contamination. Our research sites in both West Lafayette and Vincennes have high levels of FHB develop in our non-treated susceptible variety checks and initial DON testing was at 7 ppm.

Fusarium head blight management is difficult and requires an integrated approach. This includes selection of varieties with moderate resistance and timely fungicide application at flowering. We are now past implementing either of these management options, but these are important to remember for next year. In addition, it will be important to assess your fields this season to determine if you have FHB. FHB can cause direct yield loss creating seeds that are shriveled and have a rough, sunken appearance to complete head loss (which I have already seen in multiple fields around the state). FHB infection can also reduce seed quality and feeding value of the grain due to the risk of mycotoxin (DON/vomitoxin) production in infected seed.

The question now is “I have scab in my field what do I need do?” Here’s a short list.

  1. Document the issues in each field, so you have records for making decisions on future disease management. FHB is easy to see when the head is still green – it will be much more difficult to rate as the heads reach maturity. See images of FHB in the head both at green and more mature stages. You might be able to see the pink salmon sporulation and/or purple-black fruiting bodies on mature heads (Figure 1).  In addition, it is good to note during the season what management tools were attempted – spray date and growth stage of crop, was there variability in the growth stages, weather conditions after fungicide applications. These all can play a role in effective disease management.
    Figure 1. Wheat spikes showing bleached florets affected by scab. Salmon to pink sporulation may be visible and can help confirm once the spikes have reached maturity (pink arrows). Dark purplish-black fruiting bodies can also occur mature wheat heads (black arrows),

    Figure 1. Wheat spikes showing bleached florets affected by scab. Salmon to pink sporulation may be visible and can help confirm once the spikes have reached maturity (pink arrows). Dark purplish-black fruiting bodies can also occur mature wheat heads (black arrows).

  2. Harvest fields with lowest disease first; adjust combine settings to blow out the smaller, shriveled kernels and chaff; and separate loads from healthy and disease fields. Mycotoxin contamination is usually the highest in the more heavily disease kernels and if they can be removed that would help reduced mycotoxin level.
  3. Test for DON levels in both kernel and straw before feeding to livestock. Scabby kernels do not always indicate high DON and vice versa. It is important to test and know what your DON numbers are in your grain, even if you don’t see a high level of disease. Straw can also contain DON. DO NOT use straw for bedding or feed from fields with high level of scab.
  4. Understand your elevators inspection and dockage procedures (each one can have a different practice). Levels of DON greater than 2 ppm may lead to price discounts.
  5. It is not recommended to store grain from field with high levels of scab – accumulation of DON and other mycotoxins can continue in stored grain. Suspect grain, if stored, should be dried to 5% moisture as soon as possible after harvest and kept separate from the good quality grain.
  6. Planting seed from fields that had moderate to heavy scab is not advisable. The infected seed will have low germination and poor vigor resulting in a thin stand. If going to use this seed, it should be cleaned thoroughly to remove the scabby seeds, and a fungicide seed treatment would be advised to protect germination and reduce seedling blight.

 

The next question “Why was is it so bad? I followed the guidelines applied my fungicide at flowering but we still have poor control.”

Here’s a few of my observations:

  1. Highly favorable environmental conditions for Fusarium head blight (FHB)/scab occurred all spring.
  2. Many wheat varieties have moderate resistance that help can reduce the risk of sever disease, and fungicides can help suppress the development, but this may only provide about 50% suppression. Therefore, even with the best management programs in place the extremely favorable conditions for FHB have led to high levels of infection this season.
  3. There was extremely variable growth in individual fields this year – plants ranged from boot to full flower when trying to make a decision on fungicide timing. In addition, fungicides may only provide partial suppression of FHB and timing is a significant issue for obtaining moderate levels of control.
  4. Frequent rains not only complicated planting, but any and every other trip across the field. Rain events closely following fungicide application may have diluted or washed off applications further reducing expected efficacy.

Additional references:

US Wheat and Barley Scab Initiative https://scabusa.org/

Cowger, C., and Arellano, C. 2013. Fusarium graminearum infection and deoxynivalenol concentrations during development of wheat spikes. Phytopathology 103:460-471. https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-03-12-0054-R

De Wolf, E. 2019. Fusarium head blight. https://www.bookstore.ksre.ksu.edu/pubs/MF3458.pdf

Salgado, J. D., Wallhead, M., Madden, L. V., and Paul, P. A. 2011. Grain harvesting strategies to minimize grain quality losses due to Fusarium head blight in wheat. Plant Dis. 95:1448-1457. https://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-04-11-0309

Wise, K. et al. 2015. Diseases of Wheat: Fusarium Head Blight (Head Scab) https://www.extension.purdue.edu/extmedia/BP/BP-33-W.pdf