C.O.R.N. Newsletter: 2021-23

Not only has rain delayed the harvest of some fields for grain, it has also delayed the baling of straw in several fields that have been harvested. In Ohio, wheat straw is sometimes just as important or even more important than grain, as it is used as bedding for livestock, and in some cases, as a feed ingredient. Delayed baling due to excessive rainfall could cause the quality of the straw to deteriorate as a result of mold growth. To fungi (molds), wheat straw is nothing more than dead plant tissue ready to be colonized. Under warm, wet conditions, saprophytic fungi (molds that feed of dead plant materials) readily colonize wheat stubble, resulting in a dark moldy cast being formed. This problem is particularly severe in lodged fields.

Some of the molds growing on the wet straw in the field, including the fungus that causes head scab (Fusarium graminearum), also produce mycotoxins such as vomitoxin. This could be a concern when using straw for bedding or as a feed ingredient. In general, vomitoxin levels are much higher in straw than they are in the grain, and even when the straw is used as bedding, a small portion of it is consumed. For instance, depending on the production system, pigs may obtain up to 12% of their total feed intake from straw. Pig are particularly sensitive to vomitoixin – it causes vomiting (hence the name), feed refusal, and low weight gain. A good way to tell if the straw is colonized by the scab fungus it to look at the color of the mold growing on it – a pinkish-white mold would be a good indication that the straw could be contaminated with vomitoxin.

Vomitoxin contamination of straw tends to be highest when the straw is baled from fields with high levels of head scab. In addition, straw from fields that were planted with head scab resistant variates usually contain less vomitoxin than straw from fields plated with susceptible varieties. Thankfully, head scab levels were very low in most fields across the state this year, as a result, initial vomitoxin levels in the straw may be low. However, straw from scab-free fields can still be colonized by the fungus and contaminated with vomitoxin, especially if it goes through repeated wetting and drying cycles before being baled. This would be even more of a problem for those fields that have not yet been harvested, as both the grain and the straw could be contaminated with vomitoxin. Moldy straw could also contain other mycotoxins that are harmful to livestock. So, in addition to examining the straw for mold growth (pinkish color), get a sample tested for mycotoxins before using it for bedding or feed.        

The R3 soybean growth stage is a common time to consider foliar application of fungicide, insecticide, and fertilizer. Before we jump into the potential yield outcomes of these products, let’s define the R3 growth stage. A soybean plant is at the R3 growth stage when there is a pod at least 3/16 inch long (but less than 3/4 inch long) at one of the four uppermost nodes on the main stem with a fully developed trifoliolate leaf. A leaf is fully developed, and the node is counted, when the trifoliolate leaf at the node immediately above it is open (Figure 1). Soybean plants within a field may be at different growth stages. Over half of the plants need to be at a certain growth stage for the whole field to be considered that growth stage. For more information on soybean growth stages R3 through R6, see this YouTube video: https://www.youtube.com/watch?v=Z0A1fkU4oBU.

When should I spray a foliar fungicide?
First, consider the disease triangle. For a disease to develop, there must be a 1) host (Is your soybean variety resistant or susceptible?), 2) pathogen (Is there a history of a certain disease in your field? Do you see any visual symptoms of disease?), and 3) conducive environment. Most foliar diseases, such as brown leaf spot and frogeye leaf spot, are favored by wet conditions.

In the soybean agronomic trials, brown leaf spot and frogeye leaf spot tend to be the two most common soybean diseases (Figure 2). In these trials, we’ve measured a yield response to foliar fungicide applied at R3 in 9 out of 28 environments, ranging from 4 to 8 bu/acre. At the responsive locations, which tended to be in central and southern Ohio, there was foliar disease present (brown spot and frogeye leaf spot). Additionally, these positive yield responses occurred in years with greater precipitation. Very little to no yield response occurred in dry years and in years when soybeans were flooded. If you have visual symptoms of disease, a conducive environment, and susceptible variety, R3 is a good time to spray a foliar fungicide.

When should I spray a foliar insecticide?
Often, if a farmer plans on spraying a foliar fungicide, they will tank-mix a foliar insecticide. From last week’s article on soybean defoliation, “It is very rare that we reach economic levels of defoliation here in Ohio.” (Click here to read the entire article: https://agcrops.osu.edu/newsletter/corn-newsletter/22-2021/soybean-defoliation-it-takes-lot-really-matter). Over the past several years, we’ve tested foliar insecticide in 28 Ohio environments. Out of those 28 environments, we’ve only measured a yield response (+5 bu/acre) to foliar insecticide applied at R3 one time. In the other 27 environments, soybean yield was unaffected by foliar insecticide with defoliation levels in the mid- to upper canopy at <15%. As mentioned in last week’s article, “…treatment is advised when defoliation levels reach 40% in pre-bloom stages, 15% in bloom, and 25% during pod fill to harvest.”

When should I spray foliar fertilizer?
In Ohio, the most common micronutrient deficiency is manganese. However, even then, we’ve only measured a yield response to manganese foliar fertilizer in two out of 20 Ohio environments. Soybeans are most likely to respond to foliar fertilizer when there are visual symptoms of deficiency. Interveinal chlorosis is a visual symptom of manganese deficiency (Figure 3). Manganese deficiency tends to occur in fields with high pH or high organic matter (muck), especially if soils are droughty. In dry soil, manganese is converted to a form that is unavailable for plant uptake.

Recently, soybean agronomists across the U.S. evaluated foliar fertilizers in 46 environments and found no soybean yield increase when the products were applied prophylactically (e.g., no visual deficiency symptoms). For more information on this study, see: https://www.youtube.com/watch?v=0DtNagk6ghI. In most situations, foliar fertilizers are unnecessary.

Summary
When soybean plants are yellow due to saturated soil conditions as seen in northern Ohio last week, it can be tempting to apply foliar products to help the plants ‘recover’ (Figure 4). These fields often already have a lower yield potential and are unlikely to respond to foliar products in the absence of disease, insects, and nutrient deficiency.

Multiple trials in Ohio and across the U.S. have shown that prophylactic applications of foliar fungicide, insecticide, and fertilizer provide no yield benefit. Before applying these products, it’s important to scout your fields for disease, insects, and nutrient deficiencies.

Many corn fields in Ohio are rapidly approaching silking (R1), and foliar diseases such as Gray leaf spot have been observed where the environment has been conducive to disease development, prompting growers to consider fungicide applications. The information below was developed by the Corn Disease Working Group (CDWG) as part of the Crop Protection Network. The Crop Protection Network is a a multi-state and international collaboration of university and provincial extension specialists, and public and private professionals who provide unbiased, research-based information to farmers and agricultural personnel. 

The CDWG developed ratings for how well fungicides control major corn diseases in the United States. The CDWG determined efficacy ratings for each fungicide listed in the table below by field testing the materials over multiple years and locations. Ratings are based on the product’s level of disease control and does not necessarily reflect yield increases obtained from product application. A product’s efficacy depends upon proper application timing, rate, and application method as determined by the product label and overall disease level in the field at the time of application. Differences in efficacy among each fungicide product were determined by directly comparing products in field tests using a single application of the labeled rate. The table includes marketed products available that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products. Additional fungicides are labeled for disease on corn, including contact fungicides such as chlorothalonil. Other fungicides may be available for diseases not listed in the table, including Diplodia, Gibberella and Fusarium ear rots. Many products have specific use restrictions about the amount of active ingredient that can be applied within a period of time or the amount of sequential applications that can occur. Read and follow all use restrictions prior to applying any fungicide.

If you are interseted in learning more about fungicide application in corn and reducing the risk of vomitoxin, listen to the recording of the July 8th CORN Live Session featuring Dr. Pierce Paul at https://youtu.be/8eN7qPmbp-A or play the embedded video below.

Find more crop disease resources at CropProtectionNetwork.org