Fungicides are an important part of plant health, protecting crops from disease. Corn tar spot, southern corn rust, and frogeye leaf spot are a few diseases that can be costly if not treated with an effective fungicide application. Planning an effective fungicide application with adjuvants specifically targeting these diseases is imperative to reducing disease pressure.
Corn tar spot has become a pressing issue for midwestern corn farmers, especially in eastern corn-belt areas where abundant rainfall in mid-summer allowed disease establishment and increased spread. Heavy tar spot infections cause corn to mature early and not complete grain fill leading to smaller and lighter kernels and reducing yields.
Tar spot (Phyllacohra maydis) was first confirmed in the US in 2015 in Illinois and Indiana but originated in Latin America, specifically Mexico. Since 2015, corn tar spot has spread from a few counties in Illinois and Indiana to much of the corn-belt and continues to be an active concern.
Growers east of the Mississippi River have an established inoculum base that can produce issues again if conditions are wet this summer. Tar spot has been recorded as far west as eastern Nebraska. With heavy use of irrigation, corn tar spot has the potential to become a seasonal issue for Nebraska.
Like other crop diseases, cultural practices, like crop rotation to non-host crops, degrading corn crop residue after harvest via tillage that buries residue, or rotating away from corn until the residue has decomposed, can reduce tar spot presence.
Fungicides are an important in-season tool to help prevent yield losses from tar spot. However, with tar spot being a newer disease, not all fungicides are labeled to control corn tar spot, so it is essential to know which fungicides are labeled for control and work with your local agronomist or trusted advisor to understand what products, rates, and application timing will work best.
Corn leaf infected with tar spot
After harvest, fields with a heavy amount of tar spot are likely to have less stalk integrity and should be prioritized for harvest to avoid harvest issues with downed corn. In addition, the hybrids in those fields should be noted for their susceptibility to tar spot and actual performance to help plan future hybrid selection.
As seed companies develop and understand more about hybrid tolerance and resistance to tar spot, planting corn hybrids that are not highly susceptible to this fungus is also essential.
Southern corn rust causes more yield loss than common rust. In recent years, southern corn rust has become more common further north into the Corn Belt, however, the disease overwinters in the southern US and requires storm systems to move it north into the Midwest.
Corn leaf infected with southern corn rust
Southern corn rust is more reddish-orange in color than common rust. Since rust is typically present in the upper leaves of corn plants and infections usually occur later in the season in August, it should be scouted for two weeks before to two weeks after tasseling. Later-planted corn and susceptible hybrids are more at risk for southern corn rust.
Frogeye leaf spot is a fungal pathogen that has typically impacted soybeans in the southern US but is now present in the Midwest.
Frogeye leaf spot overwinters in soybean residue and infects plants when rain and wind move spores from the residue onto plants. With less tillage being done, more inoculum is present in fields, resulting in increased disease presence. Weather can increase this presence since warm and wet conditions favor frogeye leaf spot.
Soybean leaf infected by frogeye leaf spot
After initial infection, frogeye leaf spot spreads into the upper leaves, becoming noticeable as circular lesions that are light grey in the center and darker around the edges. The disease can later move to pods as the season progresses. Therefore, fungicide applications around stage R2 (full flower) can be warranted depending on the severity of leaf lesions.
Crop rotation and tillage help reduce the presence of frogeye leaf spot after it has been found in a field.
While fungicides aren’t paired up for diseases, choosing the right active ingredients can effectively manage the disease. There are several fungicides available that contain triazoles, strobilurins, or a combination of both. These actives are effective against a broad range of fungal diseases in various crops. Veltyma®, Headline®, and Quadris® are some brand names for these broad spectrum fungicides.
It’s important to note that the effectiveness of fungicides can vary depending on several factors, including the specific fungicide used, the stage of the disease, environmental conditions, the severity of the infection, and how well it reaches it target. It is always recommended to consult with local agricultural extension services or agricultural experts to determine the most appropriate fungicide and application timing for your specific location and situation. Always follow the label instructions and guidelines when using fungicides.
Ensuring maximum application effectiveness is crucial when using fungicides to control fungal pathogens. Adding adjuvants increases the likelihood that the fungicide achieves its goal by helping the actives achieve full efficacy potential. Fungicide labels are sometimes ambiguous about any specific adjuvant recommendations, but they do all warn against drift and give drift mitigation. Adjuvants allow tank mixes to be tailored to local conditions, and the behavior of the targeted diseases should be considered to alleviate the disease pressure best.
The three major categories of fungicides are contact, translaminar, and systemic. There is an Exacto adjuvant for each or one may cover all categories. Exacto adjuvants for fungicide applications offer properties like spreading and wetting, drift reduction, deposition, UV protection, or rain fastness. While some products like deposition aids, drift reduction agents, or surfactants perform a single benefit, others are multifunctional delivering multiple functionalities in one easy and effective formulation.
Fungal diseases can be present deeper in the canopy, so it is vital that a fungicide application will be able to get deposited on all leaves, also the ones lower in the canopy. A deposition aid improves canopy penetration and gets the fungicide deep into the plant’s lower leaves. Deposition aids increase the amount and uniformity of fungicide active ingredients deposited onto a plant surface and deeper into the canopy.
Since corn tar spot, for example, infects plants from the lower leaves upward, initial fungicide applications benefit in particular from deposition aids helping the lower corn canopy receive active ingredients. This protects corn plants before the upper leaves, which capture the most sunlight, are affected.
Drift reduction agents keep spray applications on target by fostering a more uniform and consistent spray pattern with less influence from environmental factors. Drift reduction agents “Right Size” the spray droplets leading to a reduced drift potential. Right sizing means that these formulations significantly reduce the amount of driftable fine droplets, while preventing droplets from becoming too large and bouncing off of a leaf surface. This keeps the fungicide in the sprayed field and on the leaf for optimal protection.
A good surfactant will help spread the actives over the target area ensuring that the active ingredient will come in good contact with the target. This makes it possible for the active to do its work no matter what type of functional category the fungicide falls into. If the spray application does not cover the leaf, the results of a fungicide treatment will not be as effective.
Multifunctional adjuvants offer convenience and ease of use by combining multiple functionalities in one adjuvant. For example, Exacto’s SYNTHEX® L 954 is a multifunctional adjuvant with nonionic surfactant, deposition, and penetrant capabilities. When added to a tank mix with fungicides, it is proven to increase the droplet size of a spray mixture and reduce the amount of drift (figure 1).
Figure 1: SYNTHEX L 954’s drift reduction capabilities tested in a Battelle Wind Tunnel study. The study used an XR11003 nozzle at 40 psi. The results showed that adding SYNTHEX L 954 to the fungicide tank mix, droplet size increased and the percentage of driftable particles reduced.
SYNTHEX L 954’s nonionic surfactant properties give superior spreading and wetting characteristics to improve the performance of the active ingredient by ensuring uniform surface coverage with reduced bounce and run-off. This is especially important for contact fungicides. Innovative technologies limit driftable fines by managing droplet size modification without pump shear or solution fatigue. This formulation includes Penetration Factor™, a patented technology that increases penetration through waxy, wetting-resistant cuticles and enhances the permeation of the active ingredient to the mode of action site. These properties are beneficial for improves the effectiveness of systemic fungicides.
Another type of multifunctional is a spreader sticker, an adjuvant that combine a sticker with a surfactant to ensure that more active ingredients stay on and spread across leaf surfaces to optimize control. The surfactant provides efficient spreading on the leaf. At the same time, the sticker can protect the active ingredient against wash-off (figure 2), UV degradation, and wind erosion so it stays longer on the leaf, ensuring better efficacy.
Figure 2: Droplets for nonionic surfactant (NIS), NIS/sticker mix, and sticker adjuvants before (left) and after (right) a rainfall event. The NIS almost completely washes away with the water while the sticker remains mostly in place.
These sticker spreaders combine a sticking and surfactant component to improve contact and adhesion of crop protection products by sticking the application to the target surface and inhibiting ultra-violet light degradation, irrigation wash-off, and wind erosion. The surfactant wets and spreads the active ingredient and distributes pinene evenly across the leaf surface, increasing the ability to protect a greater leaf surface area important to contact fungicides. The most common used sticking materials are pinene and latex, with pinene providing longer lasting protection as it allows for plant growth due to its flexibility.
When spraying corn during the season, it is crucial to prioritize the positive outcomes of applications while being mindful of any undesirable side effects. While some basic adjuvants with surfactants may contain NPE (nonylphenol ethoxylates), chemicals known to be associated with Arrested Ear Syndrome and harmful to aquatic life, there is good news. There are adjuvants readily accessible that offer the same functionalities mentioned earlier, without the inclusion of NPE. These alternatives present a safe and responsible choice for in-season applications, ensuring both effective results and environmental well-being.
Be sure to read the label of active ingredients to ensure essential adjuvants like surfactants, deposition aids, drift reduction agents, and spreader stickers are included or not prohibited for use in your fungicide tank mix.
Other useful information:
Fungicide Efficacy
Exacto Activator Adjuvants
Exacto Utility Adjuvants
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