disease – Page 2 – Maryland Agronomy News

To Spray Or Not To Spray? Fungicide Considerations For Soybeans

Andrew Kness, Agriculture Agent
University of Maryland Extension, Harford County

Syptoms of Septoria brown spot on soybean — *Symptoms of Septoria brown spot on soybean. Image: Daren Mueller, Iowa State University, Bugwood.org.*

With the majority of full season soybeans in the region somewhere around R1-R3, growers may be considering a foliar fungicide application; however, with the current prices of soybean, is it worth the expense? That is a million dollar question and it is very difficult to answer. This article will outline a few points to consider as you try to decide if a fungicide application is worth the investment.

First and foremost, the purpose of a foliar fungicide application is to manage foliar diseases. This is what they are designed to do, and with the exception of where pathogens have developed resistance, they are very good at accomplishing this task. However, in order for a fungicide to achieve disease control and a yield boost, disease must be present and yield-limiting. The predominant foliar diseases of soybean in Maryland include Septoria brown spot, Cersospora leaf blight, frogeye leaf spot, and to a lesser extent, target spot and downy mildew. These diseases are all generally favored by moist, warm weather and typically do not appear in our region to any significant extent until our soybeans reach approximately reproductive phases. Historically, these foliar diseases are not typically present at high enough levels to cause a reduction on yield—but sometimes they do. It should be noted that the diseases that typically cause the major yield reductions in soybeans are stem and root diseases that infect soybeans very early in the growing season such as sudden death syndrome, charcoal rot, brown stem rot, soybean cyst and root knot nematodes, and cannot be managed with an in-season fungicide application. These diseases require a different management strategy.

If you look at the body of research on foliar fungicide effect on soybean yield, you’ll find the results to be quite variable from year-to-year. One year you may see a positive and significant yield response to a fungicide application and nothing the next year, or even at a different location within the same year. Current research on soybean indicates a positive and statistically significant yield response from a fungicide application about 30-50% of the time.

Generally, a significant yield response is achieved when foliar disease pressure is high; which goes back to the purpose of a fungicide, which is to manage disease. A positive and statistically significant yield response is more likely when disease pressure is high. The trick is trying to figure out when conditions will favor situations of higher disease pressure. If we can do that, then we can do a better job of determining when and where foliar fungicides are needed. To do this, take into account the foundation principal of plant pathology; the disease triangle.

Like any triangle, the disease triangle has three legs, which are a visual representation of the three criteria that must all be present, at the same time, in order for a disease to occur. They are: 1) a susceptible host, 2) a virulent pathogen, and 3) a conducive environment for the pathogen. If all three are present at the same time, disease will occur and this is when fungicides are a valuable tool in our toolbox. Assessing your risk in each one of these three categories can help you determine if a fungicide application is necessary. Here are some considerations for each.

Susceptible Host

Know your varieties and check the disease resistance ratings. Host resistance will significantly decrease, and in some cases completely eliminate, your risk for developing that particular disease. If you are growing susceptible varieties then you will have an increased likelihood of developing disease.

Virulent Pathogen

In the case of the major foliar diseases of soybean in Maryland, a virulent pathogen is always present. These pathogens persist on residue, waiting for a susceptible host and conducive environment to sporulate, infect, and grow.

Conducive Environment

This is the most difficult leg of the disease triangle to determine and the reason why fungicide applications can be “hit or miss.” Each pathogen requires slightly different conditions for which it can grow; but in general, research indicates that temperatures within the range of 75-86°F; coupled with accumulated rainfall greater than 0.5 inches in 5 days or 1.0 inch in 10 days; or 3 days with 10+ hours of relative humidity at or above 95%, favor the development of the major foliar diseases of soybean. If these temperature and moisture conditions are met, your risk of developing foliar diseases increases.

However, as you might imagine, predicting these conditions is very difficult and the micro-climate within the soybean field itself can vary greatly from conditions outside the field (i.e. where a weather station may be located). Factors that can influence the micro-climate within a field that favor higher humidity and thus greater disease potential include: narrow row spacing that restricts airflow, higher plant populations, taller/larger plants, hedgerows, wood lines and topography that restrict airflow, fields that are prone to frequent and prolonged periods of dew or fog (for example, fields near streams/rivers), and overhead irrigation.

As a general rule, warm temperatures coupled with moisture around the time of flowering through pod development will favor disease development that could significantly reduce soybean yield.

An additional consideration is economics. If the application costs you $15 per acre, then you will need at least 1.76 bushel increase to pay for that application if your sale price is $8.50 per bushel. This is certainly attainable if disease pressure is high, but if it turns hot and dry, you would be out $15 per acre.

Also consider fungicide resistance when determining a fungicide application. From a resistance-management standpoint, it is better to only use fungicides when the probability of developing disease is likely. Indiscriminately spraying fungicides when not needed exposes the pathogens to the active ingredients and pushes fungal populations towards resistance, even if disease pressure is low. Also, cutting fungicide rates accelerates resistance. It is also important to rotate modes of action to slow the pace of fungicide resistance. Fungi can quickly develop resistance to fungicides; for example, there are several populations of Cercospora sojina, the causal agent of frogeye leaf spot, that are already resistant to Qoi fungicides (such as Quadris, Aproach, Headline, etc.).

Determining if a foliar fungicide application will provide a yield bump is difficult, but taking into consideration the above points as it relates to the disease triangle can help. For more information regarding which fungicides to use for which disease, consult this table.

Corn Disease Updates

Alyssa Koehler, Extension Plant Pathologist
University of Delaware

The beginning of this year was very conducive for Pythium species. Seed treatments typically have efficacy for about two weeks. In many cases this year, stands came up well, but then suffered from post-emergent damping off or stunted, slower growing plants following favorable environmental conditions for disease 3-4 weeks after planting (Figure 1). Infected plants typically have a brown to rotted mesocotyl when dug up (Figure 2). In severely infected plants, the top of the plant may become completely separated, resulting in death of the plant. In less severe conditions, the plant is able to set new brace roots and continue growing. Damping off from Pythium is common in low field areas that hold more moisture, but wet, cool spring conditions favored development across entire fields this season.

corn dying in field caused by pythium — **Figure 1.** Damping-off of corn caused by Pythium (Photo A. Koehler).

Over the past few weeks, corn has been growing well and we are now beginning to see some foliar diseases. Reside-borne leaf diseases like Grey Leaf Spot (GLS) and Northern Corn Leaf Blight (NCLB) are common across our area. It is possible to select hybrids with good/excellent resistance to these diseases, which will reduce the amount of foliar disease observed. Resistant hybrids typically have smaller lesions and reduced spread of spores. Continuous corn, no/minimal till systems, rainy weather or heavy and high plant populations can all create environments that favor foliar disease development.

corn disease symptoms — **Figure 2 (left).** Corn plant with brown mesocotyl tissue due to Pythium (Photo A. Koehler). **Figure 3 (middle).** Young Grey Leaf Spot Lesions and **Figure 4 (right).** Northern Corn Leaf Blight lesions (Photos N. Gregory).

Grey Leaf Spot, caused by the fungus Cercospora zeae-maydis has begun to show up in fields. Under favorable conditions, this disease can significantly reduce yields. GLS usually begins on lower leaves with small, tan, rectangular lesions with a yellow halo (Figure 3). When lesions are young, they can be difficult to distinguish from other common corn foliar diseases. At maturity, lesions are grey to tan in color, with a long rectangular shape; partially resistant hybrids can have more jagged margins than lesions on susceptible cultivars. Northern Corn Leaf Blight has not been as common yet this year. Symptoms from this disease are long, tan to grey lesions that become oblong to “cigar-shaped” with age (Figure 4). For both of these diseases, lesions can coalesce to form large necrotic areas and yield reductions are often observed when lesions are present on the two leaves below the ear leaf of higher.

If applying a fungicide, application is typically recommended at VT/R1 timing for greatest chance of economic return. As a general threshold, if 50% of plants have lesions on the third leaf below the ear or higher prior to tasseling, a spray may be considered. Other considerations when making a fungicide application include hybrid susceptibility, environmental conditions, grain prices, and cost of application. The Fungicide Efficacy for Control of Corn Diseases table prepared by the Corn Disease Working Group provides information on fungicide efficacy for various corn diseases (https://crop-protection-network.s3.amazonaws.com/publications/cpn-2011-corn-fungicide-efficacy-for-control-of-corn-diseases.pdf).

FHB Update: May 10, 2019

*72-hour risk for Fusarium Head Blight development on moderately resistant wheat varieties for May 10, 2019.*

Wheat in the Eastern Shore of Maryland is finishing up flowering. The risk
for FHB is high. If you haven’t sprayed and you are still within 4-6 days
of flowering, you can still do so. Wheat in the Northern parts is heading
now and will soon start flowering. The FHB risk for this part of the state
is also predicted to be high, and the farmers should be prepared to spray
fungicides on their wheat when it flowers (50% of the main tillers showing
yellow anthers). The fungicides effective for FHB are Prosaro/ Caramba/
Miravis-Ace. The best stage recommended for spraying fungicides is early
flowering or within 4-5 days of that. These fungicides do not need to be
tank mixed with another product for spraying. The fungicide products should
be applied at the full rate recommended by the manufacturers. Strobilurin
containing fungicides should not be sprayed at this stage. Aerial
application at a rate of 5 gallons per acre or ground application at 15
gallons per acre with 300-350 um droplet size is recommended. Spray nozzles
should be angled at 30°-45° down from horizontal, toward the grain heads,
using forward- and backward mounted nozzles or nozzles with a two
directional spray, such as Twinjet nozzles. There has been no other major
disease being seen anywhere across the state in wheat so far.

–Nidhi Rawat, Small grains Pathologist, University of Maryland

For more details, go to the FHB Risk assessment tool at
http://www.wheatscab.psu.edu

For the latest news and updates from the U.S. Wheat & Barley Scab
Initiative, go to https://www.scabusa.org

New Section 3 MD Label: Miravis Ace for Scab in Barley and Wheat

wheat infected with head scab — *Wheat infected with Fusarium head blight. Image: A. Kness, University of Maryland.*

A new Section 3 MD label for Fusarium head blight (head scab) control on barley and wheat was just awarded for the Syngenta fungicide Miravis Ace, (see the label here: MIravis Ace Syngenta Label-ue1r8d).

See this article for more information on Fusarium head blight management, and stay on top of scab this year by subscribing to scabalerts and using the Fusarium Head Blight risk assessment map.

Free Corn Tar Spot Webinar

Tar spot is a disease of corn that is new to the United States and could eventually make its way to Maryland.

Join Dr. Nathan Kleczewski from the University of Illinois Extension for an update on Tar spot in corn. This disease was first observed in the United States in 2015 in Northern Illinois and Indiana. In 2018, the disease significantly affected corn production in the Midwest and Florida. What is tar spot of corn? How does it work? What is our current understanding of this disease and its management? These and other questions will be addressed through this free webinar.

Registration is Free, but capped at 100 participants.

To register for this meeting, click the following link: https://web.extension.illinois.edu/registration/?RegistrationID=19924

Guess the Pest! Week #23 Answer: Sudden Death Syndrome of Soybean

Bill Cissel, Extension Agent – Integrated Pest Management, University of Delaware; bcissel@udel.edu

Congratulations to Lamar Witmer for correctly identifying the disease as sudden death syndrome of soybean and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #23 Answer: Sudden Death Syndrome of Soybean
by Nancy Gregory, Plant Diagnostician, University of Delaware; ngregory@udel.edu

Sudden death syndrome of soybeans (SDS) is caused by the fungus Fusarium virguliforme. We started seeing this disease in Delaware in 2002 in cool and wet seasons, but have seen it more often in the past few years. SDS can be confused with other stem diseases such as Phomopsis stem canker and charcoal rot. Leaf symptoms of yellowing and browning between the veins are typical, and leaves shrivel and fall off, leaving petioles still on the stems. If stems are pulled up and placed in a plastic bag overnight, blue spore masses of the fungus may be seen at the base of stems. The internal stem tissue (cortex) may show dark discoloration. There is a toxin produced by the fungus that is responsible for the symptom pattern showing up at the top of the plant. The fungus overwinters in debris, and disease is most severe when infection occurs early. Improving drainage, alleviating compaction, and treating seed may help get seedlings established.

Top 6 Tips For Managing Soybean Diseases

By Andrew Kness, Agriculture Extension Educator

University of Maryland Extension, Harford County

As we move into the 2018 growing season, here are some tips for managing soybean diseases. They are all important points to consider, and are in no particular order of importance.

Know what you’re up against. Don’t just guess at the cause of the problem; use the resources at your disposal to confirm the cause. The University of Maryland Extension Plant Diagnostic Lab is a great resource, as well as private labs. Your local extension agent can also help you diagnose a problem. Also, remember that other abiotic disorders such as drought, herbicide damage, nutrient deficiencies, etc., can sometimes resemble a biotic disease. Proper identification will help you implement the correct management tactics.
Know your soybean variety. Pay particular attention to the disease and resistance (if any) ratings. Selecting varieties that are more tolerant or resistant to a particular disease will be a key part of your management moving forward. In fields where a particular disease is a recurring problem, selecting varieties that have resistance or tolerance (if available) should be a priority. Use University variety trial information (when available), in conjunction with the seed company ratings, to select the best seed for each field.
Plant at the proper population. Pushing plant populations too high will result in a denser canopy, creating a much more humid and moist micro-climate within the canopy, which is an ideal environment for most plant diseases to develop. Research in Maryland has shown that full-season soybean plant populations should be targeted at 140,000 plants per acre and 180,000 plants per acre for double-crop production.
Protect the top ⅓ of the canopy. The top third of the soybean plant contributes to nearly all of the carbohydrates necessary for grain fill, which is what determines your yield. Foliar diseases that infect the top third of the plant, if severe enough, can significantly affect yield. If foliar fungal diseases begin spreading to the upper third of the plants that are approaching flowering and pod fill, a fungicide application may be necessary to protect yield; however, economics and return on investment should be considered. Fungicides are relatively cheap now but you should still consider the economics; so before spraying, figure out how many bushels you need in return to warrant that application. During average years in Maryland, diseases are typically not severe enough to warrant a fungicide application for most of our foliar diseases of soybean. If they are, we generally achieve the best control of most foliar diseases when an appropriate fungicide is applied between R1 and R3. Always remember to read and follow the label, as well as rotate fungicide modes of action to curb disease resistance. The Take Action Pesticide-Resistance Management website has some great resources and tools to help make rotating modes of action quick and easy, including a searchable database and the Take Action Soybean Fungicide Classification Chart.
Plant in ideal field conditions. Avoid having seed and seedling plants in soil that is cool and waterlogged. Waterlogged soils can lead to the development of stem and root diseases (Figure 1), which have no in-season cure once infection occurs. Also, if you’re using fungicide seed treatments, realize that they are only effective for approximately 2-3 weeks after planting. Practices that help reduce compaction and increase water infiltration will prevent your seedling soybeans from having “wet feet”, which is the driving cause of root and stem disease development. Planting into soils that are the proper moisture and temperature will go a long way in helping you manage many soybean diseases.
Crop rotation and residue management. Since many of the major soybean diseases in Maryland overwinter on old soybean residue, planting soybeans back-to-back is not recommended. Breaking up the rotation with a small grain, corn, or another crop before returning to soybeans will allow the old soybean residue more time to decompose and kill off the pathogens that are overwintering in and on them. Likewise, a light tillage pass with a vertical till or turbo till tool to size old soybean residue will increase residue decomposition, reducing inoculum load in your field for future soybean crops.
Figure 1. Classic symptoms of post-emergence damping off in soybeans caused by root-rotting pathogens during an extended period of saturated soil conditions. Image: Cary Hicks, Bugwood.org.

signs and symptoms of charcoal rot on soybean stems — **Figure 2**. Signs and symptoms of the charcoal rot pathogen, Macrophomia phaseolina. Notice the tiny black specs, which are microsclerotia, structures that allow the pathogen to overwinter in old soybean residue. Image: Martin Draper, USDA-NIFA, Bugwood.org

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