2023 Maryland Tar Spot of Corn Research

Andrew Kness, Senior Agriculture Agent | akness@umd.edu
University of Maryland Extension, Harford County

Summary

Tar spot is a new foliar fungal disease of corn first discovered in the United States in 2015 and confirmed in Maryland in 2022 and was estimated to be the most significant yield-limiting disease of corn in the US in 2021 and 2022. As a new disease for our state, this project collected preliminary data on the distribution of tar spot in our state and compared the efficacy of different fungicide application timings. Through field surveys we identified and confirmed tar spot in eight Maryland Counties at a frequency of approximately 47% and at a relatively low severity rate (not exceeding 30%). These observations suggest that the tar spot pathogen can overwinter in Maryland, as it has expanded its range from two counties in 2022 to at least eight in 2023. Field evaluations of two fungicide programs: one pass program at VT and a two-pass program at VT followed by R2, we observed a significant difference in tar spot severity and plant lodging compared to the control; however, there was no difference in yield. Additional research on fungicide timing and the spread of this disease should be conducted in the future to help develop improved management recommendations.

Survey of Tar Spot Distribution in Maryland

Several fields were scouted for tar spot starting during late vegetative growth stages and frequency and intensity of scouting was increased from tassel through harvest. Initial scouting was focused in fields in Harford County near fields where tar spot was confirmed in 2022. In addition, reports were solicited from other Extension Agents and crop consultants/scouts throughout the state. Suspected positive samples were confirmed by laboratory technique and all positive samples were uploaded to the tar spot tracker map on corn.ipmpipe.org.

The first reported and confirmed incidence of tar spot in Maryland for 2023 came from a corn field in Cecil County on August 22. The second came from Carroll County on August 31, followed by Harford County on September 3. We confirmed tar spot in the additional counties of Kent and Queen Anne’s on September 19; Baltimore County on September 22; Caroline County on September 25, and Dorchester County on October 6 (Figure 1).

Figure 1. Map showing confirmed distribution of tar spot for the 2023 growing season (yellow). Map from corn.ipmpipe.org.

Several fields were scouted in Northern Harford County throughout the year surrounding fields where tar spot was confirmed in 2022. By the end of the season, tar spot was found in over 50% of these fields (9/16) at levels ranging from 2% to 25% severity (Figure 2). It was observed that tar spot severity continued to increase after black layer for as long as there was green, living tissue remaining on the plants. This increase in severity after physiological maturity does not affect yield but does make for a notably increased level of severity present at harvest and thus the potential for an increase in overwintering spores that will provide inoculum for the following year.

Figure 2. Corn leaf with approximately 10% tar spot severity. Tar spot symptoms include raised, black specks on the leaves.

An additional survey of 12 fields on Maryland’s Eastern Shore from Cecil to Queen Anne’s County was conducted on September 19. During this time, two fields were confirmed with tar spot. Severity was very low (<2%) in the field in Queen Anne’s County, and high in the field in Kent County (30%).

Altogether, tar spot was confirmed in 16 out of 34 fields (47%) scouted/reported throughout the state (Figure 3), with samples coming from as far west as Washington County (no confirmed samples) east to Cecil County (two confirmed samples) and south on the Maryland Eastern Shore as far as Dorchester County (one sample confirmed).

Figure 3. Google Earth map of fields scouted (blue markers) and confirmed (red markers) presence of tar spot. Markers are approximate locations and not precise to protect the identity of the landowner and/or farmer.

Weather conditions were favorable for tar spot on the Eastern Shore and Northern Maryland; however, severe drought conditions from Frederick County west may have prevented its widespread establishment in Western Maryland.

Based on this survey, tar spot appears to be established in all the northern counties east of Frederick and south on the Eastern shore to at least Dorchester County, at a frequency of approximately 40-50%. Judging by the confirmed occurrences in other counties in different states, it is likely that tar spot is present in more Maryland Counties than determined by this survey.

On-Farm Fungicide Trials

Fungicides are an effective management tool for foliar diseases of corn, including tar spot. Research from the Midwest has shown a positive response to fungicide applications in fields where tar spot disease severity is high. However, there is debate as to if one fungicide application made around VT is sufficient to control tar spot, as yield losses have been reported as late as R4. In 2023 we established an on-farm trial to evaluate the response to a single fungicide application compared to a two-pass program for managing tar spot in corn.

Field plots were established at a farm in Harford County, MD in a field immediately adjacent to where tar spot was found in 2022. Corn (Revere Seed ‘1307 TCRIB’) was no-till planted into soybean residue with a John Deere 1775 NT ExactEmerge™, 30-inch, 16 row planter at the rate of 35,000 seeds/acre. Rows 1, 2, 15, and 16 on the planter were shut off to create alleys between adjacent plots and to eliminate treatment overlap, as well as to ensure harvest accuracy. This resulted in 12-row plots that were between 75 and 150 feet long. Plots were arranged in the field in a randomized block with three treatments and five replicates (Figure 4).

Figure 4. Tar spot fungicide timing research plot layout.

Fungicides (Table 1) were applied at the VT and R2 growth stages using a DJI T30 drone calibrated to deliver 2.8 gallons per acre spray volume to the entire length of the 12-row plots. VT applications were made on July 12 and R2 applications were made on August 5. Trivapro 2.1 SE was used for all applications. Trivapro was selected because previous research has demonstrated that multi-mode-of-action products have the best efficacy against tar spot.

Table 1. 2023 Fungicide Treatments.

Treatment Product Name

Active Ingredient(s)

Application Rate (& Timing)
Nontreated Control N/A N/A
1X Pass Trivapro 2.1 SE

Benzovindiflupyr + Azoxystrobin + Propiconazole

13.7 fl oz/A (VT)
2X Pass Trivapro 2.1 SE

Benzovindiflupyr + Azoxystrobin + Propiconazole

13.7 fl oz/A (VT) & 13.7 fl oz/A (R2)

Foliar diseases were rated prior to fungicide application and approximately every two to three weeks following until harvest. Disease severity from tar spot was visually rated as the percent leaf area infected in the canopy from 10 random plants from the center two rows of each plot.

Lodging scores were collected at harvest by conducting a “push test” on 10 plants from the center two rows of each plot. The push test consists of pushing a corn plant approximately 30 degrees from vertical; plants that break have compromised stalk strength and were considered lodged.

Yield data were collected by harvesting 12 rows of each plot using a John Deere S780 combine on October 13, 2023. Yield data was exported from the combine monitor and RTK was used to correlate yield with plot locations since we were not able to collect individual plot weights. All yields reported are adjusted to 15.5% moisture. All data were analyzed using ANOVA and significant differences between treatments were separated using Fisher’s Least Significant Difference (LSD; α=0.10).

On-Farm Trial Results

Tar spot was first observed in the plots on August 29 present at a very low level (less than 2% severity). Overall tar spot disease severity was low throughout the season in these plots. One possible explanation for this is the early planting date, which likely allowed the corn to complete its critical reproductive growth stages before weather conditions were favorable for tar spot development.

Early disease ratings revealed a significant difference in tar spot severity (p=0.0176) in treated plots vs nontreated plots (Table 2). However, late disease ratings collected at harvest show an overall increase in tar spot severity, but no difference between treated and nontreated plots. This is likely due to the fact that fungicides can only offer around 14-21 days of protection. In this trial, the second fungicide application did not provide improved tar spot control compared to the single pass treatment; however, the single fungicide application at VT delayed tar spot infection compared to the nontreated control.

Table 2. 2023 Tar Spot Disease Rating and Harvest Data.

Treatment Tar Spot Severity (%) Lodged Plants (%) Grain Yield (bu/acre) Grain Moisture (%)
9/11/23 10/12/23
Control 3.05 a* 3.75 10.0 a 192.56 19.06
1X Pass 1.18 b 2.88   5.0 a 199.05 19.41
2X Pass 0.85 b 4.00   0.0 b 201.56 20.31
p-value 0.0176 0.4133 0.0680 0.2123 0.4343

*Treatments connected by the same letter are not significantly different from each other (α=0.10).

The control plots averaged 192.56 bu/acre with a low of 169.7 and high of 214.6; the single pass (1X pass) program yielded an average of 199.05 bu/acre with a low of 177.5 and high of 228.6 bu/acre; and the two-pass (2X pass) fungicide treatment yielded an average of 201.56 bu/acre with a low and high of 194.4 and 222.7 bu/acre, respectively. However, there are no statistically significant differences in yield between treatments (p=0.2123). Likewise, there was also no significant difference in grain moisture. Tar spot disease severity was relatively low; likely too low to impact yield in this trial, leading to no yield response.

The 2X pass fungicide program did improve standability of the crop at harvest, with 0.0% lodging, significantly better than the 1X program (5.0%) and the control (10.0%).

This work was supported by funding through the Maryland Grain Producer’s Utilization Board and in-kind support from The Mill. Special thanks to Clear Meadow Farm for their use of land and equipment making this research possible.

Preparing for Tar Spot of Corn in 2023

Andrew Kness, Senior Agriculture Agent | akness@umd.edu
University of Maryland Extension, Harford County

Figure 1. Signs and symptoms of tar spot on corn. Black raised areas are tar spot and long rectangular grey lesions are from grey leaf spot.

Tar spot is a foliar disease of corn caused by the fungus Phyllachora maydis and we confirmed it for the first time in Maryland from a grower field in Harford County in 2022; however, it is likely that it has been present in fields at low levels earlier than the 2022 growing season. Weather conditions across northern Maryland and Southern PA in August and September were favorable for tar spot development and pockets of disease outbreaks were reported, leading to much discussion about the disease amongst farmers and ag service providers over the 2022-2023 winter months about what to do to manage this disease in the future.

The pathogen that causes tar spot is favored by cool, wet weather. Tar spot spores overwinter in old corn crop residue and it seems to survive our winters just fine, as demonstrated by winter survival in Pennsylvania, as well as many northern corn belt states.

Temperatures between 60-70°F, coupled with 7+ hours of leaf wetness from dew, humidity, rain, or irrigation, trigger sporulation and subsequent spore germination on susceptible corn plants. Roughly 14-21 days later, signs and symptoms of tar spot will develop on corn plants in the form of small, raised black spots that have the appearance of tar or splattered black paint (Figure 1). These spots are the reproductive structures which provide secondary inoculum that repeatedly infect more tissue for as long as temperature and moisture conditions remain conducive.

In the Midwest where tar spot has been present since 2015, yield losses have been reported upwards of 60 bushels in bad years. It is also important to note that tar spot can make corn plants senesce and dry down much faster than normal, going from green to brown in 10-14 days under optimum conditions. This can make silage harvest tricky, which is why scouting is so important.

We do not know how prevalent and severe this disease will become in Maryland, so I encourage farmers to diligently scout corn fields to get ahead of it and also to determine where the disease is distributed. Scouting will also help you determine if a fungicide application is warranted. Fortunately, most fungicides that are labeled for corn do a fairly good job of protecting against tar spot, but there is data that suggests that the two and three way mode of action (MOA) products work better than single MOA products.

Fungicides should be applied as close to disease onset as possible; for tar spot this can be tricky because it can infect corn at any growth stage and it can still have significant yield impact as late into the season as R4. University research in the corn belt has found that the best chance for an economic return on investment is a single application around VT-R1; however, there are some instances where a second application was necessary but these were only when weather conditions for tar spot remained favorable during these later reproductive growth stages.

A few things to consider for tar spot management as we go into the 2023 growing season are as follows:

  1. Avoid highly susceptible hybrids, especially in corn-on-corn fields or if you have a field with history of tar spot. There is no complete resistance to tar spot in commercial corn hybrids, but we do know there is some variability in susceptibility. Work with your seed dealers to try to identify your best hybrids and plant them in these fields where you think tar spot may be a problem.
  2. Tillage and residue management appears to play a minor role in the management of this disease. Tillage may slightly reduce primary inoculum, but we need to keep in mind that tar spot spores can blow in from neighboring fields; so, I would not roll out the heavy tillage and blow up your no-till system just to try managing tar spot because it will only have a marginal effect.
  3. Corn-on-corn has a higher risk for developing tar spot, especially if the previous corn crop was infected. Rotate with other crops to break up this cycle. P. maydis only infects corn (including dent, sweet, and popcorn); all other crops are not hosts.
  4. Hybrid maturity also plays a role in disease severity. Research from the Midwest has shown that longer maturing hybrids suffer greater yield loss than shorter maturing hybrids. This is because the longer you push the grain fill period into the cooler late summer/fall months, the more likely tar spot will infect during earlier grain fill growth stages.
  5. Scout fields this year starting a little before tasseling through to maturity. As mentioned above, this will help you determine if a fungicide is likely to pay off or not.

We will be conducting some field trials this year looking at tar spot management in Maryland with funds from the Maryland Grain Producer’s Utilization Board. Part of this project will also include a survey of corn across the state to determine the distribution of tar spot. If you think you find tar spot in a field this year, I would be interested in knowing about it. You can call or email me (410-638-3255, akness@umd.edu), or report a sighting at corn.ipmpipe.org.

FHB RISK ASSESSMENT MARYLAND: 5/24/22

Nidhi Rawat, Small Grains Pathologist
University of Maryland

Flowering is finishing up across all of Maryland now, except for a few late planted fields in the northern part of MD that may still be flowering. FHB risk across the state continues to be high. So, if your wheat is currently flowering or has flowered within last 4-5 days, you can still spray FHB fungicides. I have started scouting the wheat variety trials for FHB symptoms on the Eastern shore and am already observing moderate FHB incidences in untreated plots there. The weather conditions have been conducive for FHB this season so far. If you have planted resistant varieties, the situation is expected to be better. Natural infections of leaf rust and stripe rust are also being seen sporadically. I do not expect any major losses due to these two now though. So, it’s okay not to spray any fungicide specifically for these diseases.

FHB RISK ASSESSMENT MARYLAND: 5/18/22

Nidhi Rawat, Small grains Pathologist
University of Maryland

Date 5/18/2022

With the recent showers and warming weather, FHB risk in the state is increasing. Wheat in the Eastern Shore of Maryland has finished or is just finishing flowering. If your wheat has just flowered last week, and you have not sprayed FHB fungicides you can still do so, as fungicide application 4-6 days after flowering is also known to provide some control. Wheat in the Northern parts is either flowering/ will soon start flowering. The FHB risk for this part of the state (Frederick, Carroll and Harford counties) is predicted to be moderate to high, especially if the wheat variety planted is susceptible. If the planted variety is resistant, the risk is not predicted to be particularly high. The recommended stage for application of fungicides on wheat is at flowering (50% of the main tillers showing yellow anthers) or within 4-5 days of that. The fungicides effective for FHB are Prosaro-pro/ Caramba/ Miravis-Ace. 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.

Corn Disease Update

Alyssa Koehler, Extension Field Crops Pathologist
University of Delaware

As tasseling in corn approaches, now is a good time to scout fields to decide if a fungicide will be applied. When considering the economics of a fungicide application, it is important to know your potential for disease based on field history, environmental conditions, and hybrid selection.

Many of the foliar pathogens of corn can survive in residue, so corn-on-corn fields carry a higher potential for disease, especially if disease has been observed in previous years. Hybrids with higher resistance ratings may not need a fungicide. Resistant hybrids typically have smaller lesions and reduced spread of spores. In dryland fields, hot, dry weather will keep disease pressure low. Reports of foliar diseases have been minimal so far this season. Irrigated fields keep enough moisture to favor environments for disease and may see development of Grey Leaf Spot (GLS) or possibly Northern Corn Leaf Blight (NCLB).

GLS is one of our most common diseases of corn and usually begins on lower leaves as small, tan, rectangular lesions with a yellow halo. When lesions are young, they can be difficult to distinguish from other common corn foliar diseases. As lesions mature, they become more diagnostic. At maturity, lesions are grey to tan in color, with a long rectangular shape (Figure 1); partially resistant hybrids can have more jagged margins than lesions on susceptible cultivars. Lesions often join to form large necrotic areas under favorable environmental conditions.

grey leaf spot lesions on corn leaf
Figure 1. Rectangular lesions of Grey Leaf Spot on corn.

Yield reductions are typically observed when lesions are present on the two leaves below the ear leaf or higher, so these are the leaves to pay close attention to when scouting. If over 50% of plants have lesions on 5% or more of this leaf surface, you may want to consider a fungicide application. If applying a fungicide, VT/R1 timing has shown the greatest chance of economic return.

Click here to see fungicide recommendations for corn

The Fungicide Conundrum When It’s Hot and Dry

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

As corn and soybeans begin to enter reproductive growth stages, foliar fungal diseases can have a serious impact on yield if growing conditions favor their development. However, with the hot and dry weather, disease pressure is low and will remain low if these weather patterns persist. Weather patterns in the Mid-Atlantic can switch rapidly and in general, fungicides need to be applied as a protectant, so spray decisions need to be made before the onset of disease. Coupled with the fact that many new fungicides on the market today are promoted to help plants cope with stresses, such as drought and heat, can complicate the decision.

I like to remind folks that fungicides are designed to do one thing—and that is to manage fungal diseases; they do this job very well. Hundreds of university trials have demonstrated that the most likely yield response and economic return occurs when fungicides are applied at the correct time and used when disease pressure is high. When disease pressure is low, yield responses sometimes occur, but are far more inconsistent than when disease is present.

There is also limited university data regarding fungicide utility to help plants manage drought stress. In the lab, strobilurin and triazole fungicides have been shown to regulate stomatal conductance and photosynthesis intensity in some corn hybrids, which improves the plants response to drought. However, when conducted at the field level, results are inconsistent and do not occur predictably. More research is needed to determine if fungicides can improve yields in drought conditions, and if so, when the application should be made. Existing studies indicate that the fungicide needs to be applied before drought stress occurs in order to stimulate the appropriate response in the plant; which again, makes spray decisions difficult without a crystal ball in hand to see into the future.

Another consideration, and perhaps the most important, is economics. While a fungicide application may provide some measurable differences in appearance and/or yield, is it enough to cover the application cost? Again, this is a tricky question to answer, but the body of research indicates that a single fungicide application in corn around VT-R1 and R1-R3 in soybean is the most likely to provide an economic return on investment when disease is present. In the absence of disease, the probability of an economic return is very low. Here are some additional factors that you should account for when considering a fungicide:

  • Crop history and tillage: Many fungal pathogens of corn and soybean are residue-borne. If growing corn-on-corn or soybeans after soybeans, those fields will be prone to higher disease risk. Conservation tillage fields are also at higher risk.
  • Irrigation: In dry years, fields that are overhead irrigated will be at higher risk of developing fungal diseases than dryland fields.
  • Disease progression: Scout your fields and see if and where fungal lesions are present on the plants. Many of the most important diseases of corn and soybean start low in the canopy and progress up the plant if environmental conditions are conducive (Fig. 1). For soybean it is critical to keep the top ⅓ of the plant clean of disease from flowering to pod fill; and for corn, it is critical to keep the ear leaf clean. If you notice fungal pathogens encroaching on these upper plant parts as they begin to flower and tassel, a fungicide application may be beneficial to protect yield.

    Grey leaf spot lesions on a corn leaf and froyeye leafspot lesions on a soybean leaf
    Figure 1. Two economically significant diseases of corn and soybeans: grey leaf spot lesions on corn (left) and frogeye leaf spot lesions on soybean (right).
  • Hybrid and variety resistance: Pay attention to your corn hybrid and soybean variety foliar disease ratings (resistance ratings are disease-specific); those that are more susceptible will have the greatest chance of an economic return on investment to a fungicide application than resistant varieties. Planting resistant varieties and hybrids is one of the most effective disease management tools.
  • Resistance management: It is generally considered bad practice to spray a fungicide when disease potential is low, as you are exposing pathogens to unnecessary chemistry, and with each exposure you drive the population towards resisting that chemical. This is why it is important to rotate modes of action and use full label rates, as cut rates can accelerate the development of resistance. The Take Action website (https://iwilltakeaction.com/) can help you choose products with differing modes of action.

The take-home message is to realize that you have the greatest chance for an economic return on your investment with a fungicide application when disease actually develops. Applying a fungicide to help plants cope with drought stress will likely not help to a degree that covers the cost of the application; as replicated field trials have yielded inconsistent and unpredictable results.

 

 

Web-Based Resistance Management Tools

Kurt Vollmer, Weed Management Specialist 
University of Maryland

If you attended the agronomy meetings this past winter, you heard me talk about the importance of using multiple strategies to mitigate herbicide resistance. Rotating and using multiple sites-of-action (SOA), is one strategy that helps prevent weeds and other pests from adapting to a single pesticide group. This can be challenging with so many products on the market. Take Action (https://iwilltakeaction.com) is a farmer-focused education platform designed to help farmers manage herbicide, fungicide and insect resistance. Several tools can be downloaded from this website to aid in your pest control decisions. Among these is an app (https://iwilltakeaction.com/app) that allows the user to

  • Quickly identify herbicide, fungicide or insecticide brands or active ingredient SOA numbers,
  • see a list of other SOA numbers to help diversify his or her weed control program,
  • and search the herbicide, fungicide or insecticide last used to prevent the use of similar SOAs

In addition, the GROW IWM website (https://growiwm.org) provides excellent information on how to use integrated weed management practices for herbicide resistant weeds.

Considering an Insecticide For Your Small Grain?

Alan Leslie1, Agriculture Agent; Kelly Hamby2, Extension Specialist; and Galen Dively, Professor Emeritus2
1University of Maryland Extension, Charles County
2University of Maryland, Department of Entomology

This time of year, anyone growing small grains will be planning to apply fungicides to manage Fusarium head blight, and many will consider tank-mixing an insecticide to control any insect pest problems at the same time. These tank mixes are an appealing option to reduce the time, fuel, and damage to the crop from having to make a second pass over the field later on in the season. In addition, with many synthetic pyrethroids now available as cheaper generic versions, the costs associated with adding an insecticide to the tank may seem like cheap insurance against possible pest outbreaks. However, to ensure that this added investment gives you a return with increased yields, you should still follow an integrated pest management approach and base the decision to add an insecticide on scouting and documentation of an existing pest problem. Below, we outline several possible insect pests that could be controlled with an insecticide applied with fungicides over small grains, and summarize situations where that application may be warranted, and when it may not.

Aphids. Aphid populations need to be controlled in the fall to reduce Barley Yellow Dwarf Virus incidence in small grains. Spring insecticide applications will not reduce incidence of the disease. Only a few aphid species tend to feed on grain heads, and can reduce yield from head emergence through milk stage (Fig. 1). After the soft dough stage, no economic losses occur. Aphid populations are generally kept in check by insect predators and parasitoids, and thresholds for chemical control of aphids in the spring require at least 25 aphids per grain head (with 90% of heads infested) or 50 per head (50% heads infested) and low numbers of natural enemies. Applying a broad spectrum insecticide when aphid pressure is not above threshold tends to kill off beneficial predatory and parasitic species, which can allow aphid populations to flare up, as they are no longer being suppressed by their natural enemies.

aphids on wheat head
Figure 1. Aphids feeding on wheat head.

True armyworm and grass sawfly. Both true armyworm (Fig. 2) and grass sawfly (Fig. 3) are sporadic pests of small grains and their pest pressure and feeding damage can vary widely from year to year. Automatically applying an insecticide to target these pests is not likely to be a cost-effective strategy since they are not pests that reliably cause economic injury. When these pests are present in high numbers, they are capable of causing significant yield loss through their behavior of clipping grain heads. Scouting should be done to check for the presence of these two pests and insecticide treatment is only needed if they exceed threshold values of one larva per linear foot for armyworm and 0.4 larvae per linear foot for grass sawflies.

armyworm and sawfly larave
Figure 2. True armyworm larva (top). Figure 3. Grass sawfly larva (bottom).

Hessian fly. Cultural methods are the best way to control Hessian fly in small grain, such as planting after the fly-free date, selecting resistant varieties, and using crop rotation to disrupt their population growth. Spring feeding by the fly larvae can cause stems to break, reducing yields. There are no effective rescue treatments for Hessian fly; insecticides targeting fly larvae are ineffective since they are well protected from sprays by feeding inside of the leaf sheath (Fig. 4). If this year’s crop is damaged, it is imperative that fly-resistant varieties are planted after the fly-free date next year.

Hessian fly larvae feeding inside wheat stem
Figure 4. Hessian fly larvae feeding inside of wheat leaf sheath.

           Cereal leaf beetle. This species is widespread in Maryland and is typically present in small grains, though it only occasionally reaches levels that injure crops. Cereal leaf beetle larvae chew the upper surfaces of leaves, leaving them skeletonized (Fig. 5). Larvae can cause yield loss if the flag leaf is severely skeletonized before grain-fill is completed. Insecticides with good residual activity tank mixed and applied with fungicides can potentially control populations of cereal leaf beetles, protect the flag leaf, and improve the yield of the crop if beetle pressure is high. However, predicting whether populations will reach damaging levels is not straightforward, and scouting should be used to guide spray decisions. If a field has 25 or more larvae plus eggs per 100 tillers, and there are more larvae than eggs, then chemical control is needed. In Maryland, a parasitoid wasp species (Anaphes flavipes) may parasitize 70-98% of cereal leaf beetle eggs, so if a field is dominated by eggs with few larvae, insecticide may not be needed. Additionally, feeding by cereal leaf beetle will not cause economic damage after the hard dough stage. So far, we have received no reports of economic levels of cereal leaf beetle in the region.

Cereal leaf beetle feeding on leaf
Figure 5. Cereal leaf beetle larva and feeding damage.

In conclusion, tank mixing an insecticide with your fungicide application can pay off if you have economically damaging levels of an insect pest, but applying any insecticide without a pest problem will not pay off. If populations are present, seem to be increasing, and you will not be harvesting soon, you could gamble. The risks of that gamble include losing money on an unnecessary input cost, secondary pest outbreaks if natural enemy populations are wiped out, or the target pest outbreaks anyway because the application was poorly timed. Scouting fields regularly to document pest pressure and using IPM thresholds as a guide for using chemical controls is the best way to hedge your bets when deciding whether to add an insecticide to the tank this spring.

For more information on tank-mixing insecticides with small grain fungicide applications, check out current research updates from Dr. Dominic Reisig at North Carolina State University: https://smallgrains.ces.ncsu.edu/2019/03/aphids-in-wheat/

https://entomology.ces.ncsu.edu/2015/04/should-you-spray-cereal-leaf-beetle/

And Dr. David Owens at the University of Delaware:

https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/cereal-leaf-beetle/

 

 

Managing Fusarium Head Blight

Dr. Alyssa Koehler, Extension Field Crops Pathologist
University of Delaware

With the mild winter, wheat and barley are moving right along. Planting behind corn is common in our region, but this maintains inoculum for Fusarium Head Blight (FHB). Fusarium species that cause FHB can infect both corn and small grains. Walking through fields with corn stubble, you may see orange growth on old debris (Figure 1). Wet spring conditions favor fungal sporulation that can lead to infected wheat heads. As the pathogen grows on debris, spores are released that can be rain dispersed or moved through air currents. As the grain is flowering, spores land on the head or anthers, colonize these tissues, and move into the grain head. Once inside the grain, water and nutrient movement is disrupted, which results in the bleached florets we associate with FHB (Figure 2). Shriveled and wilted “tombstone” kernels can reduce yield and result in grain contaminated with mycotoxins. Deoxynivalenol (DON), also referred to as vomitoxin, is a health hazard to humans and animals. Wheat heads colonized later in development may not show dramatic symptoms, but can still have elevated DON.

Figure 1 (left). Corn stubble with Fusarium sporulation that can contribute to FHB in wheat. Figure 2 (right). Wheat head showing bleached florets from Fusarium Head Blight.

As we approach heading and begin to think about in-season disease management strategies, a well-timed fungicide application can help to reduce disease severity and DON levels. It is important to remember that fungicides can help to reduce disease levels and DON (traditionally around 50% reduction on a susceptible variety), but they do not eliminate FHB or DON. To try to maximize the efficacy of fungicides, it is important to apply at the correct timing. Fungicides for FHB are most effective when applied during flowering in wheat and at head emergence in barley. The Fusarium Risk Assessment Tool (www.wheatscab.psu.edu) is a forecasting model that uses current and predicted weather forecasts to predict FHB risk. The model is currently being configured for this season and should be accessible at the link above by the end of the first week of April. Historically about 70% accurate, this tool aids in assessing FHB risk as wheat approaches flowering and fungicide application decisions are made. The pathogen that causes FHB infects through the flower and rainfall 7 to 10 days prior to flower favors spore production and increases risk of infection. Optimal wheat fungicide application is at early flowering (10.5.1) to about 5 days after. Although new products like Miravis Ace can be applied earlier, it is still best to wait for main tillers to be at 10.5.1 or a few days beyond so that secondary tillers have a greater chance of being at 10.3-10.5.1. If you spray too early, heads that have not emerged will not be protected by the fungicide application. When wheat heads begin to flower, look for yellow anthers in the middle of the wheat head. When at least 50% of main stems are flowering, you will want to initiate fungicide applications. As the flowering period continues, anthers will emerge from the top and then the bottom of the wheat heads. Anthers can stay attached after flowering but usually become a pale white (Figure 3, next page). Triazole (FRAC group 3) fungicides that are effective on FHB include Caramba (metconazole), Proline (prothioconazole), and Prosaro (prothioconazole + tebuconazole). Miravis Ace (propiconazole + pydiflumetofen) offers a triazole + SDHI, FRAC group 7. As a reminder, fungicides containing strobilurins (QoI’s, FRAC 11) should not be used past heading because these fungicides can result in elevated levels of DON. Flat fan nozzles pointed 90° down are great at covering foliage but they do not provide good coverage on heads, which is the target for FHB management. Nozzles that are angled forward 30-45° down from horizontal (30 degrees is better than 45) or dual nozzles angled both forward and backward give better contact with the head and increase fungicide efficacy. For ground sprays, fungicides should be applied in at least 10 gallons of water per acre.

Figure 3. From left to right: Feekes 10.3, Anthesis; Feekes 10.5.1 (yellow anthers beginning flowering); 4 days after anthesis (white anthers post flowering). Image: A. Koehler, Univ. of Delaware.

Thinking beyond this season, an integrated approach can improve management of FHB and help to keep DON levels low. In your field rotation plan, avoiding planting small grains into corn residue will help to reduce the amount of initial inoculum in your field. If you have soybean fields that can be harvested early enough for a timely wheat planting, this rotation helps to break up Fusarium inoculum. In addition to rotation considerations, seed selection is another important piece of FHB management in wheat. There is no complete host resistance against FHB, but you can select wheat varieties with partial resistance. The University of Maryland sets up a misted nursery to compare FHB index and DON levels across local wheat varieties to aid in variety selection decisions. Results from 2019 can be found at https://scabusa.org/pdfs/UMD_Misted-Nursery_Factsheet-2019.pdf. Remember that these trials are conducted under extreme disease pressure and you want to look at relative DON performance. Unfortunately, barley does not have any resistance to FHB. In UMD’s 2019 trial, Calypso had the lowest DON content in local barley varieties tested.

 

At-planting treatments for controlling early-season insect pests in corn

Maria Cramer, Edwin Afful, Galen Dively, and Kelly Hamby
Department of Entomology, University of Maryland

Slug feeding damage: characteristic long, thin holes made by a rasping mouthpart.

Background: Multiple insecticide options are available for early-season corn pest management, including neonicotinoid seed treatments (NSTs) and in-furrow pyrethroids such as Capture LFR®. In addition, many Bt corn hybrids provide protection against seedling foliar pests such as cutworm and armyworm. Given that almost all corn seed is treated with neonicotinoid seed treatments (NSTs), Capture LFR® may not provide any additional protection.

Methods: In this study we compared four treatments: fungicide seed treatments alone; Capture LFR® (active ingredient: bifenthrin) applied in the planting furrow with the fungicide seed treatment; Cruiser Maxx® 250, an NST (active ingredient: thiamethoxam), which includes a fungicide; and Capture LFR® + Cruiser Maxx® 250 together. We evaluated the amount of soil and foliar pest damage after emergence. Yield was measured at harvest.

Preliminary results: Our results suggest that when wireworm pressure is high, Capture LFR® and Cruiser Maxx® 250 protect against damage and significantly increase yields. Neither treatment is superior, so we recommend using only one, and only in fields where pest pressure is known to be high. As most corn seed already contains NSTs, use of Capture LFR® at planting is unlikely to be warranted.

Sampling for soil and foliar pests

Background: Capture LFR®, an in-furrow pyrethroid product, is marketed for control of early-season corn pests, including soil pests such as white grub and wireworm and above-ground pests such as cutworm and armyworm. However, the insect pest management systems already adopted in corn may provide sufficient protection. Most corn seeds are treated with NSTs, which provide seedlings with systemic protection from many soil and above-ground pests. Additionally, most Bt corn hybrids express proteins with efficacy against cutworm and armyworm in the seedling stage, although they do not affect soil pests. Unlike NSTs and Bt traits, pyrethroids are not systemic and do not provide protection beyond the soil area to which they are applied.

While in-furrow applications of bifenthrin (the active ingredient in Capture LFR®) can effectively reduce wireworm damage in potatoes1 and provides white grub control in field corn2,3, it does not consistently increase yield in corn3 or soybeans4. Yield benefits are likely to be seen only where there is known soil pest pressure. Meanwhile, preventative applications of pyrethroids have been linked to declines in natural enemies 5,6, including carabid beetles, which are important predators of slugs.

Objectives: Our objectives were to determine whether in-furrow applications of Capture LFR® (bifenthrin) provided 1) protection against soil pests, 2) protection against seedling pests, and 3) yield benefits compared with fungicide alone, Cruiser Maxx® 250, or combined with Cruiser Maxx® 250.

Methods: This study was conducted in 2018 and 2019 at the University of Maryland research farm in Beltsville, MD. We planted 4 replicate plots of a standard Bt field corn hybrid, TA 758-22DP (VT Double Pro insect control) in 2018 and LC1488 VT2P (SmartStax RIB complete insect control) in 2019 at 29,999 seeds per acre. Plots were planted late in 2018 (June 18) but on time in 2019 (May 20). Standard agronomic growing practices for the region were used. We compared the following four treatments, applied at planting:

  No in-furrow application In-furrow Capture LFR®

Applied at 13.6 fl oz/ac

Fungicide seed treatment Fungicide (F) seed treatment alone

2018: Maxim Quattro®

2019: Vibrance Cinco®

Fungicide +

Capture LFR® (F + Cap)

 

Cruiser Maxx® 250 Cruiser Maxx® 250

(Cru)

Cruiser Maxx® 250 + Capture LFR® (Cru +Cap)

We sampled plants 24 days after planting in 2018, and 18 days after planting in 2019. In 2018, we recorded the number of stunted plants (indicating potential soil pest damage), and in 2019, we dug up stunted plants and recorded those for which soil pest damage could be confirmed. In both years, we assessed rates of above-ground feeding by pests such as cutworm and armyworm.

Wireworm (left) and characteristic above-ground symptoms of wireworm feeding (right). Note wilted center leaf.Results: Soil Pests. In 2018 there was no difference in the percent stunted plants between treatments (Figure 1), with less than 5% stunting in all treatments. This low level of pest damage may have been due to the late planting date, which could have avoided peak soil pest pressure. In 2019, all of the insecticide treatments had significantly lower soil pest damage than the fungicide control (Figure 1). Combining Capture LFR® with Cruiser Maxx® 250 was not more effective than Cruiser Maxx® 250 alone, but was more effective than Capture LFR® alone, suggesting that treatments involving Cruiser Maxx® 250 are somewhat more effective against the soil pests at this farm. In both years, plots were located in a field with a history of wireworms; however, damage was only observed in 2019. In a field without pest pressure, such as we saw in 2018, these treatments did not improve plant stand.

Foliar pests. In both 2018 and 2019, rates of foliar damage were extremely low (below 5% of plants) in all treatments and there were no differences between treatments.

Yield. In 2018, there were no yield differences between the treatments (Figure 2). Overall, we had low yields in 2018, likely a result of the late planting date. In 2019, all of the insecticide treatments had significantly higher yields than the fungicide control, with no differences between any of the insecticide treatments (Figure 2). Combining Capture LFR® with Cruiser Maxx® 250 did not increase yield.

Figure 1. 2018 and 2019 soil pest pressure, Beltsville, MD. Mean percent plants damaged for four treatments: F=Fungicide, F+Cap= Fungicide + Capture LFR®, Cru=Cruiser Maxx® 250, Cru+Cap= Cruiser Maxx® 250 + Capture LFR®. In 2018, treatments did not impact stunted plants (N.S.) In 2019, all insecticide treatments significantly reduced soil pest damage (columns with different letters have significantly different mean damage).
Figure 2. 2018 and 2019 yields, Beltsville, MD. Mean yield for four treatments: F=Fungicide, F+Cap= Fungicide + Capture LFR®, Cru=Cruiser Maxx® 250, Cru+Cap= Cruiser Maxx® 250 + Capture LFR®. Yields were not significantly different in 2018 (N.S). In 2019, all insecticide treatments had significantly higher yield than the fungicide only treatment (columns with different letters have significantly different mean yield).

Conclusions: In 2018 and 2019 we did not see sufficient foliar pest pressure to justify an insecticide application. This may be due to effective control by Bt proteins in the corn hybrids and/or low foliar pest pressure.

In a field with established wireworm pressure, all three insecticide treatments reduced soil pest damage and improved yield relative to a fungicide only control in the 2019 field season. While there were differences in pest damage levels between the different insecticide treatments, no one treatment provided superior yield benefits. Because nearly all corn seed is treated with NSTs like Cruiser Maxx® 250, additional applications of Capture LFR® may not be necessary. Preventative applications increase costs and present risks to beneficial insects without providing yield benefits. Additionally, soil pest pressure tends to be low throughout Maryland. We sampled untreated corn at five locations across Maryland in 2019 and found on average less than 3% soil pest damage. Unless a field has a known history of wireworms or white grubs, we do not recommend using at-planting insecticides.

Acknowledgements and Funding. This project was funded in both years by the Maryland Grain Producers Utilization Board. We appreciate the help provided by Rachel Sanford, Madison Tewey, Eric Crandell, Gabriel Aborisade, and Kevin Conover.

Sources

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