Andrew Kness, Senior Agriculture Agent | akness@umd.edu University of Maryland Extension, Harford County
Figure 1. Regional map of tar spot of corn for the 2023 growing season. Map generated from ipmpipe.org/tarspot.
Tar spot of corn has been confirmed in Maryland for the 2023 growing season. The first report came from a field in Cecil county on August 22, followed by several additional reports in Harford and Carroll county (Figure 1).
Tar spot is a relatively new fugal disease of corn in the United States and it was confirmed for the first time in Maryland in August of 2022. As daytime and nighttime temperatures begin to decline, now is a good time to look for symptoms in your corn fields. Tar spot is favored by cooler temperatures (60-70s), as well as prolonged periods (7+ hours) of leaf wetness from rainfall, dew, or humidity. Tar spot can cause infected plants to senesce prematurely, which can adversely affect yield, especially if infection occurs early in the reproductive stages. Yield losses are not as severe if infection occurs later in the reproductive stages.
Tar spot spores overwinter in old corn crop residue and are deposited onto corn leaves via splashing rain or wind (spores are only wind-blown for very short distances). Once a spore lands on corn tissue, it will germinate and infect the plant as long as the environmental conditions remain conducive. After an incubation period of about 14-21 days, black reproductive structures, called stroma, are visible on the leaf surface (Figure 2). These structures resemble black paint or tar, hence the name “tar spot.”
Figure 2. Symptoms of tar spot on corn leaf. Black specks are the reproductive structures of the fungus.
If you find tar spot in your field, you may want to take precautions to try to prevent its spread during harvest, as you could potentially inoculate new fields by bringing infected residue into the next harvested field. If you’re harvesting an infested field, it would be a good idea to try to remove as much corn fodder off of the equipment before moving to the next field.
Preparations for managing tar spot in 2024 should start in the winter with good seed selection. If possible, choose hybrids with good tar spot resistance (there is no complete resistance); seed companies are starting to rate hybrids for their tar spot resistance/tolerance.
If you find tar spot in your field, I would be interested in knowing about it. We have a grant from the Maryland Grain Producer’s Utilization Board and one objective of the study is to determine the distribution of tar spot in Maryland. Call (410-638-3255) or email me (akness@umd.edu); or submit a report at corn.ipmpipe.org.
Results from the University of Maryland Small Grain Variety Trials are in the tables below. To download a pdf copy of the report, click here.
University of Maryland also rates the relative susceptibility of select varieties of wheat and barley to Fusarium head blight. The results from this trial can be found here.
For questions regarding the small grain trials, contact Dr. Vijay Tiwari (vktiwari@umd.edu) or Dr. Nidhi Rawat (nidhirwt@umd.edu).
If you’d like to learn more about how to interpret variety trial data, view our fact sheet by clicking here.
Alyssa Koehler, Extension Field Crop Pathologist | akoehler@udel.edu University of Delaware
Seedling diseases are common across many crops, including soybeans. The most common seedling pathogens Pythium spp., Phytophthora sojae, Fusarium spp., and Rhizoctonia solani are often naturally present in the field, but certain environments, like compacted, wet, or poorly-drained soils and cool weather that slows emergence can favor disease development. Infected seedlings may fail to emerge before coming out of the ground (pre-emergence damping off) or die after coming out of the ground (post-emergence damping off), surviving plants may appear stunted with other symptoms visible on the seedling.
Pythium spp. can cause pre- and post-emergence damping off. Development of Pythium is favored by wet soils and we have species in the area that favor both cool and warm weather. Tissue will be brown in color and appear soft and rotting. This can look very similar to Phytophthora in seedlings, but can be confirmed by submitting a sample to the diagnostic lab. Surviving plants may be stunted and have less vigor.
Figure 1. Soybean seedling with post-emergence damping off due to infection by Pythium.
Phytophthora is also able to cause pre-and post-emergence damping off. Tissue will be soft and tan-brown in color. Stems may look bruised and with rotten roots, plants will usually wilt and die. It is also possible to see Phytophthora symptoms develop mid to end of season. Later in the season, dark brown discoloration of the stem will extend from just below the soil line up into the plant (Figure 2).
Figure 2. Soybean plant with root rot and dark brown discoloration moving up the plant .
Rhizoctonia can cause damage pre- or post-emergence. Often rust-brown lesions will be present on the roots or lower stems. In some cases this may girdle the stem stunting or killing the plant.
Fusarium spp. can infect the seed or seedling. There will often be brown lesions on the roots and the root system may appear shrunken. Numerous Fusarium species can be associated with root rot of soybean. Another soybean disease, Soybean Sudden Death (SDS), is caused by Fusarium virguliforme. While F. virguliforme will infect early in the season, symptom of SDS do not become present until the plant reaches reproduction stages.
Seed and seedling diseases can be difficult to manage. Waiting for fields to be relatively dry and delaying planting until soils are warmer than 55°F can favor quicker emergence and growth, which can be beneficial. Genetic resistance is available for managing Phytophthora. Fungicide seed treatments may reduce seed and seedling diseases. In our work over the past three seasons, we have observed faster emergence and improved stand when using seed treatments in April planted soybeans. Although collectively called fungicides, not all fungicide products are effective against all pathogens. Pythium and Phytophthora are fungal-like organisms called oomycetes. Active ingredients, mefenoxam and metalaxyl, have activity on oomycetes, while strobilurins (azoxystrobin, trifloxystrobin, pyraclostrobin, etc.) have activity on Fusarium and Rhizoctonia. For this reason, seed treatments typically have two or more active ingredients. Each year the Crop Protection Network releases a “Fungicide Efficacy for Control of Soybean Seedling Diseases” publication to aid in decision making for seed treatments. The full publication can be found at https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-soybean-seedling-diseases.
The Crop Protection Network (cropprotectionnetwork.com) is a national working group comprised of Extension agents and specialists from across North America that provide data through publications regarding pest management in agronomic crops. The website and publications can be a great resource to your operation. The following are the most recent fungicide efficacy tables for foliar diseases of corn and soybean.
Andrew Kness, Agriculture Agent | akness@umd.edu University of Maryland Extension, Harford County
Figure 1. Powdery mildew growth on lower leaves/stems of a wheat plant.
Spring is a busy time on the farm that demands a lot of different tasks; one of the tasks that can be overlooked is scouting your small grains now for the presence of diseases, especially this year considering many areas of the state are off to a wet start. For a few select diseases, scouting now could pay off later in the season.
Most of the wheat in Maryland is somewhere between Feekes 5 (green up) and Feekes 6 (jointing), and most has received its first shot of nitrogen. This is the perfect time to scout your fields for foliar diseases like powdery mildew and the leaf blotch complex diseases.
Powdery mildew (Figure 1) is one of the more common diseases of wheat in our region, although it is typically a non-issue unless it gets out of hand. The fungus that causes powdery typically colonizes wheat in the fall when the plants are small, then goes dormant inside the plant over the winter. Green up is the perfect time to scout for powdery mildew because you will be able to see the powdery white tufts of fungus growing on your wheat plants (Figure 1). These signs are typically observed close to the crown deeper in the canopy and/or on plants that are in double-planted pinch rows or headlands. The disease is often more severe on over-fertilized fields, too. It is good to scout for the presence of powdery mildew now, but do not treat until the wheat begins to joint. Powdery mildew will not begin actively growing until the wheat plant comes out of its winter slumber and begins rapid growth (jointing, Feekes 6), and fungicides have little-to-no activity on dormant fungi. Instead, hold off on a fungicide application until your second nitrogen application at jointing. If you only notice a few sporadic instances of powdery mildew now, you may not even need to treat at jointing, especially if weather conditions become warm and dry. If you decide to hold of on a Feekes 6 application, you’ll still want to keep an eye on it, especially as the flag leaf emerges. The top three leaves, especially the flag leaf, contributes to nearly all of your yield.
Also, know your wheat variety because there are significant differences in resistance and tolerance. Data from University of Maryland, University of Delaware, and Virginia Tech can help you determine your wheat’s susceptibility. In addition, varieties containing the pm6 resistance gene have broken down and are no longer providing adequate resistance here in the mid-Atlantic region, so consider those varieties susceptible.
Similar management should be taken against the leaf blotch complex diseases. If you find them now, hold off on a fungicide application (if at all) until at least Feekes 6. And again, protecting the flag leaf is your main priority, so if the disease is slow to progress due to inadequate weather conditions or host tolerance and the disease is present at low levels, then a fungicide application at Feekes 6 may not be warranted at all. Also, the fungicides we commonly use to manage Fusarium head blight/head scab will also work on powdery mildew and the leaf blotch complex; so our fungicide applications at early flowering typically do a good job at keeping these diseases from progressing to the flag leaf.
For help with identifying diseases on wheat, you can send samples to the University of Maryland Plant Diagnostic Lab, or call your local Extension agent.
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.
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.
Andrew Kness, Agriculture Agent University of Maryland Extension, Harford County
As we get into the swing of the 2020 growing season, it can be helpful to have access to a quick reference for fungicide recommendations for if/when diseases become a problem on your farm. As you are aware, there are several products available for disease management and it can be difficult and confusing to select the appropriate product. Also remember that just because a pesticide is labeled for use on a particular crop to manage a specific pest, does not necessarily mean or guarantee that the pesticide will work to manage it. Pest populations are constantly evolving and therefore develop resistance to products over time. A good example of this is the fungicide, propiconazole; once very effective for managing head scab of wheat, is now ineffective against the pathogen.
To help aid your fungicide selections, the Crop Protection Network has some great resources on fungicide efficacy that they update each year (and soon to come, insecticides and herbicides). The Crop Protection Network is a multi-state and international partnership of university and provincial Extension specialists and public and private professionals that provides unbiased, research-based information.
These publications list the relative fungicide efficacy for the major diseases of corn, soybeans, and small grains and are linked below. If you have trouble accessing or interpreting the information, feel free to contact me.
Nidhi Rawat, Small Grains Pathologist University of Maryland, College Park
With low temperatures and intermittent showers, this wheat season has been unusual in Maryland. The cool wet season was conducive to the growth of several foliar fungal pathogens. At the same time, several physiological issues caused by the unusual weather conditions manifested themselves on leaves of the plants. The conditions were not too supportive for extensive symptom development for Fusarium Head Blight so far, although DON contents should be analyzed before declaring it a FHB-free year. The major diseases/ physiological disorders observed in the state this year so far are described below to help in identification of the underlying factors.
Leaf Diseases
Tan Spot
The fungal pathogen, Pyrenophora tritici-repentis, produces host-selective toxins that causes yellowing and death of leaf tissue. Symptoms of tan spot early in the season include diamond-shaped or oval spots with a yellow halo mostly on the lower leaves. These spots turn brown and progressively enlarge and coalesce damaging large areas of leaf tissue. These tan colored spots have a small dark brown center resembling an “eye-spot” that is particularly distinctive when holding the leaf up to the sunlight (Fig 1). Tan spot may develop any time during the growing season, but well-developed lesions are frequently observed later in the season. After the season is over, the fungus overwinters in straw and stubble as black fruiting bodies called pseudothecia.
Figure 1. Typical symptoms of tan spot on a wheat leaf (A) and wheat plants (B). Images: N. Rawat, University of Maryland.
Septoria/Stagonospora nodorum Blotch (SNB)
The causal agent of SNB is the fungus Parastagonospora nodorum. Symptoms usually appear after head emergence and are mostly found on the upper leaves, although all leaves are susceptible. SNB starts as small, water soaked dark brown flecks, which later expand to make larger irregular dark brown lesions (Fig 2). As the disease progresses, the lesions develop an ash gray-brown center containing brown-black pepper grains (pycnidia), which are sometimes difficult to see in brown lesions. Glume blotch, which refers to the SNB on glumes of wheat heads, is caused by the same pathogen (Fig 2). Pycnidia, which are brownish-black pepper grain-like dots, are the asexual reproductive structure of the fungus and are diagnostic of this disease.
Figure 2. Stagonospora nodorum blotch on a leaf (A) and glume blotch (B).
Septoria tritici blotch (STB)
STB is caused by fungus Zymoseptoria tritici (previously named Septoria tritici). Symptoms of STB are usually detected on lower leaves in the fall and early spring as yellowish or chlorotic flecks on leaves. Symptoms start as yellowish flecks, especially on the lower leaves, which are in contact with the soil. These flecks enlarge and develop into brown to reddish brown lesions, which are initially restricted to the leaf veins, giving the appearance of parallel sides. More importantly, lesions are associated with the presence of visible pycnidia that are sphere or ball-shaped, gelatinous and gray-brown. These small black pycnidia in lesions are the most reliable in-field characteristic for identifying SNB. Differentiating STB from SNB can be difficult and often requires microscopic observations.
Figure 3. Septoria Tritici Blotch on a wheat leaf (A). Notice the brown lesions restricted by the leaf veins (A). Black dots (pynidia) in the lesions in the enlarged section (B). Images: N. Rawat, University of Maryland.
Barley Yellow Dwarf (BYDV)
BYDV is a viral disease caused by a Luteovirus transmitted by aphids. The typical symptom of BYDV, as the name suggests, is yellowing of leaves that starts from the tip and progresses towards the base (Fig 4). The yellow color changes to purple and red as disease develops. Severe infection leads to stunting of plants. Oftentimes the flag leaves of infected plant stand out prominently as a reddish flags. The symptoms of BYDV may often be confused with nutrient deficiency or leaf streak mosaic virus. The disease occurs in patches in the field indicating the area of infestation by aphids, as compared to mineral deficiency, which is present more or less uniformly in the field. Aphid vectors may be visible on the plants infected with BYDV. Unlike leaf streak mosaic virus, BYDV does not have a distinct mosaic pattern on the leaves. Definitive confirmation of BYDV can be done by lab tests.
Figure 4. Symptoms of Barley Yellow Dwarf Virus on wheat leaves (A). Notice the pattern of disease distribution (B). Images: N. Rawat, University of Maryland.
Physiological Disorders
Leaf Tip Necrosis
Leaf tip necrosis (LTN) is a physiological disorder and can be caused by several factors namely: hot dry air, contact fungicides, foliar fertilizer burn, soil salinity, cool evening temperatures. Even some genetic factors such as leaf rust resistance gene Lr34 or stripe rust resistance gene Yr18 are known to cause leaf tip necrosis. LTN normally progresses from the margins of the flag leaf tip and lower leaves should show some signs of it as well (Fig 5). The severity of the leaf tip necrosis is dependent on both the variety and the growing conditions during flag leaf emergence and early grain-fill and thus some varieties have a tendency to show a lot more leaf tip necrosis than others.
Figure 5. Leaf tip necrosis on a leaf (A) and in a field (B). Picture Credit: B. Beale, University of Maryland.
Physiological Leaf Spotting (PLS)
Physiological leaf spotting (PLS) is another issue that we observed this season. There are majorly two factors/ triggers that may cause PLS, namely: chloride deficiency and tissue oxidation due to sun damage. PLS is bit of a black box when it comes to diagnosing or predicting when it will appear. Chloride is an essential micronutrient for plants, and is generally not tested for in tissues or soils. In case of sun damage, spotting occurs after a period of prolonged cloudy weather that is then followed by sunny weather. Such symptoms appear first, and more uniformly, on upper leaves exposed to direct sunlight and little is seen further down in the canopy; but in severe cases lower canopy leaves can express symptoms as well. PLS may be mistaken for spotting caused by the previously described diseases. However, the pattern of distribution of symptoms is more uniform and spread over to a larger geographical area than the diseases. Unfortunately, there is no ‘test’ that can be done to confirm this type of PLS; only the elimination of all other possible causes.
Figure 6. Physiological flecking on wheat leaves.
Management of Wheat Foliar Fungal Diseases
It is important to diagnose the correct underlying disease/physiological factor of a disorder in order to make productive management decisions. Following are some of the management strategies that are helpful in the cases of the diseases described above.
Disease prevention by growing resistant varieties is the most economical method of control of fungal foliar diseases. Varieties with a good resistance level may be available for some of these diseases, for others a moderate to low level of resistance may be available.
Good-quality and pathogen-free seed should be used for planting. Planting seed infested with these fungal leaf spot pathogens can result in reduced germination and poor seedling vigor.
Seed treatment fungicides can reduce the risk of seedling infections.
Crop rotations can reduce the initial inoculum load of fungal leaf spots.
Management of volunteer weeds that serve as green-bridge for these diseases from one year to the next is helpful.
Several fungicides are labeled for the management of tan spot and the Septoria complex. Early season fungicide application is not recommended in the absence of disease or in an unfavorable environment. A good source of information is the fungicide efficacy table organized by the North Central Regional Committee on Management of Small Grain Diseases (NCERA-184) and can be found on the UMD Plant Sciences/Extension website.
Andrew Kness, Agriculture Agent University of Maryland Extension, Harford County
Given current commodity prices, growers may be considering increased corn acreage. Continuous corn presents additional risks in terms of disease that you need to proactively manage. Here are some points to consider so that you can try to stack the deck in your favor in case conditions become favorable for pathogens that threaten corn yields.
Here in our region of the world, we typically have plenty of humidity and moisture that favor disease development in our crops. Furthermore, our most common and potentially most severe yield-limiting diseases, such as grey leaf spot (GLS), northern corn leaf blight (NCLB) (Figure 1), as well as stalk rots, are residue-borne and overwinter in corn stover. The following are points to consider for disease management in a corn-on-corn system:
Variety Selection: You can do yourself a big favor right off the bat by selecting hybrids that have good stalk integrity ratings and resistance to NCLB and GLS. Genetic resistance is one of the most cost-effective ways to manage disease. If you’re planting corn into corn, especially in a reduced or no-till situation, place hybrids with good NCLB and GLS resistance in these fields and save your more susceptible hybrids for fields that have lower disease potential (for example, after soybeans).
Residue Management: Several pathogens of corn survive and overwinter in corn residue; therefore, residue is the primary source of infection. Not surprisingly, more corn residue present on the soil surface means that the following corn crop is at a higher risk of developing these diseases. While no-till and reduced-till systems afford us many benefits in crop production, harboring pathogen inoculum is one of the drawbacks. Chopping and sizing corn residue in the fall into smaller pieces can help accelerate its decomposition and may reduce inoculum (spores) slightly at best. In order to reduce inoculum significantly, more aggressive tillage is necessary to bury the residue. If you’re trying to build soil health and utilize no-till or reduced tillage, this may not be an option and you will need to make sure you are doing a good job in all other areas of disease management.
Planting: Getting planting equipment and planting conditions correct is important, especially for seedling and root-rotting diseases of corn. Ensure you’re achieving proper planting depth, as the longer the seed sits in the ground, the more prone it will be to rot and dampen off. Soil temperature is important for getting the seedings off to a quick start, so it is advisable to wait until soil temperatures are at least 50 degrees and rising to plant; this is especially important for corn-on-corn. Fields with reduced tillage, cover crops, and substantial cover will warm up slower than tilled fields or fields with less cover (i.e. last year’s soybean fields). You may consider planting corn in last year’s bean fields first, then your corn after corn fields to ensure soils are warm enough for rapid and uniform emergence. Additionally, do not push plant populations too high, as excessive plant populations can cause dense canopy humidity which favors disease development and can stress plants if nutrients and/or water become limiting, which will predispose plants to pathogen infection.
Seed Treatments: Nearly all commercial corn hybrids come pre-treated with seed treatments, which typically contain a fungicide. These fungicides will provide some protection from many seed and root-rotting pathogens for about two weeks.
Weather and Scouting: Weather plays a crucial role in disease development. Cool, saturated soils in the spring favor the development of our seed and root-rotting diseases. Moisture, humidity and excessive leaf wetness, coupled with moderate to warm temperatures favor the development of NCLB and GLS (64-81°F for NCLB, 70-90°F favor GLS). Both of these pathogens will infect susceptible and moderately susceptible hybrids throughout the growing season as long as the weather is conducive for their development; however, you want to keep an eye on them as to where their lesions are present on the plant. Infections on the lower leaves have no impact on yield; however, if they infect the ear leaf and above, there is a potential for significant yield reduction. Scout your fields at least weekly as plants approach tasseling to make sure NCLB and GLS are not encroaching on the ear leaf. Look for the presence of lesions as shown in Figure 1. If infections are approaching the ear leaf, then you may want to consider a fungicide application.
Fungicides: Fungicides can be an important management tool for foliar fungal pathogens, in particular NCLB and GLS. If temperatures remain between 65-90°F in conjunction with high humidity and excessive leaf wetness as the plants approach reproductive stages, then a fungicide application around tasseling (VT) may be beneficial to protect yield. Determining whether a fungicide application will be economically beneficial is the difficult part, and knowing your cost of application can help you make a decision so that you know how many bushels you need in return to pay for the fungicide. There have been hundreds of University fungicide trials conducted on corn over the years, and less than 50% of the time are fungicide applications economical. It is important to realize that there are conditions where a fungicide application is more likely to pay; they are: 1.) Applied at VT to a susceptible or moderately susceptible corn hybrid, 2.) Corn-on-corn, especially in no-till fields, 3.) environmental conditions are favorable for disease development (warm, humid, and leaf wetness) at the time around VT.There is also interest in applying fungicides for perceived stalk strength benefits. In general, fungicides will not improve stalk strength directly, rather indirectly by managing foliar diseases. Stalk rots are strongly correlated to disease severity on the flag leaf. When photosynthetic area of the flag leaf is reduced due to pathogen lesions on the leaves, the corn plant cannibalizes the carbohydrates stored in its stalk in order to fill the grain, thereby compromising stalk integrity. Therefore, if you keep the ear leaf clean, you will greatly reduce stalk rots and improve standability, which is where fungicides can help. This is why it is important to scout your fields, look for disease, then determine if a fungicide application is warranted.
Planting corn after corn poses additional risks that favor the development of disease in your crop. By talking into account these steps, hopefully you can better manage your crop and put more dollars in the bank.
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.
