Phyllis Bongard, Educational content development and communications specialist, Jochum Wiersma, Extension small grains specialist, and Dean Malvick, Extension plant pathologist
An early, but rainy start to the 2024 growing season resulted in a wide range of planting dates and associated issues. Drs. Jochum Wiersma, Extension small grains specialist, and Dean Malvick, Extension plant pathologist, joined moderator Anthony Hanson, Extension educator-crops, to discuss the issues to watch for in the May 22 session of Field Notes.
Small grains update
The earliest small grains seeding dates ranged from late March in southern Minnesota to the second week in April in northern Minnesota. Then weather delays limited planting opportunities to the 3rd week in April and the 2nd week in May in the northern region of the state.
Overall, small grain stands are very good, with the earliest seeded grains well into the tillering stage. Winter rye seems to be ahead of most years, having reached heading in southern Minnesota nearly two weeks ago.
However, in fields where heavy rains closely followed seeding, drowned-out spots may need to be replanted. Seeds that are imbibing moisture are vulnerable to rotting in saturated soils, resulting in very thin field stands. Once wheat has emerged, it can likely survive a few days of flooding as long as leaves are above water.
What to watch for
With the earliest crops tillering, it's time to look at weed control programs and start scouting for diseases. While it's still relatively early in the season, start looking for powdery mildew and tan spot in winter cereals, particularly if a field is wheat on wheat. Tan spot symptoms start as small chocolate-colored lesions that are eventually surrounded by yellow halos and look something like nitrogen deficiency. Dr. Andrew Friskop provides a good summary in the current issue of NDSU’s Crop & Pest Report of the economics of early-season control of tan spot. The risk for Fusarium head blight (FHB) on winter rye is very low due to cool temperatures, so there is no need to treat for it at this time.Populations of English grain and bird-cherry oat aphids were picked up three weeks ago. These aphids are the vectors of the barley yellow dwarf virus (BYDV). Once they start feeding, viruliferous aphids can transmit the virus to the plant, which eventually appear stunted and yellowed. Management of BYDV is through aphid control. The economic threshold for aphids is one or more aphids on 80% of the stems. North Dakota State University recently updated their aphid thresholds (page 97) from vegetative stages through head emergence. Depending on thresholds, treatment might coincide with an herbicide program. Keep in mind that tank mixes of an insecticide with contact herbicides and/or fungicides can increase leaf burning.
Populations of English grain and bird-cherry oat aphids were picked up three weeks ago. These aphids are the vectors of the barley yellow dwarf virus (BYDV). Once they start feeding, viruliferous aphids can transmit the virus to the plant, which appear stunted and yellowed. Management of BYDV is through aphid control. The economic threshold for aphids is one or more aphids on 80% of the stems. North Dakota State University recently updated their aphid thresholds (page 97) from vegetative stages through head emergence. Depending on thresholds, treatment might coincide with an herbicide program. Keep in mind that tank mixes of an insecticide with contact herbicides and/or fungicides can increase leaf burning.
Other causes of yellowing
Yellowing can have several causes. Tan spot and barley yellow dwarf have already been mentioned. Nitrogen (N) deficiency, particularly early in the season, is a common cause of temporary yellowing in the field. Cool temperatures, wet soils, and still-developing, small root systems can lead to early season N stress. During the 1990s, Russ Severson conducted trials to see if topdressed 28% UAN during wet years would eliminate this early-season slump. What he found was that the additional N application made a yield difference 50% of the time on sandy soils. Unless deficiencies are severe, the crop should recover quickly once temperatures warm and soils dry.Could the yellowing be from a sulfur deficiency? Sulfur (S) deficiency also causes yellow leaves, but symptoms occur in new growth rather than old growth like an N deficiency, because sulfur is not as mobile in the plant. If ammonium sulfate (AMS) wasn’t used in the starter blend and you’re seeing sulfur deficiency symptoms now, an AMS application may be in order. Dave Franzen at NDSU just wrote a nice article in that same issue of NDSU’s Crop & Pest Report that details what happens with nitrogen, sulfur, and wheat in wet soils.
Asters yellow is another insect-vectored disease, but this one is vectored by leafhoppers. Like the black cutworm and true armyworm, leafhoppers generally migrate from southern states into the upper Midwest. Crop consultants in the central portions of the Red River Valley are reporting large numbers of aster leaf hoppers. At this time, an economic threshold for leafhoppers in cereals hasn’t been determined.
Finally, crop injury and yellowing may be caused by herbicide carryover after a dry year, even when rotation restrictions have been followed. Herbicide degradation depends on active microbia in the soil, which in turn requires moisture. Authority and Outlook are examples of residual soybean herbicides that may cause carryover issues this year.
Corn and soybean disease issues
Since wet springs are associated with a greater risk of disease, what kinds of issues can we expect? There are two different timeframes to think about. The first is more immediate and focuses on seed and seedling disease.Seed and seedling disease
Most of our seed has very high germination rates and can tolerate sitting in the soil for a long time and remain viable. However, this season’s wet conditions could bring on some issues with seedling disease that we normally don’t see on a wide scale.The ”big 4” pathogens that cause the most seedling disease issues are Pythium, Phytopthora, Rhizoctonia, and Fusarium. The first two are oomycetes – or water loving pathogens. Rhizoctonia thrives in warm, moist soils – it doesn’t require saturated soil – so in years when planting is delayed and soils have warmed, it may be a significant problem in some areas. Three of these pathogens or some variation of them can cross between corn and soybeans. The exception is Phytopthora sojae which only seems to infect soybean; however, Phytophthora sansomeana can infect soybean and corn - although this appears to be uncommon in Minnesota.
Now is a good time to scout fields to see what is happening in the field. Look for plants with differing heights or discoloration. Are there missing or dying plants? These pathogens can kill seedlings when they’re emerging or soon after, but other factors may also be at play, such as wireworm or cutworm. Scouting will help tease out the cause. If seedlings have been infected but are surviving, there’s a good chance that many will pull through. While scouting, this is also a good time to assess seed treatments to see if they’re working adequately.
Phytopthora survey
If you encounter problem soybean fields and you suspect Phytopthora root and stem rot is the cause, the Department of Plant Pathology would like your help. One of the more widely used resistance genes in soybean varieties, Rps1K, has been breaking and not protecting the crop well anymore in many fields. We would like to know how widespread the pathogen is that can overcome this trait, so are requesting soil samples from suspected problem fields. Please contact Megan McCaghey (mmccaghe@umn.edu) or Dean Malvick (dmalvick@umn.edu) if you have questions or can help.Infection now - issues later
The second timeframe deals with early-season infection that doesn’t become apparent until later in the season. It’s widely thought that soybeans can be infected with the fungus that causes sudden death syndrome (SDS) in very early stages of seedling growth – perhaps even before emergence when the radicle is just an inch or two long. If the early-season infection window is missed altogether, we may not have problems later; however, the flip side is also true. We may have more SDS infection now which could lead to more problems later. Whether or not SDS develops will depend on conditions during June and July that promote the disease.What about hail?
Since most of the seedling diseases infect through the roots, there seems to be little association between hail wounding and seedling disease.Cover crops and disease
Do cover crops increase or decrease the incidence of disease? The answer is it’s complicated and it can work both ways. One Iowa study showed that some cover crops could increase the populations of Pythium in the soil, which could lead to greater risk of infection of other crops. Winter rye grown as a cover crop can close the green bridge for a few diseases, such as wheat streak mosaic virus, and insect pests like armyworm.On the other hand, some cover crops can increase antagonistic organisms that reduce plant pathogen activity. Cover crops may also physically interfere with a residue-borne pathogen by disrupting spore dispersal. Like so many things, whether it helps or hurts depends on the crop, cover crop, growing conditions, and disease organism.
Disease Diagnosis
Every year is different and we never quite know what we’re going to find or what’s going to develop. That’s where Digital Crop Doc comes in. When disease is developing in one of your fields and you’re wondering what it might be or you’re looking for confirmation, submit photos to the online form and one of our experts will contact you with a diagnosis. This service is free and open for corn, soybean, small grains, sugarbeet and forage submissions in the state of Minnesota. If we can’t give a definitive diagnosis, we might request more information or suggest that samples be submitted to the U of Minnesota Plant Disease Clinic (https://pdc.umn.edu/).Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
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