Even though few, the raindrops Saturday night were so refreshing after a hot week! I don’t know that it’s even really that hot compared to past years, but the sun seems extra intense to me this year. Cooler temps are welcome this week!
Crop Update: There is very little disease pressure thus far in both corn and soybeans. For corn, the most common thing I’ve seen this year is physoderma brown spot/purple leaf sheath, which is something we don’t worry about in Nebraska. Bacterial leaf streak is common on certain hybrids as always and is one we don’t worry about. A fungicide will not help against it and won’t protect against it. Gray leaf spot is very minimal to date in lower canopy, if it can be found. Same for common rust. The closest southern rust has been found is in southern Arkansas. So short story, fungicide isn’t necessary yet unless one is saving a trip for corn insects. For corn insects, there are still hot areas of Japanese beetles feeding on silks in addition to corn rootworm beetles. Spidermites are also flaring in some fields. Also be aware that spraying a fungicide can flare corn leaf aphids as it kills the fungus that attacks them.
For soybean insects, there’s still some Japanese beetles feeding and some spidermites flaring. For disease, have seen very minimal phyllosticta leaf spot and frogeye leaf spot and not anything close to levels for spraying. Seeing lots of phytophthora root rot this year in fields that is continuing to kill plants and there’s nothing we can do about that this year. Fields with a history of white mold may have been sprayed to help reduce disease pressure.
Tar spot has not been found in Nebraska yet this year. A great resource to track diseases such as southern rust and tar spot is: https://corn.ipmpipe.org/diseases/. Click on the disease of interest to see a U.S. map of where the disease has been found. Suspect samples can be submitted to the Plant and Pest Diagnostic lab in Lincoln for free. You can also get your samples to me locally if you’d like. There’s been a lot of confusion so far with insect “poop” and tar spot. A quick field test is to either get your finger wet or spit on the leaf and rub the spot to see if it comes off. Physoderma and tar spot won’t rub off but insect excrement will.
Fungicides and insecticides are helpful when we need them. Everyone’s trying to make the best decisions possible. Concerned we let fear of ‘protecting the crop’ drive decisions. Crop prices continue to be volatile and economics should be considered. Sometimes fields are sprayed a second time when disease comes in later once the residual wears off (regardless of product and because the product only makes it so far into the canopy unless chemigated). In terms of resistance management, we have fewer modes of action with fungicides than herbicides available to us, and we use those same modes of action in all our crops. We already have resistance to the quinone outside inhibitor (group 11 formerly strobilurin) class of fungicides to frogeye leaf spot in soybean. Concerned it’s only a matter of time before this impacts us on the corn side too.
I realize I’m continually an outlier in saying to wait and not automatically apply at tassel. Based on the Nebraska research (shared last year here) and observation I feel we can wait till disease pressure warrants applications and allow them to help with stalk strength. I also realize this column would’ve been more timely last week with the spraying that’s occurred.
South Central Ag Lab Field Day Aug. 4 will begin with registration at 8:30 a.m. with program from 8:45 a.m.-3p.m. at the South Central Ag Lab near Harvard/Clay Center. There are several tracks to choose from throughout the day including the latest in weed, disease, insect, nutrient, irrigation management, and soil health. Free lunch and CCA credits available. More info. and RSVP at: https://go.unl.edu/scalfieldday.
The first picture on the left has been common of both physoderma brown spot and insect poop. With physoderma brown spot, most often these purplish spots are more prevalent on the midribs, leaf axils, and leaf sheaths (as seen in the middle photo). Often the spots outside of the midrib are more yellow/tan in color and are often confused with southern rust. Photo 3 on the right-hand side was tar spot that was found in Oct. 2021 in York Co. Would recommend getting your finger wet or spitting on the leaf and rubbing the spot to make sure it’s not insect poop as several samples looking like the first photo have been that instead. Physoderma won’t rub off and neither will tar spot. Feel free to submit any suspect samples to the diagnostic lab for free.