September 21, 2023

Tar spot here to stay

Management tips

Initial symptoms of tar spot are brownish lesions on the leaves of afflicted corn plants. Black, spore-producing spots appear later, making the leaf feel rough or bumpy.

EL PASO, Ill. — Tar spot was an unknown corn disease in the “I” states less than a decade ago, but no longer. A severely infected field can reach yield losses upwards of 60 bushels per acre.

The disease and its management was a topic of discussion at the Beck’s Central Illinois Field Day question-and-answer session featuring Jim Schwartz, director of research, agronomy and Practical Farm Research; Jon Skinner, regional agronomy manager in Illinois and Iowa; and Chad Kalaher, field agronomist in east-central and northeast Illinois.

Once a field gets tar spot, is it in that field forever?

Skinner: More than likely, yes. We’re going to have to deal with tar spot from here on out in our corn production. Anything you do from a localized perspective, meaning tillage or burying residue or removing residue from the field is going to have minimum benefit to it.

Tar spot is an aggressive disease. It has what they call a steep dispersal gradience. When it shoots its spores, it can go 250 feet in the absence of any wind or environmental condition.

As far as recommendations for tar spot, one thing I always do is look at genetics. If that’s our primary concern, different genetics have different tolerances to the tar spot infection.

I also would have a planned fungicide application of a high quality product in the reproductive stages. The products I typically recommend are Trivapro, Miravis Neo and Delaro Complete.

We talk about generic fungicides in those situations, and they don’t have a lot of activity against tar spot. So, I’m being very specific in the fungicide I select.

Yes, it’s going to be there forever. It overwinters on the residue and when conditions are conducive the following year it’s going to keep coming.

Are there any soil tests that can show that you have that pathogen?

Kalaher: I’ve worked with a company that does some soil testing to identify pathogens. Right now, I’m not aware of a test that exists where they can identify tar spot, but I bet they’re working on it.

Can tar spot be more virulent if there is nitrogen deficiency, gray leaf spot or other problems in the field?

Skinner: It can attack itself. It does not need an open wound or other delivery mechanism. It can infect from the ground up with those spores. Typically where we see tar spot have the highest yield reductions is when we have comorbidity.

So, either we have another foliar disease like gray leaf spot or northern corn leaf blight, rust of some sort, that helps move that process along of senescence.

One big thing we see oftentimes is if we have a heavy infestation of tar spot, it effects that leaf and colonizes the vascular bundle. So, the xylem and phloem that’s contained in that tissue is basically pinched off and so then you can’t move photosynthates and stuff back through the plant, even all the way down to the root.

When that happens, the plant gets stressed and you typically see and increased level of stalk decline through disease, either gibberellin, diplodia, fusarium stalk rot, something like that, which then causes stalk decline and plant tip-over. So, by itself it can effect and it can have a pretty drastic effect.

When one looks at the seed product book and sees hybrid ratings for tar spot, what should be considered when seeing a rating of 5 versus 8, for example?

Skinner: As far as the disease ratings go, it’s not if that corn plant is going to get tar spot, it’s how that plant reacts to it after it comes in contact with that infestation.

If tar spot is my No. 1 concern, I wouldn’t select a product just based on one concern because I can give you stuff that’s very, very good against tar spot, but emergence may be a little bit lower, standability may be a little bit lower and overall yield may be poor, but I can assure you tar spot is not going to take a big stranglehold on you.

If tar spot is a huge concern of yours and it’s going to be in the top two or three things that you’re selecting a hybrid based on, I’d go with a 7 or 8 because they’re exponentially better than a 6 or a 5.

Kalaher: A specific example of that, one of our top-selling products in Illinois, one of our highest-yielding products in Illinois, is not the best rated product for tar spot. It doesn’t have like and 8 or 9 rating for tar spot. It’s more like a 7 or a 6. We would recommend a product like that and spray it with a fungicide to protect it against tar spot or once tar spot shows up, spray it.

Now, if you’re a farmer that will never consider to spray a fungicide and you have the risk of tar spot, then, yes, maybe that tar spot rating does become more important for you operation and your criteria for hybrid selection.

Do I ever consider a sequential fungicide application? Would it ever pay to do one at VT and R3?

Kalaher: There’s been a lot of talk about that now we’re going to have to spray corn twice because of tar spot. We don’t necessarily want to plan for a double application of a fungicide. We would rather scout then spray. Not everybody scouts their fields, but we don’t think every field is going to need a sequential application of a fungicide for foliar diseases.

This year, in both of our areas for the most part, I don’t think we’re going to see any positive return on investment from spraying a dual pass of fungicide.

Skinner: I agree wholeheartedly of what Chad said. There were instances in 2020 where we did see some sequential applications have a big increase in yield. We had really good corn then and we had a high level of disease.

We had gray leaf spot, we had southern rust, we had tar spot, we had the whole cocktail of diseases where (the second application) paid off. The opportunity for it to pay off is there.

I don’t think it’s something that will consistently pay off. I’d rather do what Chad said. If you’re clean at tassel, R1, give it a little time. See what develops there, and it you’re going to make that application, do it at R3, early R4 timeframe, and then protect the rest of your grain fill.

Schwartz: In the south in 2020 where there was delayed planted corn and southern rust we saw some positive responses to a sequential application because we’re talking 50 to 60 days grain fill and these products have 20 days of protection. So, we saw some time where a sequential applications did pay when we had heavy southern rust in a susceptible hybrid.

Tom Doran

Tom C. Doran

Field Editor