Plant Pathology, Weed Science

Episode 9: Cercospora Disease Identification

Join Emily as she talks to Bob about how he identifies Cercospora leaf spot in the field and in the lab.

This transcript has been edited for clarity. 

0:22 Emily: Hi everyone, and welcome to Farm Sci-Ed, the show where we go into the science and education behind farming. I’m Emily Stine, and today we’re talking to Dr. Bob Harveson about cercospora diagnosis and what he looks for when trying to make a positive identification. So sit back, relax, and let’s go find out what he looks for.

0:37 Bob, can you tell me what cercospora looks like?

B: Well, it’s it’s a sort of an ash gray – it’s a lighter gray and they’re they’re oval to circular and they then they can coalesce and and really kill a large part of a leaf. They nearly always have some sort of a of a dark halo – dark brown to purple – that sort of thing. What you’re looking for, like with the hand lens, is is uh clear well they’re clear and they look kind of like cobwebs – you’ll see on the the lesion itself – you’ll see what looks like pepper pieces of little little pepper black and then you’ll see like the cobwebs and all that kind of stuff if it’s sporulating. If it’s not, then it’s kind of difficult to tell the difference.

2:27 E: What does cercospora look similar to in sugar beets?

B: Well I guess one of the things it can be is alternaria. Because that alternaria tends to be more circular and less oval than cercospora and it also doesn’t always have a border around it. There’s other several other diseases that foliar diseases like phoma that also can be confused if you don’t know what you’re looking for, and then bacterial leaf spot as well. When you have enough lesions that the bacterial leaf spot is starting to kill the the leaves, then that also can look like uh cercospora and been coalescing. And the reason that’s important to know is because the cercospora is the only one of this group that we really need to be concerned about. So if you if you mistakenly thought it was one of the other ones and made an application then you’re going to be wasting your money for that. Or if you thought it was one of the other ones and you didn’t make the application, then you also could take a hit from the pathogen that’s that’s damaging – or is cercospora.

2:27 E: How do you go about positively diagnosing cercospora infections?

B: You’re looking at the leaves you’re looking at it with with a hand lens, but honestly the only way to know for sure

in in most cases is to take it to the lab. And there’s just there’s a number of other things: you can plate it out, you can watch – watch, you know look at it under a microscope to see if you can see those spores, but it’s essentially you’re trying to make it sporulate so that you can identify it for certain with that. And sometimes it’s already doing that in the field sometimes it’s not.

2:57 E: And can you describe how you force cercospora to sporulate in the lab?

B: Something that’s called a humidity chamber is it’s just a paper towel that’s been dampened and put into a petri plate. And you just get a piece of the tissue that’s got the lesions on it, put it into that, close it up, and then it creates a really humid environment, which then in roughly 24 hours you can look at it again. It should be sporulating, then you can look at it in terms of that and and or you can put it put it onto media and let it grow out. And then in that way it would produce its uh the spores in that manner. Or you can also look at it directly uh with the with the microscope and see the spores themselves.

3:37 E: What do cercospora spores look like?

They they’re long and cylindrical, almost like a sword – a blade of a sword or something like that. It’s it’s very very long and thin.

3:50 E: How do the spores of Cercospora compare to the spores of other diseases?

B: Well it’s it’s hard to describe, but it’s – alternaria would to me look more like a club, you know it has a – it has a thin handle on it but then or more like maybe a tennis racket it’s got a bulbous end on one and then it’s got sort of like a handle on the other and with the cercospora, it would be cylindrical the whole way, like like a mop stick or something or broomstick.

4:20 E: What’s the next step after you have a positive diagnosis?

B: There’s not any kind of magic number, but I think in general, people look at this and say, “if you see three or more lesions on one single leaf then it’s time to it’s time to spray” or at least think about that. It also is important to know which part of the plant that – you’re not as concerned with it if it’s down in the lower part of the canopy as it would be up on the newer leaves, because that’s where the damage occurs. And and so then yeah because every time you look at that – you see three or four lesions, there’s probably more on the way that just haven’t formed yet.

4:55 E: Makes good sense to me! Well folks, today we talked to Dr. Bob Harveson about how he positively diagnosed cercospora in the field. We talked about what it looks like, what it might be confused with, and what he does in the lab to figure out if it’s cercospora or not. Join us next time as we go into detail on the other topics. Follow our twitter @TheFarmSciEd for updates on when new episodes are released and visit our blog at for transcripts and other information. Have a good one!