Late Blight Webinar 2010: 3 - Symptom Imitators

eOrganic authors:

Dr. Margret McGrath, Cornell University

Dr. Sally Miller, Ohio State University

This clip is from the Late Blight Control in Your Organic Farm Webinar

Audio Text

So we're calling our next little session our late blight symptoms imitators. So essentially there are a number of things that look like late blight and we get a lot of these in our diagnostic labs that people are concerned. They think they've got late blight but what they have is one of the many many things that can cause disease on tomatoes. So here is one we see often because it's a very common disease, it's called early blight of tomato and also it's on potato and again some of the symptoms are similar to late blight because you see this dark brown tissue or necrotic tissues. You'll see some chlorosis around that. Sometimes there's a black lesion on the fruits but if you turn the leaf over you won't see that white fuzziness underneath. Here it is on potato. And again you'll see small dark black lesions. Often times you can see these concentric rings, this is sort of a target type of lesion. Turn it over and there won't be sporulation like you see with late blight.

This is another big disease of tomato, it's called septoria leaf spot, and a lot of times it is confusing because this is another disease that occurs when there is a lot of rain fall although it does like warmer temperatures than late blight does. And again it tends to produce much smaller spots but there's still that you see around here. And if you have really good eye sight you might be able to see little dots, but certainly with a hand lens you can look in the affected tissue and it will be a light tan color usually and there will be some little dots there. Those are, this is a true fungus attacking that causes this disease and they form something called pycnidia which are little structures that hold spores and so what you're seeing is those little things. And in that case certainly that is a good diagnostic for septoria that differentiates it from anything else.

Now this is even a little tougher one, botrytis and the next one I'll show you, because these occur during similar conditions to late blight. They like to have high humidity, rain fall and coolish conditions. And sometimes you'll get a very dark lesion on the stem, it'll attack the fruits although you tend to not get bronzing, you tend to get sort of this whitish watery looking rot. And it can if the conditions are right kill young tissue. But the thing about botrytis is you almost always get a very fuzzy gray mycelium and spores and that differentiates it from late blight.

The other one that also is even a little trickier is called fulvia leaf mold. Now if you don't have a high tunnel or green house you've probably never seen this because it's generally just seen where you have extremely high humidity and mostly in green houses. Now commercial green houses often use varieties that are specifically designed for green houses so they don't see this, they have resistance to this disease. But a lot of the verities that are meant for the fields and are used in high tunnels you could see why this could be confusing because you do see some bronzing sometimes on the fruits, you have a yellowish lesion on the top but if you look underneath that you don't have dead zone in the middle surrounded by a ring, you just have sporulation all the way across. And it's a little different color, it's kind of an olivey color.

Now there are a couple of diseases, now I suppose some of you think “gosh, even more, tomatoes get so many diseases,” and they do, they get a lot of them. So this is something that is a very important disease called bacterial cancer and hopefully you've never seen it but it can cause a lot of chlorosis but you're not going to see the fruit symptoms or the stem symptoms are going to be little dots, not the big lesions that you see with late blight. Now this one's a little bit more problematic, it's called bacterial soft rot and with this in potatoes is called black leg in potato and this one is a little more difficult because they do look a little like late blight lesions however you don't see the typical symptoms on the fruits or leaves that you would see with late blight and if you kind of rub your finger along this it's extremely slimy because it's bacterial, so that's another hint that it's bacterial soft rot or black leg.

Now here's another one that can be difficult, it's called the tospovirus, a tomato spotted wilt family virus and sometimes you can see a bronzing sort of color on there, but the leaf symptoms often have these little dots and often a ring, a ring spot, so that is the best way to distinguish it, but again we get these samples and they can be confusing.

We also have some physiological or physical problems that can occur that can be confused with late blight such as blossom end rot. I think anyone who has ever grown tomatoes has seen tomato end rot. But sometimes, we just got this one a week ago, this is blossom end rot on the sides of its native fruit. So the person that served the county agent was actually concerned that this might be late blight but it's most defiantly blossom end rot.

Chemicals can cause burns like fertilizers can cause burns and various kinds of chemicals. You can get injuries that look like this and sometimes you might have to send that to a lab because you can't be sure of that one.


Published July 27, 2010

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