Diseases Caused by Too Much Water

Diseases Caused by Too Much Water

Part of Ep. 1603 Bailey's Harbor

When there is too much rain, leaf diseases  such as anthracnose, bacterial blight, leaf streak, Guignardia leaf spot, apple scab, are on the rise.  Find out how to recognize and fight them with UW-Extension plant pathologist Brian Huddleston.

Premiere date: Sep 17, 2008

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley:
Gardeners have to deal with weather all the time. in Too much snow. Too cold. Too windy. Too little rain. Today we're focusing on too much rain. I'm with UW-Extension plant pathologist Brian Huddleston. Too much rain is just as bad as not enough rain.

Brian:
It definitely can be. Certainly, when we have a lot of rain we see an increase in leaf diseases. That's what I'd like to concentrate on today. I brought this beautiful bouquet of diseases for you.

Shelley:
All I need is a vase!

Brian:
That's right. The first one we'll talk about has been common this year. We've had a lot of calls on this. It's called anthracnose, a very common disease. You can see it on virtually any plant. We tend to see it on woody ornamentals like maple, and also ash and oak. You get these little weird, dead areas on the leaves. They're blotchy. If the disease occurs relatively early before the leaves are fully expanded, you'll get a lot of leaf distortion where there's cupping and curling.

Shelley:
So the deformity there.

Brian:
That's due to the disease. Oftentimes, you'll see this kind of stick around for the entire growing season. It may cause some leaf loss in the trees. But it tends to be relatively cosmetic and the trees tend to recover fairly well.

Shelley:
Okay.

Brian:
The other one that we see, oftentimes earlier in the spring although again, it will continue through the summer.

Shelley:
This is a lilac.

Brian:
That's a lilac. The name of the disease is bacterial blight. Although here, it's really causing a leaf spot. The tendency with this disease are little angular sorts of spots, dead areas on the leaves with yellow halos. Those yellow halos are very typical for a bacterial disease. I love that this organism oftentimes occurs naturally on the leaf surfaces without causing disease. It's there all the time.

Shelley:
It's just sitting there?

Brian:
It's just sitting there on the plant surface. It's only when the populations get real large, which tends to occur when it's wet that we start to see disease symptoms develop. The other really cool thing about this particular organism is that if you've ever seen those little brown spots on your snap beans, that particular disease is caused by the same bacteria.

Shelley:
So it's affecting the food we eat, as well.

Brian:
That's right. Another leaf disease, this time on an herbaceous plant. It's on a day lily.

Shelley:
This looks dead.

Brian:
But this particular disease and none of the ones that I'll talk about really kill plants. But it will cause some defoliation, again. These long, streaky, dead areas down the leaves with this yellow border.

Shelley:
Okay, so you admit it's dead? 

Brian:
I admit this tissue is dead. The whole plant is not dead, though. This is called leaf streak. What I love about this particular pathogen and fungal organism that causes this disease, is that if you look at it under a microscope it's reproductive structure looks like little medieval maces with the little ball with little spikes on the top. It's very easy to identify.

Shelley:
You're in the right job, Brian.

Brian:
I love my job! I see all kinds of interesting things all the time. Another disease that we see quite commonly more on things like Boston ivy or on Virginia creeper is a disease called Guignardia leaf spot. Guignardia is the name of the fungus that causes the disease. It's relatively discreet. There are somewhat circular spots on the leaves. Oftentimes, they're with kind of a darker edge.

Shelley:
I see it just around it.

Brian:
It's a cosmetic disease, again, on something like Boston ivy. But it can be a more serious problem on a crop like grapes, where it causes a disease called black rot. The fungus will actually infect the fruits and cause them to shrink and mummify.

Shelley:
I've seen that.

Brian:
Yes, and it can be very serious there. Then another fruit crop actually we're showing it here on crab apple, but it occurs in apple, as well, is this disease. This is apple scab. You can see the spots caused by a fungal pathogen.

Shelley:
Too much rain.

Brian:
When we have cool and wet springs, particularly when the buds are first starting to emerge we tend to get a lot of this particular disease. Again, it will continue throughout the growing season as long as we have rain. Oftentimes, to the point on susceptible varieties, where we end up with a lot of leaf loss. In fact, oftentimes, by early July we'll see trees that are totally defoliated because of this disease.

Shelley:
And this will continue?  In fact, we've got leaves on the ground here. This leads to my next question. Most of these are foliage diseases. How do we deal with them?  What's the best thing to do? 

Brian:
Definitely, the first step is to get rid of all that diseased leaf material that's on the ground.

Shelley:
Fall clean-up.

Brian:
Clean up those leaves. Get them out of your yard. You can bury the leaves. Burning is an option although that tends to pollute a little bit. But you can certainly do that.

Shelley:
If I'm a good composter? 

Brian:
Hot composting works very, very well to eliminate these sorts of disease causing organisms. That's probably the best way to deal with the problem. Now with trees, you can do a little bit of pruning to open up the canopy to dry leaves more rapidly.

Shelley:
So more air through there, okay.

Brian:
Then the other option, although I don't normally recommend this only in rare cases, would be a fungicide treatment. But only on very special trees that perhaps have been stressed for several years. Because you have to worry about the stressed trees getting more serious diseases that will actually kill them.

Shelley:
Otherwise, it's cosmetic.

Brian:
Quite frankly, if you want to use a fungicide treatment, you should really contact your county Extension office and get a specific recommendation for the disease that you're dealing with.

Shelley:
Okay, good advice, Brian, thanks.

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