Dr. Margret McGrath, Cornell University
Dr. Sally Miller, Ohio State University
This clip is from the Late Blight Control in Your Organic Farm Webinar
Thank you all and thank you Meg, my part of the presentation today is to talk about the biology of late blight and its pathogen, how you diagnose late blight and then how we scout for it, how we find it out in the field. This is the culprit, this is the micro-organism that causes late blight. This is a microscope picture magnified about 200 times. The fuzzy white stuff you saw is partly these strands that you're seeing, these oval shaped sporangia. And when I get to the life cycle in a moment, you'll get to see a little more clearly what these sporangia are and what they do. We used to call this a fungus, it looks like a fungus, but it's not really a fungus. This is called an oomycete pathogen, and it really is quite similar to algae but without chlorophyll.
So, this is the disease cycle for late blight. Draw your attention to the right hand side of this cartoon at this point, and remember we talked about those sporangia. Here is a picture or diagram of what a sporangium looks like. Essentially, a sporangium has two ways of infecting plants. It may germinate with something called a germ tube and that is something that is going to infect the plant or if it has good conditions for zoospore production, which is free moisture, it will produce these swimming spores. And these swimming spores then can multiply the effect, so there will be a dozen or twenty of them in a sporangium. So they'll be released and you can get actually amplified the number of inoculation units that end up on plants.
So, whether its the sporangium or the zoospores, they land on the plants and they can go down, percolate in the soil and get in the crown and into the tubers of potatoes, the tops of tomatoes. On the tomatoes they can get to any part of the top of the plant. Fruits, stems or leaves can be infected. On the potatoes, just about anything on the plant can be infected. So, essentially, what happens after the plant becomes infected, is that then the mycelium moves all the way and uses the nutrients that are inside the leaves and then produces these what are called sporangia spores. They are essentially just a little branch structure that produces those sporangia on the bottom side of the leaf. Now, on the stem they will just be on the stem surface, or on the fruits they will be on the fruit surface. But the interesting thing about this is the very highly evolved critter, you might say, and it drops down these sporangia spores. The sporangia can then be open and available to the wind or the breeze to pick these things up and more them elsewhere, or droplets of water can move them. So, then they will move up to another plant or a different part of the plant and can cause disease.
Now, if these are on potatoes and the tubers are infected, and the tubers aren't harvested, or they end up on a cull pile, we'll talk about that later, they can survive in that condition in association with living tissue over a winter. For example, if the potato doesn't freeze or isn't destroyed by some other means, the Phytophthora can survive in there and then when the potato sprouts in the spring time and the Phytophthora can colonize and produce those spores again, the sporangia, and the cycle starts all over again. So that is very important to understand so that so that you can understand why we're talking about the ways we suggest for control.
Now again it's a very clever beast, this Phytophthora, because it has another means of over a season survival. And this is through another process. Phytophthora has two different mating types. We call them the A1 and A2 mating types. They live separately from each other, but if both of them are present in the same plant, they can mate and produce this thing called an oospore. An oospore is a very resistant structure that is impervious to a number of various environmental highs and lows and can last a number of years. When the time is right, it will produce sporangia and enter into the disease cycle again. Now, I should mention that for U.S. Growers we don't commonly have those two mating types here, although they are beginning to show up and I think Meg pointed that out. So where we are right now, most of the time we don't believe we have this oospore phase that allows overwintering. If that becomes common that can really change the way we try to manage this disease. So, that's the disease cycle.
Let's talk about symptoms. And we saw some pictures already. In fact they are very similar to the ones Meg showed except these are from Ohio and hers are from New York. You see the very necrotic tissue. Now I think I did mention but it's very important that late blight is essentially a disease of cooler season or cooler situations although that pathogen does not die if it gets hot and drive it does not die particularly if it's associated with stem tissue or fruit tissue or tubers. So it likes to have very cool or relatively cool temperatures, high humidity and rainfall. So what will happen here you see this brown dark tissue, that is dead because Phytophthora infestans is associated with living tissue where you are going to find that mycelial mat you saw and that white fuzzy stuff will be on the outside.
So, again, this is the top of the leaf, and I'll show you the bottom here in a moment. And this is just again showing the kind of devastation, this could just be one sporangium that caused this big lesion here and this happened just in a few days. Similarly, you will see something like this on potato again. This is the back side of the potato, and this is where the sporulation is occurring in the sporangia. In tubers you'll see kind of a brown necrosis on the tuber tissue and eventually secondary bacteria and other things will rot the tuber. So, these will essentially just melt away.
Here are some more symptoms. This has unfortunately become more typical as a symptom, a stem lesion. Essentially, it's a dark brown to black color. If the humidity is high in the canopy you may see this fuzzy white stuff that is the mycelium, those spores of the late blight pathogen. This is very typical of the kind of lesion you'll see and this kills the tissue all the way through the stem. And this turns out to be quite a good place for this to survive if conditions are not optimal for Phytophthora to sporulate. Here's just another picture on the right showing you the devastation you can get.
Here is a pretty typical symptom on fruit, it's kind of a bronze color, and I'll show you in a moment that there are some other things that will do this but the color is quite bronzed and it can attack both green and red fruit. If again the conditions are right you may see this white sporulation on the surface of the fruit. This is a good one, this was in a green house it's a good example of the stem lesions as well. So this again is what you will see on the back side of the leaves and is very typical. Remember this because you're going to look for this when you're doing scouting because it's a very good indication that this is Phytophthora infestans. Look for this ring of sort of a fuzzy white stuff and that's the mycelium and the sporangia. And if you have a hand lens you will see something more like this and you might be able to see that little branching if you have a very good hand lens. That will help you make the diagnosis.