Dr. Margret McGrath, Ohio State University
Dr. Sally Miller, Cornell University
This clip is from the Late Blight Control in Your Organic Farm Webinar
There are basic requirements in order to have disease and we plant pathologists talk about it as this disease triangle and with late blight we always have susceptible hosts. There are a lot of us growing tomatoes and potatoes in the U.S. so there are plenty of susceptible hosts. Conditions are often favorable but the pathogen is what is limited and generally speaking that is what our limiting factor for late blight occurrence. And the reason for that if we start thinking about what the sources are the most important are infected potato tubers and that pretty much is going to be in your major potato production areas and anyone who has had late blight and doesn't take care of those potato tubers the next year. A very important source. And then plants in frost-free areas and that would the southern part of Florida and that's pretty much it in the U.S. We've got a limited area where this pathogen can survive because remember it's a obligate pathogen, it has to have living host tissue to survive.
So from those plants in frost-free areas they are going to produce those spores that Sally showed you, they're wind dispersed, they can easily move. And from there the pathogen can easily move to other production areas during the course of the season. And we had a couple cases, not to many over the years, where plants that are infected have been moved to other areas and the pathogen has come with it. Hasn't happened too often. With tomato transplants this has happened and we had one situation where we had a strain of petunia plants. Those are our sources in the United States. It is important to realize that in Europe that we now have a new source. A number of years back they brought in the pathogen equivalent of the other mating type, the opposite sex, they now have both mating types in some parts of Europe. They have sexual reproduction occurring and as Sally pointed out, that results in a structure that can survive in the soil in the absence of living host tissue. So if that happens in the U.S. As Sally pointed out and I'm just reiterating, we will start seeing late blight much more regularly. So that's the reason to be really aggressive about how we manage late blight because we sure don't need that to happen here.
So a key fact about this pathogen is that it produces these wind dispersed, asexually produced spores. So it is just cloning itself which is continually reproducing these spores that are all the same, it can infect and produce a new spot with some of these aggressive strains as few as four days. But typically it's about four to ten days to go from a spore landing on the leaf to a lesion like this that is producing more spores and you can see from the numbers there are an awful lot of spores on that leaf and that is where the disease can just escalate, where you've got the right kind of conditions for that amount of inoculant being produced. Now typically those who study the disease feel that it move about fifteen to twenty miles with these spores under normal conditions but it can move further than that.
We had a situation back in 2007. We had an outbreak on Long Island where I am, saw symptoms on the twenty-sixth of August and the nearest know source was a report our of Pennsylvania back in the beginning of August. So that is a pretty big leap that the pathogen apparently has made. And you can just see how devastating it was for this particular grower on these particular plants. It hit, he did have some fresh fruit, fruit looked good when he picked it but you can see the softness at the top of the fruit, those symptoms all appear while they sat on the stand. That's when he really realized he had a problem when he watched customers picking up this fruit and putting it back down again because of those symptoms. So there is an example of where late blight, the pathogen, made a huge leap so that can happen, doesn't happen that often.
Ok, so in the U.S. in recent years we've had late blight occurring every spring in Florida, at least since 1993 and in recent years it has continued developing later into the spring than it has in the past. So they are still seeing late blight into May since 2005. That means the pathogen is present and is producing spores at a time of the year where we do have production north of Florida. Now those spores have a place to be dispersed and to land on crops which was not so much the case prior to 2005. In Maine we've had, every summer pretty much, they've had late blight in the major potato production areas in the north because there is so much potato production the chances of there being infected potatoes is pretty high. So they've seen it pretty much every year they've been battling it up there. Most else where, it's pretty sporadic and rare and this next slide illustrates that there are differences in jost virulence which is important to realize. The potato strain really hits the potatoes, usually the potatoes are hit pretty hard, but this past year we had stains that hit the tomatoes incredibly hard, very aggressive on the tomatoes and not so aggressive on the potatoes. It's a very opposite situation now we've got strains that are real potato strains where we're used to this disease being a real potato disease, things are changing.
Now this slide illustrates just how sporadic late blight can occur in some regions. This is where I am, I've been here working as a vegetable pathologist since 1988 and the first time I saw late blight was in 2002 and you can just see over the years it has very much varied when we've first seen late blight and often not until quite late in the season, October and at that point where it's almost frost time, it's not such a big issue when it appears that late in the season. And contrast that with Maine up in the major potato production area. They see late blight every year and it's around about the same time depending on when the environmental conditions become favorable for the pathogen and this is it coming out of the seed and that's when the disease is getting started. So it's very regular in occurrence when it's coming from seed potatoes or volunteers, very sporadic and erratic otherwise in other areas such as where I am.
Last year we had detected our usual, what we call the usual potato strain, it's the one that has been named US-8 and has been occurring in Maine the past few years that occurred again. But we had five mostly new genotypes that had not been seen before and they were all on tomato. In most of the north-east it was this US-22 but a couple new strains had occurred. Most of these genotypes, most of these strains that we were detecting were one of the mating types. A second mating type was detected in a couple areas but no evidence yet that we've go oospores forming in the U.S. but it does show us that the possibility is there, it's knocking on the door that one of these days those two opposite mating types will by chance come together and we will have oospores formed.
This year so far occurrences in Florida as usual, they've had late blight through May, not quite as much as past years because of the really cold weather back in January knocked the late blight back quite a bit, but it still occurred. Has some occurrences in Louisiana and then since then we've had a couple of very limited outbreaks often in these areas it has just been one sighting so far. For instance here in New York we've had one garden confirmed of having late blight and also in Connecticut, just one outbreak so far. So pretty limited to date and it will just be a matter of time to see where things go. Most of these reports have been on tomato and all various places. We've found it in greenhouses, high tunnels, retail stores predominately in Canada this year, gardens, fields. So a real variation as to where we are seeing late blight, that is important to keep in mind.
What we haven't figured out yet is the source for all the late blight outbreaks so far this year. We think it's infested potato tubers but we haven't been able to pinpoint sources for any of these fields. Part of the challenge is since this tomato strains don't like potatoes all that much, we're not going to see a lot of symptoms in an infested field. Another complication is since these strains don't like potatoes that much when they are infesting the tuber they aren't destroying the tuber quite as much as the really aggressive strains so much better chance for the pathogen to survive in these potato tubers. And I think this is part of the community aspect to this disease. We really need to all work together and when we have outbreaks this year, talk about it, try to figure out what the source was because from that understanding of where the pathogen is surviving we are going to be much better able to control it in the future.
So that ends my little section on occurrence so we can move on to management.