Dr. Margret McGrath, Cornell University
Dr. Sally Miller, Ohio State University
This clip is from the Late Blight Control in Your Organic Farm Webinar
First and foremost is to start with resistant varieties. Unfortunately, at this time we don't have things available on the market but they're coming. There have been reports of some varieties that are a little less susceptible. Matt's Wild Cherry was one last year that was reported. That information is up on the web, you can get some information about what's available. But there are some varieties coming, there is some work being done at the universities and some of our seed companies have developed new varieties, so that's going to be an option. A couple of the first ones that will come out; Bejo Seeds which comes out of Cornell and North Carolina State University program. There is Mountain Magic which has stood up very well and they have a plum variety, I know Johnny's is working on some. So we will see some varieties with some resistance in the future, thats going to be an option. It's going to be limited seed next year so if you're interested in resistant varieties next year you are going to want to get a hold of seed as quickly as you can.
Very important is to make sure that you have certified potato seed, so there is a much lower chance that this pathogen is in that seed. You want to destroy cull potatoes because that's a real primary place where the pathogen overwinters. Make sure that there are none in existence at the start of the next production season each year. Likewise, you want to control potato volunteers, a primary place where the disease gets started every year coming out of those infested tubers that didn't get harvested the year before. Controlling tomato volunteers and susceptible weeds is also important, not because they can be primary over-wintering sources of the pathogen but because they aren't being looked at and managed. They can be harboring the pathogen and you don't see it because you're not looking at those on a regular basis. But if you got a tomato volunteer that you want to grow and you're going to manage it, that's a different story. So tomato volunteers can be really common; here's one hiding down in my own garden peas. You just got to be diligently out there looking for these volunteers and managing them if you're not going to grow them as a crop.
Fortunately there are not many plants that are hosts for the late blight pathogen. There are a couple of important crops also petunias sometimes and a number of weeds. So not too many things out to be on the lookout for. Very important one last year that seemed to be an excellent host for these new strains is this bittersweet night shade. You can see from a close up of the symptoms that very typical big late blight kind of a lesion and this I found in my own back yard last year which was a bit alarming. We don't see this weed to often in a farm situation where there is a lot of tillage going on but in the land scape it is an extremely common weed. So that is a real concern that the pathogen can get multiplying on that weed.
You want to scout regularly for late blight as Sally pointed out. Another thing to do is monitor reports of late blight so you know where it is occurring. Extension bulletins will have this information, there is a late blight report sheet up on Google Docs so anybody can log reports of where late blight is occurring. You can see where reports have been so far this year. Cornell is mostly focused on New York and the neighboring states. There is a listserv, this listserv also posts the updates on the web. You can get late blight updates, you can see what has happened in past years as well as this year. And Abby who now runs these has a listserv specifically for growers. So if you're interested in being on her listserv or if you are in that region you can contact her and get on her listserv. And I show this page because it illustrates important things about late blight. So here's a report from the twenty-sixth of September in 2006 and it shows you that there is no amount of late blight that is too little to report. Here we got a report of a single late blight lesion being found. This is an important disease, no amount is too little to report. And if you head further down that page you'll see all the rest of the reports from 2006, very few reports. That is a typical year in the north-east. Very few reports of late blight.
Sporadic disease. There is a forecast model that you can get off the web to see when conditions are favorable for late blight development. Here is a picture of the website so you can see right now, or at least on the twenty-seventh of June reporting most areas mostly pretty low conditions for the disease. A couple areas where they are predicting that spores could be formed, infection was possible. Realize that this is strictly looking at environmental conditions and telling you when it is favorable for certain diseases. It's not considering wether inoculant was there. So helpful to tell you when conditions are favorable. I think what's really particularly valuable about this site is the fact that this pathogen can infect when humidity is at least ninety percent. And often when we are thinking about diseases that need moist conditions, we are focusing on rain and if there hasn't been any raining, we are thinking no diseases. But with this pathogen, high humidity is enough.
To look back at this case back in 2007. Prior to that outbreak in August, there had only been two episodes of rain. The temperatures had been pretty high; we weren't thinking that much about lat blight and so it illustrates high humidity is enough. Just two rain events you would think was late blight weather. And in fact this grower had cut back on his sprays because he wasn't feeling like it was a disease weather. We've often thought about late blight liking cooler temperatures but notice what those August average temperatures were for us. Average day temperatures of 82. This illustrates that pathogen strains have changed a little bit. They pointed out earlier with Florida late blight continuing to develop into May. It's tolerating these warmer temperatures. It's not our cool Irish potato famine pathogen any more. Very important thing to keep in mind.
Another illustration is that outbreak I mentioned to you that occurred last year at the end of August. If you look at the conditions in August that year; we hadn't even had an inch of rain prior to that outbreak. Once again, temperatures were fairly warm, defiantly not thinking about diseases and that's why this particular had stretched his spray intervals. I mentioned before, it had been ten days since he had sprayed. I went back in time and I looked and on the twenty-sixth of August, just a week prior to when we saw symptoms, just look at how many places conditions were either favorable for infection spores to be forming or infections possible. And actually that one little red dot that is right there, that is pretty much in the area of where this farm was. So this forecasting system can be really helpful for you to tell you when conditions are favorable but realize if the pathogen is not there, you're not going to have disease.
Next on the late blight management is to apply fungicides. As I pointed out to you, it had been really hard to control late blight if you wait until you've already got symptoms. Unfortunately you already have to be on a preventative schedule usually when risk is high. Next step as Sally pointed out. There are lots of ways you can get your symptoms diagnosed. If you think you might have late blight it's really important, not only to help you out, but because this is a community disease and we need to know where it is occurring.