Late Blight Lessons

Back in June, American Vegetable Grower (AVG) announced that there was an early season outbreak of late blight sweeping the Northeast. This destructive disease impacts potatoes and tomatoes and it had been found in South Carolina, North Carolina, Virginia, West Virginia, Ohio, Maryland, Delaware, New Jersey, Pennsylvania, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Vermont, and Maine. Recently, it has moved into states further West.


According to researchers, the occurrence of late blight this year is unprecedented. Until now, there was no record of it being so widespread in the eastern U.S. so early in the growing season, and growers were unprepared for its onslaught. This also marks the first time that late blight appeared on tomato plants for sale in garden center stores.

AVG had a conversation with Cornell University researcher and late blight expert Margaret McGrath to gain some insight into this year’s outbreak.

Q: What do you think will be the fallout from the late blight epidemic?
McGrath: It will take time before we know how much money has been lost due to the impact of late blight, as well as the amount spent on fungicide
applications to save crops. Loss to home gardeners probably won’t be included in the calculation. Many gardeners lost their tomatoes and potatoes because they bought affected plants or, like growers, their plants were exposed to wind-dispersed spores.

There might be more than an economic impact with this outbreak. Gardeners have experienced a very devastating disease. Hopefully now they realize just how difficult it can be to grow crops, and the value of fungicides.

Growers are very mad about what has happened and are asking what will be done to ensure it can’t happen again.

Q: Is the source of the outbreak still being disputed or are pathologists certain of its origin?
We know that tomato plants with late blight were being sold in garden centers at big chain stores early in the growing season, when this disease had been detected in very few commercial crops. We do not know how the plants became infected. Back in June, I was told the infected plants had been traced back to a greenhouse in the south. This was a logical explanation because late blight has been occurring there during the spring. But since then we learned the plants were not produced in the south.

Q: Given the nature of how this epidemic got started, do you think it could have been avoided?
McGrath: Yes I think so. First, late blight should be an easy disease to avoid during production of tomato plants. It is not seedborne. There are very few weeds and other crops that are hosts. It is an obligate pathogen only known to be able to survive over winter in the north in potato tubers.

Second, I think detection of affected plants and their removal from the chain stores could have been faster. This was partly impacted by how the plants are marketed. They are sold by consignment. Since the stores don’t own them, they are not responsible for them. At most they might water them, which is what the store manager I spoke to told me. This manager also told me he couldn’t move the diseased plants because he doesn’t own them. I saw tomatoes with symptoms of late blight in that store on June 26. Affected plants were still there on July 2. On July 15, New York State Agriculture and Markets inspectors brought me plants, so I knew the plants were still in stores.

Q: Would you describe the conditions for this outbreak as a perfect storm?
I look at this and, to me, it is beyond a perfect storm. We had a very devastating pathogen easily dispersed by wind on plants throughout the Northeast early in the production season when conditions, being unusually cool and extremely rainy, were very favorable for disease development. What made it even worse than a perfect storm is that home gardeners unknowingly bought affected plants.

So now we have unknown sources of inoculum. From gardens where affected tomatoes were planted, the pathogen spreads out to other gardens and to farms and the growers aren’t expecting it because this is not the way late blight normally develops. It usually starts in growers’ potato fields and people like myself, who work with growers, know about the initial sources and can warn other growers. What is frustrating to me is that this is a very controllable disease if you are on top of it with preventive fungicide applications. But it can be a very devastating disease if you are not.

Q: What are the main symptoms of this disease?
The main symptoms are leaf spots and stem lesions — large leaf spots and large, dark brown stem lesions. It depends on where the spores land as to where the disease shows up. It also depends on what people have done treatment-wise. If they have a really good funcigide spray that hit the leaves really well, the grower may see less of it on the leaves and more on the stem.

We also know that there are different strains of this pathogen. At this point, there is the usual strain in potatoes and it appears we have a new strain in the tomatoes. This information is very preliminary.

Q: What makes the disease so highly contagious?
It is the wind dispersable spores. Many spores can be dispersed easily. It is like cucurbit downy mildew, which is now all over the Northeast. As you compare the two epidemics you can see the difference from the source. The spores are similar and are easily moved by wind for both diseases. The epidemic with late blight occurred so much earlier and is so much more widespread, which goes back to the home gardeners as the source.

Q: How long did it take for the disease to jump from tomatoes to potatoes?
We don’t have a good feel for that. I think it is more that it started in gardens. We also had some late blight start in potatoes with the traditional strain, and from there it has moved in all directions.

Q: For organic growers, how much were they able to salvage?
They have had much better control than we thought they would be able to get. They were able to spray copper regularly and that helped. If they were on a very good spray program and were on top of getting rid of affected plants, they were OK. There were some challenges getting the copper. For the distributor, they need to bring the copper in on whole pallets. Are you going to bring in 2,000 pounds of copper if you are not sure you can sell it? Now late blight breaks out and the distributor still wonders if he can sell 2,000 pounds. When late blight broke out, our conventional grower could request a fungicide delivery that day, where the organic grower couldn’t do that quite as easily.

Q: How will this outbreak impact tomato prices?
I don’t think it will impact prices as much as it should. It has been my gut feeling that growers aren’t in complete control over there prices and the buyers tend to set the prices more. I’ve had growers comment to me that it doesn’t matter this year. They have to maintain their reputation with buyers. I don’t think you will see prices go up to offset the additional cost growers had to spend to buy fungicides.

Q: If you have it in your fields, what should you do?
Recognize that you have it, and scout routinely. Hopefully you are using good, preventative fungicides in advance and have a late blight fungicide ready when it hits. Don’t delay.