fighting late blight – organic copper spray

Pests & Pathogens, Tomatoes


Well, today I bought a bottle of copper sulfate spray (Concern copper soap fungicide). My dad says this is the only organic remedy for Late Blight. I’m planning to use this on my tomato and potato plants at my plot tomorrow.

My dad’s friend Tom, a serious gardener, uses mancozeb in regular applications on his tomatoes starting in early spring. He says the only problem with using copper sulfate against Late Blight is that it doesn’t work. Hmmm. But mancozeb is a pretty serious chemical.

My potatoes don’t seem to be too badly off. A bit of leaf damage here and there. I’m gradually removing the plants and so far the tubers are great.

But Late Blight has already killed half of my 16 tomato plants (11 different heirloom varieties grown from seeds at home started on March 21st). Unlike last year’s fungus in my garden, Septoria Leaf Spot, which gradually defoliated my plants, Late Blight this year has gotten into the stems of my tomato plants.

This is what Late Blight looks like on my tomatoes. This is what it looks like on my potatoes.

I suspect that gardeners with nice looking tomatoes right now in our community plots are using mancozeb or a similar chemical fungicide.

Its just hard to fight fungi when the weather is so wet. (And I hear that Late Blight infected seedlings were distributed by chain retailers in the NE, like Home Depot and Walmart.)

In a community plot, these fungi are spread all over by the wind. My tomatoes at home are looking beautiful, isolated from other gardens by many houses and yards. Maybe serious chemicals are the only way to go in a community plot? After a second bad year, it seems like a consideration for next year. Alternatively I could forgo tomatoes in my plot.

If you have Late Blight symptoms, it’s recommended that you remove affected plants, bag them in plastic and dispose in the trash (or bury at least 2 feet deep). Pretty serious methods if you need to cart several big bags out of your plot. And this only works if EVERYONE in the community garden also does this. If not, everyone should either forget the tomatoes next year or all get some chemical fungicide.

I’d sure love to hear your comments!

In any case, I’ll be spraying away with my copper sulfate tomorrow….

potatoes (Solanum tuberosum)
late blight (Phytophthora infestans)

13 Comments. Leave new

  • Sorry to hear about your tomatoes. I can't seem to find the copper soap fungicide in any of the local garden centers. I found garden dust which has copper sulfate in it and I've been using that … keeping my fingers crossed. -Jackie

  • dear Kathy, i am watching every day your blog,it is fantastic.
    Try to use copper sulfate spray but to know next time you cultivate potatoes after the rain try to spray your tomatoes and your potatoes immediately.
    take care to ventilate your plot,when you plant potatoes and tomatoes try to put each plant away from the other and choose the next year different places for your new potatoes and tomatoes
    personally i use only copper sulfate spray and organic insecticides (neemazal,pyretrhins,bacillus) and i have good results.
    Sorry for my english,i am from greece,continue your blog and your vegetables,you are excellent.greetings to skippy

  • Sorry you have blight. It is a common problem in northwest England where I am, though this year's good weather has fortunately spared us for now. I have used copper fungicide but in my experience all it does is buy you some time to grow the plants on a bit and ensure you can rescue the crop – about two weeks or so. Repeated applications may help but ultimately you will have to get the potatoes out of the ground. Where I live "conventional" treatments don't do much better.

    I now grow only blight resistant tomatoes and potatoes, my tomatoes were the only ones in our allotment plots to survive last year. The resistance in the potatoes was not as good, but combines with some copper spray did allow me to harvest a good crop before the tubers were damaged. If this weather is unusual for you, you might not need to do this, but if it happens regularly it's worth considering seeking out resistant varieties, particularly tomatoes as they have such a long growing period over the worst time for blight.

    I wish you luck and hope you can rescue some of your plants.

  • I also garden at home and in a community garden (Natick, MA). The community garden has always been a problem with disease and bugs.I have somewhat solved the problem by growing tomatoes, peppers and eggplants only at home. I do not grow potatoes. Skippy would be cute no matter what garden he was in!


  • I would imagine as with many chemicals, eventually it just makes the problems worse. Too bad you're experiencing so many problems. I'm suprised you're allowed to use chemicals at your community gardens.

  • Any thoughts about using Serenade? It's approved for organic production, and has seemed to work for our tomatoes.

  • Oh no, that's horrible that you have lost half your tomatoes. I don't see anything wrong with using chemicals as a last resort.

  • Our tomatoes (in the same community garden as Kathy's) were still blight free as of Thursday evening. We're going to pick up some copper sulfate today, just in csae. I really do think it's because we're just upwind of the people who got infected plants from retail. Ours are all heirloom from seed, too.

    I really hope our plot lucks out, because we only have 2(!) tomato plants in the back yard, and they were planted pretty late.

    I have a feeling tomatoes at the farmer's markets are going to be rare and expensive this year, too!

  • Kathy, the UMASS article that youA had a link to recommend using chlorothalanil-or something to that effect, saying it was the only preventative against late blight and to consider beginning a weekly spraying.

    I also use Serenade as a fungicide when needed. It seems to work well if doesn't rain 6 days a week. 8)

  • mr finch, I'm glad to hear your tomatoes are doing well upwind from the rest of us! The funny thing is 3 or 4 of your plants are ones I grew from seed. At least my plants aren't the source of it! (I'm afraid people come by and may think so….)

    Your plot is nice and open too and no vegetables grew there last year and mostly flowers the year before. I remember that the gardeners who had my plot before me were very disappointed with it and were anxious to move out. From the debris left in it, I think they had planted it full with tomatoes and hadn't cleaned up by spring when I took it.

    I think it will be good for my soil to go tomato-free and potato-free at my plot next year. For a few years. Luckily tomatoes and potatoes grow well at home.

    Thanks for the advice from others on Serenade and resistant varieties. This weather is new for us, but has been the same for the past two years The concern is that its our new pattern. If so, it'll take a little time for us NE-ers to get used to it.

  • I've had very good luck with Serenade so far, and it's definitely organic. I like the idea of battling creatures with other creatures. P. infestans is definitely on its list. No blight yet, and it's done some good with saving some of my basil crop from the horrible new basil downy mildew.

  • I garden in a home garden in Wayland, MA. I pulled my fingerlings (banana) because of the blight. If they had had two more weeks… but I didn't want to risk it. Thanks to you I was on the alert! The other potatoes and the tomatoes seem fine so far – fingers crossed.

  • I feel your pain!! I also have home plots and community garden plots. Blight has taken over at our community garden. And the cold wet weather hasn't helped at all 🙁 I also started heirloom seeds from home, and my tomato's in my home plots are wonderful!! They look just like yours! Thanks for sharing and THANK YOU for the planting calendar! I'm also about to plant new peas and fava's in the community garden and cross my fingers, lol!

    A Little Bit of Spain in Iowa


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