The heartache of late blight (and how to [maybe] avoid it)

Late blight in tomatoes is challenging to deal with. But there are ways to manage this common plant disease.

Late blight. 

These two words strike terror and despair into the heart of tomato-loving gardeners. 

All season you’ve doted on your tomato plants like a parent of a small child. You’ve cooed over each new leaf and inch of growth, watched for insects, staked, fertilized, watered, felt your heart leap with the appearance of blooms, and got excited when tiny fruits began filling out. You’re anxiously anticipating harvest, recipes and canning jars at the ready. 

Then, one morning, you find large brown spots on a few leaves. As the days go by, they spread like the Blob throughout the plant, turning one branch after another crispy brown while you watch helplessly. It’s as though the whole plant is slowly succumbing to an internal bonfire. 

If you’re inclined to heroics, you try to avert the inevitable. You whip out your garden shears and begin clipping off leaves (or entire branches) as they wither, even as you feel a slimy despair deep in your soul. It’s a struggle against the inevitable: you already know who will win. And it’s not you. 

You may get a few tomatoes in reward for your pains (or even a lot, but nowhere near the hoped-for bounty), but in the end, you’re back to buying tomatoes at the grocery store. Or patronizing your local farmers market.

Which isn’t necessarily a bad outcome.

You may meet other gardeners in similar predicaments to share battle stories and drown your sorrows in tomatoes grown by someone who’s apparently already fought that battle and (mostly) won. But I digress. 

So what is late blight?

Phytophthora infestans, an oomycete similar to fungi, is the organism that causes late blight. The humble Phytophthora was responsible for the Irish potato famine in the 1840’s that resulted in the deaths of one million people. It typically affects nightshade crops such as tomatoes and potatoes, but can affect other crops as well under ideal conditions. Most gardeners experience late blight at one point or another in their gardening careers. 

Late blight thrives in cool, wet conditions between 60 and 80 degrees. Zones that tend to have a lot of cloud cover or long rainy periods during the growing season are most vulnerable.The first signs of infection are typically large brown blotches on the leaves that can spread to entire branches. The fruit will display large brown circular spots that become mushy as the surface area is compromised and invaded by secondary bacteria.  

Entire crops can be devastated by blight. When the weather is cool and moist, a whole field can turn brown in a relatively short period, as if hit by frost. 

The disease is spread by overwintering plant material that harbors the spores. Tomato plants that have been mixed in with compost or piles of culled potatoes keep the spores alive until the next growing season, where they can infect new growth. 

Late blight solutions

At this point you may be throwing up your hands. Is it hopeless? No. It is not. 

There are some positive steps you can take, either to save this year’s crop or to better plan for next year.  

Start by improving airflow in your tomato plot to prevent Phytophthora from gaining a foothold. Luke at MI Gardener suggests defoliating the bottom 12 – 18 inches of the plant, and removing all the leaves below the lowest fruit. This will ensure that splashing during watering will not get fungus spores on the leaves. Removing any leaves that are growing into the interior of your planting (i.e. in the shade of other plants) will help with this effort as well. 

Space your plantings — at least 2 – 3 feet between plants, 6 feet between rows. Prune the plants to a single stem.  

Don’t overwater.

Use an organic spray. Luke recommends a spray of 1 – 3 tablespoons baking soda and an equivalent amount of vegetable oil mixed in a gallon of water, with a bit of dish soap added to emulsify the oil so that the solution will stick to the leaves. This will change the Ph of the leaf and prevent spores from taking hold and multiplying. 

You can also use an essential oil based formula with a (well-diluted) mixture of Neem, rosemary, clove, and/or peppermint.

Exposed soil increases the likelihood of blight. Cover the soil around the plants with no-till or landscape fabric.

Watch for leaves that develop black or brown spots. Cut them off and dispose of them in the trash. Don’t put them in your compost. That is the perfect way to incubate the disease for the following year, and you will be fighting this battle all over again.

Keep your soil healthy. That will give your crops the best chance at fighting off disease. 

Consider choosing cultivars that are resistant to late blight. Garden catalogs should have these listed in the variety descriptions. 

Use a 3-year tomato planting rotation. Don’t put tomatoes in the same bed more than once every three years. If you have a small garden, this may mean either finding other spaces to plant, growing container varieties, or getting extra creative.

Have an alternate plan handy

Consider giving the tomato growing project a rest for a few years. That doesn’t have to mean going without tomatoes, though.

Have a gardening neighbor? If they’re willing to grow tomatoes for both of you in the years that your beds are resting — and vice versa when it’s your turn — you can ensure a steady tomato crop and build those community connections at the same time. 

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