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

This article was originally published at Extraordinary Times Gardening.

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 and a half 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's 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.
• Choose cultivars that are resistant to late blight. Garden catalogs should have these listed in the variety descriptions.
• Use a 3 - 4 year tomato planting rotation. Don’t put tomatoes in the same bed more than once every four years or so. If you have a small garden, this may mean either finding other spaces to plant, growing container varieties, or getting extra creative.

Beneficial bacteria to the rescue

“Late Blight is an inevitable fact of life here in foggy, chilly San Francisco,” says Bruce N. Goren, a freelance photographer and master gardener in the Bay Area. “I've found over the years that I can delay the onset until the very end of the growing season by applying a preventative spray of Serenade [note: this product was discontinued in 2018] soon after transplantation and at regular intervals of every three or four weeks. This is a "good bacteria" which takes up residence in the vascular system of the plant leaving little room for the infestation of damaging pathogens.”

In the absence of Serenade, there are other products that work along similar lines. The important thing to remember when using them is the key ingredient: beneficial microbes. These little buggers support plant health in many ways. According to the American Society for Microbiology, there can be as many as 52,000 different species of microbes living in the soil around a plant’s roots, doing a variety of jobs that promote plant growth and serving as a first line of defense against pathogens. Controlling late blight in tomatoes is just one way that these bacteria can help your garden grow lush and healthy.

Other considerations

If you decide to abstain from tomato growing for three or four years as a strategy for containing the fungus responsible for late blight, you will be following a time-honored tradition. “Crop rotation has been farmer wisdom for thousands of years, “ Bruce notes. “It's even in the Bible.” During that time, nothing in the nightshade family should be planted in that spot.

The reason for the three or four year abstention period is that blight spores can only survive dormancy in the soil for this long if they fail to find a host environment to trigger growth. You may be able to speed up this process by putting down black, water permeable row coverings which semi-sterilize the soil when heated by the sun.

While you’re waiting, you can pivot to container growing if you absolutely must have your own homegrown tomatoes. Just make sure to fill your containers with sterile media and locate them at a distance from the infected beds.

Have an alternate plan handy

If you do decide to give the tomato growing project a rest for a few years, it doesn’t mean you have to go without tomatoes.

Have a gardening neighbor or three? Or a local gardening community? If you’re willing to grow tomatoes for one another in the years that your beds are resting, you can ensure a steady tomato crop and build those community connections at the same time.

Late blight doesn’t have to mean the end of your tomato growing dreams. It’s an opportunity to cultivate your ability to observe closely, acquire new gardening techniques, learn from your mistakes, and rediscover how to work with nature’s cycles. In the end, greater understanding of how the natural world works is an important addition to the collective treasure trove of gardening wisdom.

And if it means more tomatoes, that’s great too.

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