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Farmers Almanac
The 2014 Farmers Almanac
Farmers' Almanac

Battling Tomato Blight

Battling Tomato Blight

Depending on who you talk to, this summer has either been a very bad gardening season or a very good one. For home gardeners in the Northeastern United States, and parts of the Midwest, who’ve had their hearts set on a bountiful crop of sun-ripened tomatoes, this has been a disappointing few months. And for farmers whose main crop is either tomatoes or potatoes, it has been downright devastating, thanks to something called “late blight.”

Late blight is a disease of the Solanaceous family of plants, which includes tomatoes, potatoes, peppers, and tomatillos. It’s caused by a fungus called Phytophthora infestans — imagine a very deadly strain of athlete’s foot for plants. Late blight spreads in wet conditions, like rainy weather or morning dew, and likes temperatures between 50 to 80°F.

This year’s occurrence of late blight is one of the earliest major outbreaks on record, and also the most widespread. This ongoing epidemic is fueled by the cool, wet, and windy weather. Normally, late blight shows up later in the season, after the harvest. However, when its favorite conditions exist, such as the rainy summer we’ve had so far, late blight can spread very quickly and wipe out an entire crop in less than three weeks.

For those whose gardens remain miraculously untouched by late blight, it’s still important to keep an eye out for it. Because blight spreads so easily, it can be passed from one person’s garden to their neighbor’s garden, or worse, to the farmer’s field up the road. It’s crucial, therefore, to check your plants for blight frequently. Luckily, late blight is very easy to recognize. As it affects the leaves, stems, and fruit of the crops, you can see signs of it on most parts of the plant, though it often strikes the leaves first. The most common symptom is nickel-sized olive-green or brown spots on leaves, with a yellowish ring around the spot. There may also be a fuzzy white coating on the underside of leaves when conditions have been moist. These spots are called lesions. Leaf lesions start out as small, oddly shaped brown spots and grow larger over time. Lesions can also be found on the stems of the plant. Once the lesions begin to affect the fruit, tomatoes or potatoes will develop rotten brown spots.

The horticulture department at Cornell University has good images on what late blight looks like on tomatoes and potatoes.

Unfortunately, the safest course of action for gardeners who find an infected plant is to pull it, seal it in a plastic bag, and throw it away. Infected plants should definitely not be composted, as this could allow the disease to survive and spread again next year.

By now, you’re probably thinking “But what about my vegetables, my harvest, all that hard work? Isn’t there anything I can do?”

In some cases there might actually be hope for those plants that aren’t too far gone — that is, those that have blight only on their leaves, and not their stems or fruit. You can pull off all of the leaves that show signs of the disease, and spray a solution of two ounces of tea tree oil to one gallon of water on the entire plant. Tea tree oil is a natural fungicide. Most fungicides, whether homemade or commercial, are only preventative, however, and won’t usually get rid of the disease once it’s progressed. With a few warm, dry days, though, and a limited number of blight-affected leaves, the fungicide could help to keep the disease at bay long enough for you to produce a decent crop.

At the urban gardening organization where I work, we just mowed over the infected potato plants, making sure to throw away the diseased parts of the plants in the garbage and not the compost. We waited a few weeks before digging up the potatoes, and found quite a few healthy looking ones. To prevent them from getting infected, we made sure not to get the potatoes wet, and washed them only right before eating.

Remember, though, that if the blight doesn’t seem to be clearing up on either tomatoes or potatoes, it’s best to properly dispose of the plants to help prevent further spread in your own garden or your neighbors’.

That sums up the bad, but what about the good? Next time, I’ll talk about some of the blight resistant plants that thrived in the wet weather we had this summer, and share some strategies for preventing blight in the future.

Until then, keep growin’!

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