Tip of the Week: How Late Blight Took Hold

Posted in Gardening Tips on November 23 2009, by Sonia Uyterhoeven

Sonia Uyterhoeven is Gardener for Public Education.

I think it is worth reflecting further on the effects of the cool, rainy weather we had this season—not to reminisce but rather to attain a deeper understanding of the practice of gardening.

Something came along this year that caught people by surprise. Late blight (Phytophthora infestans) swept through the region like a hurricane and caught everyone off guard by acting like an awkward party guest who arrived early.

Meteorologically, its early appearance was logical. Late blight is a devastating fungal problem that thrives with high humidity and cool, rainy weather. It covers a plant with white powdery spores and large green to brown spots that swell to the size of a quarter. The stem is typically marked with brown lesions. This fast-moving disease can decimate a plant within a week.

In the New York area, one of the disease’s greatest targets, tomatoes, are generally planted between Mother’s Day and Memorial Day, when the temperatures are warming up. It is generally only late in the season when they are susceptible to cool, rainy temperatures.

Late blight hit the headlines this summer when a large southern wholesale distributor purportedly sent infected plants to mass retailers such as The Home Depot, Kmart, and Lowe’s.

For locavores (proponents of locally grown produce), this was a perfect PR opportunity. It was a case in point of the evils of mass production, i.e.; problems have a tendency to explode exponentially in the system. And with late blight it was true; it swept through the Northeast, wiping out large sections of tomato crops across farming communities.

For me, it was a classic illustration of the disease triangle: a heuristic device describing disease development. Draw a triangle and place at each point one of the following: host, pathogen, and environment. The triangle indicates they are all interrelated, which they are.

The host this year was the unsuspecting tomato plant, the pathogen was late blight, and the environment was just what the disease needed in order to thrive—weather that was cool, humid, and wet.

Here is how the triangle works. If all of the pieces are in place the disease prospers. If one component is changed, the dynamic is thrown off balance. Change the wet, cool, humid weather to bright, warm, and sunny and late blight disappears; the intense rays of ultraviolet light break down its fungal spores.

Why was late blight so rampant this year? Let’s take a look at the host, the tomato. Tomatoes love even moisture, so all the early rain technically shouldn’t have been the end of the world (although I think we can all agree that it was too much of a good thing). Tomatoes also love heat. They are generally planted deep so that they can develop a substantial root system, which is important for a successful harvest.

The cool weather meant that tomatoes sat immobilized in the ground this spring and didn’t develop a strong root system. When it was time for late blight to enter the scene, it preyed on a weakened, defenseless victim.

In my next post, I’ll talk about how to successfully deal with late blight.