Tomato Late Blight: Don’t Blame the Soil

Tomato late blight only overwinters on living tissue. You can’t blame contaminated soil for its reappearance the following year.

By Larry Hodgson

Question: I had late blight on my tomato plants last year that I grew in fabric pots. I would normally have reused last year’s soil, just mixing some compost to aerate it a bit. However, I’m now afraid of spreading the disease through the soil. Should I throw out the old soil and start over with fresh soil? And how do I to sterilize the pots? I’m not even sure you can sterilize fiber pots.

Elise

Answer: Actually, you don’t have to do any of the things you suggested. You needn’t worry about the soil contaminating your tomato plants, because it simply doesn’t carry late blight!

You see, soil and pots simply don’t transmit late blight disease (Phytophthora infestans), currently most serious tomato disease in many areas. This disease can only overwinter as mycelium in living tissue. And, except in tropical regions where the tomato acts as a perennial vine, tomato plants die at the end of the season. Even if there are still roots, leaves or fruit on the ground in the spring; in fact, even if they haven’t yet fully decomposed, they’ll still be dead. And dead plants can’t transmit this disease. This will also be the case with a compost bin where the tomato residue may not have completely decomposed by spring. Any tomato bits will still be dead, and therefore any mildew will be too.

Where tomato late blight overwinters

Since late blight only overwinters on living tissue and tomato plants die in the winter, that should solve the late blight problem quite neatly. But there are a few ways around that limitation and they’re enough for late blight to come back year after year.

First, it’s important to understand that late blight (Phytophthora infestans) is actually the same disease as potato blight. Yes, the disease that caused the Irish potato famine! And, true enough, the two plants, the tomato (Solanum lycoperiscum) and the potato (Solanum tuberosum), are closely related, belonging to the same genus: Solanum. It’s not at all unusual for related plants to share the same diseases. And if all tomatoes usually die in the winter, that’s not always the case with potatoes. Some do survive … then release millions of late blight spores the next season.

Potatoes usually survive the winter in two ways, plus a third more theoretical one.

Method 1

The most common means of transmission is when gardeners use leftover spuds from the previous year as seed potatoes the next spring. So, they cut up a tuber from the previous year into sections, each with an eye. And if it’s infected, it spreads the disease when it sprouts, because the spores can be carried several miles (kilometers) by the wind. According to several studies, this is by far the most common means of transmission in temperate climates.

This would be easy to solve if gardeners always bought their seed potatoes rather than grew their own. Because, in most countries, any seed potatoes sold have to be certified free of late blight and other diseases. However, many gardeners grow their own potato starts from untested tubers … and thus cause a huge problem, both for themselves and for their neighbors.

Usually, I’m all for DIYing, recycling, reusing, saving “annuals” from year to year, etc. However, this is one case where I disagree. If there is one situation where you really need to buy fresh plant material rather than grow your own from scratch, it’s with plants prone to serious diseases. At least when certified disease-free plants are readily available, like potatoes, raspberries and strawberries. Not only are you spreading this disease to your own potatoes and tomatoes by keeping starts, but to your whole community as well. And this disease should be so easy to prevent.

Method 2

The second way late blight can survive from year to year is from from volunteer tubers. That’s the term for potato tubers that people accidentally leave in the ground over the winter and that resprout the next year. They frequently survive in mild climates, but also in protected spots in cold ones. Volunteers too can easily harbor potato blight.

To eliminate this source of infestation, try to clean up the potato bed thoroughly in the fall or leave any debris on the surface where cold air ought to kill any survivors. Also, leave the bed itself exposed to the cold of winter and certainly don’t mulch it, as that could protect any unnoticed tubers. If survivors any do come up in spring, pull them out right away and wait a few days before planting potatoes or tomatoes in that spot. Late blight spores are very short-lived, so you really only have to get all volunteer potatoes out before you plant either potatoes or tomatoes.

Method 3

The third way for late blight to survive the winter in a temperate climate is on tomatoes grown under glass. Fortunately, tomatoes grown in greenhouses since the previous year usually remain there. Rarely would anyone move them outdoors where they could spread the disease.

How to Prevent Late Blight

The best way of preventing tomato late blight, though, is to plant naturally resistant varieties. You can find a list of over 50 tomatoes resistant to this disease here: Tomatoes Resistant to Late Blight.

Of course, also give your tomatoes good growing conditions: full sun, an application of compost and mycorrhizae fungi at planting, good ventilation, watering from below without wetting the leaves, mulching to keep the soil cooler, etc., because strong plants are more resistant to mildew than weakened ones.

The fact remains, though, that choosing a resistant variety is so effective in preventing late blight that I’m always surprised to see people continue to plant, year after year, varieties of tomatoes that are prone to this disease! Isn’t that a bit masochistic?

Symptoms of Late Blight

Here’s a quick list of tomato late blight symptoms … which you won’t be seeing until the second half of the summer. Late blight doesn’t show up until then: late July, August or September. After all, that’s why we call it “late blight”!

Tomato late blight usually shows up as brown spots on the lower leaves. On the underside of the sheet, these spots may be covered with whitish “fuzz.” The spots become bigger, spread, join with others, and kill entire leaves, then petioles and stems. The disease moves successively upwards, affecting leaf after leaf, stem after stem. Worse still, just when the fruits are nearly ripe, one or more soft, black depressions form on the fruit and soon it’s only good for the trash. Often the whole plant dies.

Pot Problem

But Elise also wondered about sterilizing fiber pots.

Well, true enough, there is no particular reason to sterilize your fiber pots now that you know they aren’t spreading late blight. But if, at any time in the future, you need to reuse them and want to be sure they are free of other diseases, you can empty the soil into the garden or the compost bin and brush off the pots the remove most of the roots and soil. Then, to sterilize them, wash them in the washing machine using a weak solution of bleach, hydrogen peroxide or a stain remover such as OxiClean to kill microbes. Then rinse well. Of course, you’ll be doing it to prevent other diseases, not late blight. You can find more about washing fiber pots in the article How to Clean Smart Pots.

Top photo: thefutureis, depositphotos