Choose the Right Tomatoes to Avoid Late Blight

By Larry Hodgson

Every now and then I feel the need to publish an update on the topic of tomato late blight, by far the most pernicious disease of tomatoes and other Solanaceae in humid climates. There has been a lot of progress made in the breeding and development of varieties resistant to this devastating disease: almost 40 new varieties have been added to the list of resistant varieties since the last update 2 years ago.

Learn to use them and make your gardening life easier!

What is Tomato Late Blight?

New strains of tomato late blight (Phytophthora infestans) appeared out of Mexico, homeland of the tomato, in the late 1990s and, by 2010, had spread all over the world, taking both seed suppliers and gardeners by surprise. 

This is an old disease, the one that caused the Irish potato famine in the late 1840s (the same disease also attacks potatoes), and gardeners thought that the disease was pretty much under control, since most commonly grown potato and tomato varieties of the time had some resistance to the original strains. But the new strains that appeared, notably US-22 and US-23, are much more virulent than the older ones and particularly harmful to tomatoes. In fact, in climates with fairly humid summers, where late blight is most prevalent, late blight of the tomato is now that plant’s most devastating disease.

You can recognize late blight by its symptoms. Notably, it shows up in late summer (it’s not called “late blight” for nothing!). At first, brown marks appear on the lower leaves and grow quickly in size. White cottony growths may appear under the affected leaves … if the air is humid. The disease rises successively upwards, affecting leaf after leaf. Often stems also turn brown. Worse, just when the fruit is almost ripe, soft brown or black depressions form on it and it begins to rot. Soon it is only good for the trash.

Resistant Varieties

There are now, however, tomatoes with genetic resistance to late blight. Don’t panic: these are not OGMs. Natural resistance to the new strains of the disease has been found in certain tomatoes, notably in wild species, and has been bred into garden varieties by the same old-fashioned methods our ancestors used to create heirloom tomatoes like ‘Brandywine’ (which is terribly susceptible to late blight, by the way). Just buy and sow varieties that are resistant to the disease and follow normal tomato cultural directives (grow them in full sun, practice crop rotation, leave space for aeration, water the roots without moistening the leaves, etc.) and you ought to be able to get a bumper crop of disease-free tomatoes.

Here are some of the more and more numerous late blight-resistant tomatoes to choose from.

Or Buy Disease-Resistant Plants

If you’re not into growing tomatoes from seed, look for tomato plants resistant to late blight in local nurseries at planting-out time. Many still offer more disease sensitive varieties than resistant ones (old plant habits die hard!), but pretty much all of them carry at least one or two late blight resistant tomato plants.

Bring Your List

I suggest you print the above list or put it on your smart phone. You’ll want to have with you the next time you shop for tomato seeds or plants.

For more information on tomato diseases, read Disease-Resistant Tomatoes.

Article updated from one published on March 6, 2016.