Better Never Than Late

By Paul Hetzler, Horticulture and Natural Resource Educator

It’s not too early to start thinking about late blight. No relation to early blight, with which it shares a last name, late blight has become a perennial disease since infected tomato plants were shipped from southern greenhouses to the Northeast in May 2009. Prior to that, late blight was uncommon, but now we seem to be able to bank on its arrival each August. The fact that it is a seasonal immigrant is worth noting, since most garden diseases (such as early blight) are already here in the soil.

Gardeners and produce growers make a fuss about late blight because it has the potential to kill acres of tomatoes and potatoes in a matter of days; its fearsome reputation is well-deserved. Given the botanical name Phytophthora infestans, “highly contagious plant destroyer,” it is what laid waste to the Irish potato crop from 1844 to 1846, leading to a devastating famine.

To be fair to a right nasty microbe, however, late blight was not entirely to blame for the Great Hunger of 1845-1852. During this period, record quantities of Irish beef, pork, mutton, and grain were shipped to England. The fact that potatoes were the only foodstuff not somehow limited or confiscated by the occupying British forces is the reason the failure of that single crop caused the death of over a million Irish, and the emigration of even more.

Even though we have the luxury of growing other vegetables, we still want to avoid late blight. How to dodge a killer that breezes into town on the wind is a fair question, though. It turns out that the late blight fungus is technically a water mold, and requires moisture for a spore to germinate on a tomato or potato leaf. Planting tomatoes far apart—24 inches is good—and making wide row spacing—say, 36 inches—will improve air circulation and sunlight penetration. Most gardeners already stake or trellis tomatoes; this is also key. Pruning is less common, but worthwhile. Maintain a single stem by pinching off “suckers” that arise at every leaf junction (axil). This will help reduce disease, plus get you larger tomatoes that start to ripen earlier.

Home gardeners can use protective sprays to stave off late blight, but none of these products will stop the disease once it hits. Fungicides with the active ingredient chlorothalonil are readily available, and organic growers can use copper-based fungicides. To avoid needless spraying, usablight.org has a great tool for both home and commercial growers. You can sign up to receive alerts when late blight is found elsewhere in the region, and can set the distance you prefer. Weather plays a big part in how it spreads. Not only are wet conditions conducive for the pathogen to get started on a plant, cloud cover shields late blight spores from UV radiation. In full sun, spores are killed in about an hour, but if it is overcast they are good for many days.

Knowing how to spot the scourge is critical. Many diseases of tomatoes and potatoes can be mistaken for late blight. Early blight, a ubiquitous garden disease, is soil-borne and begins on the bottom leaves, working its way up the plant. Septoria leaf spot, another soil-borne pathogen, sometimes occurs along with early blight. Blossom-end rot, which actually is due to severe water stress and is not a disease, causes tomatoes to become blackened and rotten on the bottom.

The first obvious symptoms of late blight are large, watery lesions on leaves, giving them the appearance of having been frozen and then thawed. In moist and/or humid conditions, white fungal growth may be seen at the margins of the lesions. Since late blight is airborne, symptoms will show up throughout the plant, not just near the bottom. The disease also affects stems, sometimes killing the plant above the point of infection. On the tomatoes themselves, late blight causes large, brown, greasy-looking patches that are surprisingly firm to the touch.

If late blight is confirmed, you can try and salvage unripe tomatoes of mature size by immersing them in a 10% bleach solution and laying them out on a counter top or baking sheet where you can keep an eye on them. Discard any fruit that develop lesions. Ripe tomatoes with small late blight spots are safe to eat after removing affected areas, but the USDA recommends not using them for canning.

Obviously, late blight can spread on the wind from one garden or farm to the next. To protect other growers, diseased plants should be placed in clear plastic bags and left in the sun until no green tissue is left. Once the plants are completely dead, the late blight organism can no longer produce spores and it is safe to compost those plants, bury them, or discard them in the trash.

In northern latitudes, the only way late blight has been able to overwinter is by hiding in potatoes from infected plants that have been left in the ground. When these volunteers sprout in the spring, late blight can work its way up the stem, produce spores and touch off an epidemic. Now that we are in an age of recurring late blight infestations, it is essential for gardeners and farmers scout for and destroy all volunteer potato plants each year.

For more information on late blight, go to usablight.org, or call your local Cornell Cooperative Extension office. Let’s do all we can this year to keep it late. Or better yet, never.

Contact

Paul Hetzler
Horticulture & Natural Resources Educator
ph59@cornell.edu

Last updated June 12, 2017