Planting tomatoes and potatoes? Be wary of late blight

It's time to grow tomatoes and potatoes, and that means gardeners must be vigilant in looking for signs of late blight in their gardens. The disease has occurred every year on Long Island since 2009, and the experts tell me we should expect another appearance this year.

"People need to realize this is probably one of the worst diseases we have in the vegetable world," said Meg McGrath, plant pathologist at Cornell University's Long Island Horticulture Research and Extension Center in Riverhead.

Late blight typically occurs sporadically on farms in August and September and is controlled and contained with fungicides as the season winds down. During the past seven years, however, there have been two deviations from this pattern:

1. Late blight surfaced early in the season, allowing it to spread and gain prominence all summer long.

2. The disease was present on starter plants sold at the retail level, which means it was not recognized quickly and treated vigilantly, as it would have been in a commercial growing environment. Undetected, it became rampant.

McGrath cautions that "gardeners growing potatoes need to understand that they need to plant 'certified seed potatoes,' not potatoes they saved from last year or bought at a store or farm stand. The late blight pathogen is more likely to be in the latter."

Caused by Phytophthora infestans, an oomycete (a pathogen similar to a fungus), late blight was responsible for the Irish Potato Famine in the 1840s. In one day, a single infected plant can release millions of spores, which can be carried long distances by wind, especially in dry weather. The spores remain airborne until air currents or rain bring them down to infect more plants, potentially dozens of miles away. Theoretically, millions of spores from one infected plant in Franklin Square could wipe out an entire crop on a farm on the North Fork.

Diagnose the problem

Late blight manifests on leaves as individual, elongated lesions, and on stems as white-mold-encircled gray spots. Eventually, entire stems blacken, then plants wilt and die.

If you suspect your plants are infected, a quick confirmation is imperative to avoid rapid transmission across the region. Bring samples to the Cornell Cooperative Extension diagnostic lab in Riverhead (423 Griffing Ave., 631-727-4126) or Great River (Bayard Cutting Arboretum, 440 Montauk Hwy., 631-581-4223). Doing so will help pathologists track the disease, warn farmers and tell you how to proceed in your garden.

"The number one recommendation for home growers is to plant resistant tomato varieties," McGrath advised. The second recommendation is to apply preventive fungicides, carefully and in adherence with package directions. Fungicides "are only effective before infection," McGrath said, adding that fungicide use after symptoms present would be "like using sunblock after you are sunburned."

Diseased plants "should be destroyed if confirmed and if the variety is not a resistant one or the plants aren't being treated regularly with fungicides starting before symptoms are first seen," McGrath said. She warned that there are other causes of symptoms similar to those of late blight, so an expert evaluation is necessary.

Late blight-infected plants should be removed from the ground -- roots and all -- and bagged tightly, then disposed of in the trash. They should not be allowed to remain on the ground, even if yanked out of the dirt, because spores will continue to replicate and spread from them.

Late-blight-resistant varieties

Cornell University plant pathologist Meg McGrath recommends planting these late-blight-resistant tomato varieties:

Iron Lady (red slicer)

Defiant PHR (red slicer)

Mountain Merit (red slicer)

Plum Regal (red plum)

JTO-545 (red plum)

Mountain Magic (red Campari)

Matt's Wild Cherry (small red cherry)

Jasper (red cherry)

Lemon Drop (yellow cherry)

Mr. Stripey, also known as Tigerella (heirloom)

Pruden's Purple (heirloom)

Wapsipinicon Peach (yellow peach)

Cerise rot (red cherry)

Clou OP (yellow Campari)

Golden Currant (yellow cherry)

Resi (red cherry)

Rote Murmel (small red cherry)

Rote Zora (small paste)

Quadro (small red plum)

Where to shop

Late blight-resistant tomatoes and certified seed potatoes are carried by several local garden retailers, among them (call ahead for availability):

Atlantic Nursery in Freeport (250 Atlantic Ave., 516-378-7357)

Dees Nursery in Oceanside (69 Atlantic Ave., 516-678-3535)

Garden of Eve in Riverhead (4558 Sound Ave., 631-722-8777)

Hicks Nurseries in Westbury (100 Jericho Tpke., 516-334-0066)

Note: This is not a complete list; your local garden center may have a supply, as well.