April 14, 2010

Avoiding late blight: Beware of last year's crops

Last year, many broken-hearted gardeners in the Northeast were denied their cherished homegrown tomatoes when late blight -- the same disease that triggered the Irish potato famine -- wiped out their plants.

This year, gardeners can play a key role to help avoid a repeat of 2009.

"Anyone growing susceptible plants needs to take responsibility to ensure they don't become a 'typhoid Mary,'" says Meg McGrath, plant pathologist at Cornell's Long Island Horticultural Research and Extension Center in Riverhead, N.Y. "We need to treat this like a community disease," she adds. "If infested, even a small garden can have a devastating impact on other plantings."

The good news for gardeners is that they are starting with a relatively clean slate this year. Phytophthora infestans, the fungus-like pathogen that causes late blight in tomatoes, potatoes and other tomato-family plants (Solanaceae), requires living plant tissue to survive over winter in the Northeast. That's why the disease is relatively rare in the region.

The bad news: Potato tubers are living plant tissue. So any late blight-infested potato tubers that survived in soil, a compost pile or root cellar could harbor the pathogen and give it an early start again this season.

"Destroy leftover potatoes and any volunteer potato plants as soon as they sprout," McGrath urges. "Do not wait until you see symptoms. By then, new spores likely will have already developed and spread to other gardens or farmers' fields."

While infested potato tubers are usually the main source of the pathogen in the Northeast, spores also can be carried by wind into the region from milder climates. Typically, these outbreaks occur later in the season, hence the name "late blight," McGrath says.

And there is potentially worse news: If both mating types of the pathogen establish in the Northeast, they can produce thick-walled spores that overwinter and survive several years in the soil without a living host. This has happened in parts of Scandinavia and Europe, where late blight, as a result, occurs more commonly. While only one mating type was detected in the Northeast last year, both types appear to be established in Florida.

Even though late blight cannot yet survive in the Northeast without a living host, it's still a good idea to rotate crops and clean plant stakes and tomato cages to help prevent other diseases, suggests McGrath.

One line of defense against plant diseases is to choose such varieties as Black Plum, Matt's Wild Cherry, Yellow Currant and Yellow Pear that have excellent resistance to late blight.

But even if a variety is resistant to one strain of the disease, that doesn't mean that it's resistant to all strains, says Tom Zitter, a vegetable disease specialist at Cornell. While some are better than others, no potato varieties are highly resistant to late blight.

Also keep in mind that resistance doesn't mean full immunity. "There's no silver bullet," says McGrath. "You still need to use other management practices and hope that disease pressure is low and the weather cooperates."

Craig Cramer is communications specialist in the Department of Horticulture at Cornell.