March/April 2010

Leaflet with lesion. The first sign of late blight in this garden.
Meg McGrath / Cornell University

Late Blight & Tomatoes: What You Need to Know

By Will Rowlands

Last year was an unusual one for tomato growers in Connecticut. The cool, wet and windy weather early in the season was bad enough. Who knew it would contribute to the worst infestation of late blight in recent memory?

Even though we have a microclimate of sorts in our back yard, our tomatoes couldn't escape the poor weather. We did, however, manage to avoid the blight.

Last year we bought our plants at the Wilton Garden Club's spring sale. If you don't grow your plants from seed, get them from a trusted local source. It's not only safer, it contributes to the local economy. Still, the fact that our tomatoes were blight-free was more from luck than anything else. Like many home gardeners in the Nutmeg State, we were largely unfamiliar with the disease.

Late blight is a naturally occurring fungus-like organism called an oomycete. It's a downy mildew, a type of water mold. Some strains attack tomatoes, some attack potatoes and some can infect both. The blight spreads via wind-dispersed spores or inoculum. Wind-borne spores can travel several miles or more. Joan Allen, a plant pathologist and plant diagnostician at UConn's Home & Garden Education Center, said she's heard reports of the spores traveling as far as 40 miles.

Visible signs of infection usually occur in 3 to 5 days and plants can be dead in a week. The pathogen is Phytophthora infestans. The potato strain of the organism was responsible for the Irish and Highland Potato Famines in the mid-1800s.

Late blight is not uncommon in our area but, as you might surmise from its name, it's normally seen in August and September. Outbreaks in Connecticut are usually sporadic but, in 2009, a number of factors allowed the highly contagious spores to spread early and wide. Ultimately, except for pockets here and there, the entire state was affected.

There is general agreement that infected seedlings, sold by a wholesale gardening company to big box stores, were the primary source of the infection.

The interstate transport of plants is regulated, according to Dr. Sharon Douglas, a plant pathologist at the Connecticut Agricultural Experiment Station (CAES) in New Haven. However, it's physically impossible to inspect every plant at the point of origin. On top of that, a given plant may not have visible symptoms at the time of inspection. Once the infected plants arrived in Connecticut, some employees at garden centers may not have recognized the signs or known what to do if they had.

To complicate matters, unless home gardeners come from an area familiar with late blight, such as a potato-growing region, they might not recognize the disease. [See Caveat Emptor and photos below.] Unfortunately, the failure to dispose of infected plants properly contributes to the spread of the disease. Left alone, infected plants will continue to produce spores until they die. Douglas says a single lesion can produce hundreds of thousands of spores a day.

It was a vicious circle. Commercially grown seedlings started the ball rolling but home gardens helped it spread to others nearby, including commercial growers.

The late blight pathogen is a tricky foe. The potato and tomato strains can jump from one plant to the other, and certain other plants are susceptible as well. These include hybrid petunias, some members of the Nightshade family (Solanaceae) such as bittersweet and, rarely, eggplants and peppers.

When there is only one type present, P. infestans uses sporangia to reproduce asexually by making copies of itself. In this scenario, the pathogen can only survive the winter in the living tissue of potato tubers.

Here's the real worry for the future. According to Dr. Meg McGrath, a plant pathologist at the Long Island Horticultural & Research Extension Center in Riverhead, N.Y., when two mating types are present in proximity, sexual reproduction produces an oospore that can overwinter in the soil without a living host. There is also the possibility, however remote, that it could create a new genotype that might be even more problematic.

McGrath notes that two mating types, the pathogenic equivalent of male and female, were detected last year in Pennsylvania and Virginia. If they make it to Connecticut, late blight could become ensconced and the only way to ensure a decent crop of tomatoes would be to engage in preventative spraying with fungicides.

It is interesting to note that the asexually reproduced version produces spores that can move about easily while the sexually reproduced oospore tends to stay where it is, waiting for the next plant it can infect. We'll be in for a double whammy if we end up shackled with both.

The best way to prevent this, McGrath believes, is to adopt a no tolerance policy. "Hopefully, the only way the pathogen was able to survive in the northeast from last summer was in potato tubers. We need to make sure everyone knows this and all infected tubers will be destroyed as well as any volunteer potato plants as soon as they emerge."

Potato tubers can be destroyed in a number of ways, such as deep burial and freezing, but Douglas recommends throwing them away in the trash as the safest method for home gardeners.

If you're thinking about saving infected plants, keep in mind that William Fry, a professor of plant pathology at Cornell University, says it's normally not possible to halt the progress of the disease once 5 to 10 percent of the foliage is infected. Vigilance and a quick response are necessary if you're inclined in this direction.

TAKE ACTION

Most of these measures are best performed when the sun is shining because ultraviolet radiation can kill the spores.

• Check plants on a regular basis, daily when conditions are favorable for infestation. Check as early in the morning as possible.

• Put the infected plants (roots and all) in a black garbage bag and leave in the sun for a few days so it can heat up before disposing in the trash. Do not carry infected plants, or portions thereof, through the garden; bag them in place to avoid spreading the disease.

• Don't compost infected plants.

• The pathogen shouldn't survive in or on tomato seeds from last year's crop but it's generally a good idea to use seed from healthy plants.

• Remove volunteer potato and tomato plants.

• Avoid overhead watering and handling of wet plants.

• Plant tomatoes in an area with good airflow and drainage.

• If possible, move susceptible plants to a new area.

• Sunlight, staking, mulching and increased spacing between plants can help reduce the likelihood of infection.

• Plant some early varieties. You'll get some tomatoes if late blight doesn't arrive early.

• Keep an eye out for blight-tolerant varieties or one of the blight-resistant varieties that are presently being developed.

• Stakes and cages do not have to be thrown away to prevent late blight but they should be disinfected with a bleach solution or other disinfectant because other diseases may be present.

Hopefully, if we adopt stringent measures, we can prevent late blight from taking up permanent residence in the Nutmeg State. Additional precautions are necessary if you're growing potatoes.

Allen recommends regular monitoring of plants and a quick response to any sign of problems. She also recommends seeking professional advice when necessary. UConn's Home & Garden Education Center, for example, can often diagnose problems if you send them digital photos.

Allen suggests seeking out blight-resistant or blight-tolerant varieties and noted there is anecdotal evidence that some cherry tomatoes may be less susceptible. She also suggests watering in the morning, good spacing between plants, crop rotation and mulching. Mulching, she said, has more pros than cons and adds organic matter as it breaks down.

Douglas says misidentification is one of the critical issues. You might think your plants have late blight when they don't. It's natural, for example, for some leaves near the bottom of the plant to yellow and shrivel up. Alternatively, you might think you have late blight when your tomato plants have a disease with similar symptoms, such as leaf spot or early blight. It's a good idea, therefore, to know what late blight looks like. Please consult an expert if you are unsure.

It may be stating the obvious but don't buy plants unless they appear healthy and free of infection. It's also a good idea to keep an eye out for at least 5 days in case you purchased recently infected plants that weren't showing any symptoms.

Organic gardeners should hope for hot, sunny weather as it can slow the spread of the disease or even stop it. Some may want to consider the use of a copper-based fungicide spray.

A number of other organic methods have been suggested, such as sprays made with baking soda, neem oil, garlic tea and an antitranspirant mixed with skim milk and water. Essential oils, such as oregano and thyme, may also be useful but there is little scientific data on the efficacy of these methods.

Organic gardeners, in particular, need to keep a watchful eye and remove infected plants (and those nearby) immediately. It's also essential to understand how the disease takes hold and how it spreads so that appropriate steps can be taken.

According to Douglas, copper and chlorothalonil are fungicides that are licensed for use in Connecticut. It's important to follow instructions regarding application, dosage, coverage, washing and personal protection. "The label is the law," says Douglas. She recommends reading the label every time you use a product. Even if you've used a product before, the instructions may have changed.

Allen points out that some pesticides can only be purchased and used by certified pesticide operators. Also, keep in mind that fungicides, organic or otherwise, are preventative in nature and will not make a diseased plant healthy.

The perfect storm for late blight is cool, moist weather with wind to spread the spores and clouds to protect them from ultraviolet radiation. Forecast systems for the agricultural community, such as BLITECAST, attempt to warn growers when con­ditions are ripe.

Let's hope the weather this year is more conducive to a good crop.

Even though our tomatoes escaped the blight we still had a lot of green tomatoes on the vines when the weather turned cool. We unearthed an old Nova Scotia recipe for Chow Chow (green tomato relish) and put up 14 quarts. It's amazing how fast we're going through it. It's great with fish, eggs and a number of other dishes.

Late blight presents no known risk to humans. Theoretically, you could cut out the infected portions of fruit and eat the rest but, personally, I don't find the idea very appetizing.

Not unlike life, an important part of gardening is being flexible and learning from your experiences. Knowledge is power ... know your enemy.

CAVEAT EMPTOR

Buyers should inspect tomato plants before purchase. Here's what to look for:

• Olive brown to black lesions on leaves and stems.

• A white fuzzy growth (sporangia) underneath the leaf spots, especially when the weather is wet or early in the morning. You may also see it on the top of leaves as well as on stems and fruit.

• Stem lesions vary in size from spots to several inches in length.

• Leaf lesions are typically a half-inch or larger and may have yellow margins. They may look wet or greasy or have yellowish borders. A number of neighboring lesions may merge, ultimately causing the leaf to shrivel.

• Brown sunken spots on the fruit that rapidly expand.

NEED HELP?

For answers to questions or assistance with diagnosing late blight, contact:

• CAES' Plant Disease Information Office at 203.974.8601 or the statewide toll-free number at 877.855.2237. For more information visit www.ct.gov/caes

• UConn's Home & Garden Education Center toll-free at 877.486.6271. For more information visit www.ladybug.uconn.edu or email ladybug@uconn.edu

Other useful web resources include:

• UConn's Dept. of Horticulture at www.hort.uconn.edu/ipm/

• Cornell University Dept. of Plant Pathology's Vegetable MD Online at http://vegetablemdonline.ppath.cornell.edu

• Long Island Horticultural & Research Extension Center at www.longislandhort.cornell.edu

• Cornell University's Department of Horticulture at http://hort.cals.cornell.edu

WHAT TO LOOK FOR

Fuzzy growth on the underside of leaf lesions is produced by the pathogen
under moist conditions and consists mostly of spores.
Meg McGrath / Cornell University

Green fruit showing white fuzzy growth.
Sharon Douglas / CAES

Green fruit showing brown spots.
Meg McGrath / Cornell University

The infection eventually spreads to all parts of the plant.
Joan Allen / UConn