Late blight, the disease that devastated tomato and potato crops on farms and in home gardens last summer, is back in Vermont this year. The first cases in the state were confirmed on August 3, on tomato foliage from two different backyard gardens located in Lamoille County. It is assumed the disease originated from infected volunteer potatoes in the area. According to Ann Hazelrigg, plant pathologist at the UVM Extension, late blight spores are easily carried long distances in the wind, so anyone growing tomatoes or potatoes should be watching their plants daily for signs of the disease and act quickly to destroy them in order to limit spread of the disease to other growers.

Don't be fooled by imposter diseases

Wendy Sue Harper, vegetable and fruit technical advisor for NOFA (Northeast Organic Farming Association) Vermont, cautions that it is critical that gardeners and farmers carefully diagnose tomato and potato diseases. Harper explained in a recent phone conversation how a gardener in Maine mistook early blight for late blight and destroyed tomato plants that likely would have survived and produced fruit without harming other gardens. 

Gray Mold, drought stress, early blight and Septoria leaf spot can easily be mistaken for late blight. The Long Island Horticultural Research and Extension Center of Cornell University provides extensive information about late blight identification on its website. They urge growers to not stop at the first symptom when diagnosing late blight. Because the late blight pathogen produces most of its spores at night, it's usually more visible in the morning so that is the best time of day to inspect plants. Look for characteristic leaf symptoms which are very large spots that look water-soaked at first, then turn brown, often with a border of light green wilted tissue. The best place to look for the white, fuzzy growth of spores is on the underside of leaf lesions. Large, dark brown lesions develop on stems. Unlike many diseases, late blight does not necessarily start near the bottom of plants.

As the disease progresses in the state, it is important that cases be confirmed by the UVM Extension. Those who suspect late blight are encouraged to send leaf samples to the Plant Diagnostic Clinic, Jeffords Hall, 63 Carrigan Drive, UVM, Burlington, VT 05405. Be sure to include the location of the garden and the day the blight was first noticed as well as contact information.

Three necessary conditions for infection

New Hampshire garden writer Henry Homemeyer pointed out in a recent commentary on Vermont Public Radio that "a disease needs three things in order to infect a plant: the pathogen itself -- in this case wind-borne spores. Next there must be a susceptible plant. Lastly, the proper environmental conditions must be present. Should any of these three conditions not be met, our tomatoes are safe." According to a recent NOFA Vermont Late Blight alert, while the strains of late blight that have showed up in New England in the past were intolerant of hot weather, the strain gardeners saw last year the Northeast seems to tolerate warmer conditions. With recent heavy rains and the resulting morning fog it is suspected that late blight will be showing up in more gardens in Vermont in the coming weeks.

Neither Homemeyer nor NOFA Vermont recommends that home gardeners spray the preventative copper-based fungicides that commercial growers use because, as Homemeyer points out, despite their approved use by organic growers, they are still powerful chemicals. Instead, he suggests that home gardeners who see diseased leaves on plants cut off the foliage and dispose of it promptly in the trash. According to Homemeyer, the blight can only penetrate the leaf of a healthy plant if it's moist, so sunshine is the best preventive. Pruning tomato plants of excess foliage so summer breezes will dry the leaves more quickly is also an effective deterrent. Because the goal is to keep the foliage as dry as possible, do not water tomatoes with an overhead sprinkler; instead use a hand-held device such as a watering wand that guides a direct stream of water to the base of the plant. Finally, only water on sunny mornings when the moisture will quickly evaporate in the heat of the sun.

What to do if you have late blight

It is important to take the appropriate action immediately when late blight is confirmed. What is done this year if plants get late blight will not only affect other gardeners and farmers in the area but also will impact future growing seasons. This year's late blight is probably coming from infected potato tubers that were left in the ground last year and this cycle will likely continue for the coming years if the conditions are similar. 

NOFA Vermont instructs that home gardeners should clip off tomato and potato foliage and put it in a black garbage bag in the sun to kill the plant tissue. The bag can then be taken to the landfill or composted. However, composting diseased plants can be tricky for home gardeners. The compost should only be used on different crops or flowers, as several other tomato diseases persist in the soil and cool compost -- like early blight and Septoria leaf spot. Rotate tomato and potato beds next year into a section of the garden where you have plants from the nightshade family (tomatoes, potatoes, eggplant and peppers).
If potato foliage is mowed or clipped off immediately when late blight is found, potato tubers should be saved from this disease. However, gardeners should leave potato tubers in the ground for a few weeks to let their skins toughen before harvest. Gardeners who find some infection in their tubers are advised to cut it out and kill the infected tissue by freezing it before composting it so that infected volunteer potatoes will not sprout from the compost pile. Potatoes exposed to late blight can be consumed and sold, but they should not be saved for seed. Tomatoes exposed to late blight can be eaten but should not be canned, according to a report by the UVM Extension.

All photos courtesy of Dr. Meg McGrath, Cornell University.