Potato Blight

[Carl Russell]

Just received this from NOFA-VT

Dear growers and gardeners,

We’re so sorry to bring the news that late blight has been confirmed both in Hadley, Massachusetts and in Waldoboro, Maine.

The strains of late blight we have had in New England in the past were intolerant of hot weather. However, the strain we saw last year in Vermont and present throughout NE seems to tolerate warmer conditions. Pathologists are suspecting this warmer weather strain is the one reappearing this season. It is likely it is coming from infected overwintered tubers. The recent scattered thunderstorms and unsettled weather are likely contributing to the outbreaks and spread of the disease.

Wet weather – especially afternoon and evening rain with morning fog, which allows the foliage to stay wet for more than 6 hours – is perfect for spore production, which can spread the disease.

Commercial growers should scout diligently, and watch the weather. Ann Hazelrigg at UVM recommends preventative spraying of tomato and potato crops. Nu Cop 50 WP and Champ WG are copper fungicides approved by OMRI, and have the Agricultural Use Requirement box on the label, so farmers with employees can use these materials. Please read the label and wear proper personal protective equipment.

For home and community gardeners, we do not recommend spraying copper fungicides. Please maintain practices that help the plant foliage dry out faster. Prune off suckers, remove excessive foliage, and even consider removing plants if they are over-crowded. ATTRA recommends compost tea sprayed as a protectant, but plan to spray in the morning on a sunny day so the leaves have time to dry out. Biological sprays, like Serenade, that put beneficial organisms on the foliage may also be helpful to gardeners.

Not sure what late blight looks like? Cornell’s Long Island Horticultural Research and Extension Center has high-quality photos of infected tomato and potato plants. Be sure to scout your tomato and potato plants rigorously and often.

If you suspect late blight, please send a sample of several leaves of the suspect foliage to the UVM diagnostic clinic immediately for confirmation. It is extremely important to get lab diagnosis in order to accurately track the movement of this disease.

Send your samples to:
Plant Diagnostic Clinic
Jeffords Hall
63 Carrigan Drive, UVM
Burlington, VT 05405

Until late blight is found in Vermont, home and community gardeners can send tomato and potato samples that may have late blight to the Plant Diagnostic Clinic for free -usually this costs $15. Please look at the photo links to late blight in potatoes and tomatoes before sending in your sample, and please visit the Clinic website for directions on how to prepare your sample.

If late blight is confirmed on your plants, destroy them immediately. Late blight can only survive on living tissue. Once the plant is dead, late blight dies too. Remember that one late blight lesion can produce 100,000 to 300,000 spores per day.

Potato tubers are infected with late blight when spores wash off plant foliage and wash through the soil to the tubers. Commercial growers should mow infected potato foliage and wait two to three weeks before harvesting. Tomato fields should be plowed under. Growers can also clip off tomato and potato foliage and put it under tarps in the sun to kill it. Once the foliage is dead it can be composted, but plan to use this compost on different crops or flowers, as several other tomato disease persist in the soil and cool compost–like early blight and Septoria leaf spot. Rotate out of tomatoes and potatoes next year.

Gardeners should clip off tomato and potato foliage and put it in a black garbage bag in the sun to kill the plant tissue. You can take the bag to the landfill, or compost dead plants per our instructions above. Leave potato tubers in the ground for a few weeks to let their skins toughen before harvest.

If potato foliage is mowed or clipped off immediately when late blight is found, your potato tubers should be saved from this disease. If you find some infection in your tubers, cut it out and kill the infected tissue by freezing it before composting it, so that infected volunteer potatoes will not sprout from your compost pile. Potatoes exposed to late blight can be consumed and sold, but they should not be saved for seed.

Please alert the NOFA office if you find late blight in your garden or farm so that we can help spread the word.

Thank you for your vigilance.

With fingers crossed for dry weather,

Wendy Sue Harper, Ph.D.
NOFA Vermont Vegetable and Fruit Technical Advisor
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Carl

[Tim Harrigan]

http://www.pestid.msu.edu/DiagnosticFactsheets/tabid/218/Default.aspx

Here is a link to some updated plant disease info from MSU. There is a new late blight fact sheet with some pictures in the ‘diagnostic factsheets’ section. Might come in handy next year…

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