Late Blight

Late Blight:Guidance for this year's garden

Phytophthora infestans or late blight, the disease in the 1840’s that destroyed the Irish potato crop that led to a terrible famine and mass emigration, infected potatoes and tomatoes in many of Vermont gardens and farms last summer destroying farmers’ crop and their income. Late blight doesn’t over winter in Vermont. It enters the state by either blowing in on southern storms from infected areas or traveling into the state on infected plants. In 2009, late blight came into the state
on infected plants sold at big box stores and on storm fronts. Many gardeners have asked what they can do to help prevent late blight problems this garden season. Follow the steps below to help prevent problems in your garden.

Check Seed Potatoes for Blight before Planting
If you had late blight in your potatoes this year and you save potatoes year-after-year, be vigilant
about checking them in storage to ensure your home-grown seed potatoes are free of blight before
you plant them. Check them frequently in storage and remove any funky, black sunken, or corky
looking potatoes. If there is any question about them, don’t use them. Destroy any infected potatoes
you remove from your root cellar by burning them, freezing them or feeding them to livestock. If
you composted blight infected potatoes this fall and winter, scout early and often to identify and
remove any potato volunteers from your compost pile.

Buy Certified Seed Potatoes
To reduce the chance of using blight infected spuds buy Certified Seed potatoes. Certified Seed is
different from Certified Organic. Certified Seed means the potatoes are inspected by the industry
and grown to a quality specifically for planting; these seed potatoes are less likely to have diseases.

Buy Local Transplants
Buy tomato transplants from local, organic farmers; know the people growing your plants. The
Vermont Master Gardener program has created a web list of Vermont growers selling tomato starts
that you can find at: http://www.uvm.edu/mastergardener/. Since the disease does not survive in
seed and will not overwinter here, our transplants grown in Vermont should be free of blight. If
you buy transplants from stores that purchase plants from southern distributors, ask where the
transplants are from so you can determine if the plants are grown in an area that has late blight
present. Check transplants carefully for diseases before you purchase them.

Use Resistant Varieties
Resistant varieties can help slow or prevent the spread of late blight; these can be especially helpful if
you live in an area that consistently has the cool temperatures and leaf wetting conditions—a lot of
rain, dew and fog—required conditions for late blight. There are some potato varieties that have 
foliar and/or tuber resistance to blight. When growing your own tomato transplants, use varieties
that have some resistance to blight. Below are charts of varieties that show some resistance.

Rotate Crops
If you haven’t been rotating crops in your garden, this year is the best time to start. Crop rotation is
the first strategy for good organic management. Rotations can help build soil, increase yields, and
break pest cycles. The basic principle is to rotate out of crop families to other crops that do not share
pests and diseases. For example, rotate out of potatoes into another non-Solanaceous plant family,
which means you will not grow potatoes, tomatoes, peppers, eggplant or petunias in last years potato
bed. Choose an open canopy crop so you can see and remove the volunteers and over-wintering
tubers easily. Late blight does not infect tomato seeds, but control tomato volunteers so your
rotation really is a rotation. If you have significant build-up of a disease and it is economically
feasible, stop growing susceptible crops for one year to reduce future problems. Many diseases and
insects are able to build-up in garden soil when crops are grown in the same place year after year—
move them around!

Ensure Plants Get Proper Nutrition
Plants that get proper nutrition are healthier and have more vigor which can help them grow and
produce better and resist some pests and diseases. Test your soil so you know if your garden soil pH
must be adjusted with lime, has any nutrient deficiencies or low organic matter, or has excessive
amounts of nutrients, trace elements, and/or heavy metals. Choose amendments that help you
improve your soil’s physical, chemical and biological condition without over-applying nutrients.
Excessive nitrogen makes crops more susceptible to insects and diseases.

Reduce Leaf Wetness
If you have been crowding plants for years and calling it “intensive gardening” consider spacing
crops for more air flow to reduce leaf wetness and prevent diseases. Most fungal disease spores
require 6-8 hours of leaf wetness to germinate; wetness can be from watering, rain, dew, or morning
fog. When plants are crowded, the foliage cannot dry out, which extends the number of hours the
leaves stay wet. Good spacing helps plant foliage dry out. Pruning suckers off of indeterminate
tomatoes will also reduce foliage density and improve air flow. Also, if you need to water using a
method that wets the foliage, water in the morning so foliage has all day to dry.

Monitor for Late Blight
So many gardeners and farmers had late blight on tomatoes and potatoes in 2009 that everyone
should be especially vigilant looking for it on tomatoes and potatoes in 2010. This is especially true
if weather conditions are cool and wet. If you find late blight lesions on your crops, remove and
destroy tomato plants, and clip off and remove potato vines to prevent the spread of this disease.
One lesion can produce 100,000 to 300,000 spores per day! Wait 2-3 weeks before digging up
tubers; the tubers in the soil develop a tougher skin. Please let NOFA-VT or UVM Extension people
know so we can warn others late blight is in the state as early as possible.

Good management techniques can help gardeners and farmers alike have a successful growing season
and a bountiful harvest.
Many thanks to Vern Grubinger and Ann Hazelrigg of UVM Extension and Jean-Paul Courtens of Roxbury
Farm for their helpful suggestions.

Resources

Photo Galleries

McGrath, Margaret Tuttle. Late blight on potato. Cornell University Extension.
http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/...

McGrath, Margaret Tuttle. Late blight on tomato. Cornell University Extension.
http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/...

The following resources were used to write this article and provide more detailed information on late blight.

Cornell University, Plant Diagnostic Clinic. Fact Sheet Late Blight.
http://plantclinic.cornell.edu/FactSheets/lateblight/late.htm

Fry, William E. 1998. Late blight of potato and tomato. Cornell University Extension.
http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm

Grubinger, Vern. 2009. University of Vermont Extension. Late blight on potato and tomato in 2009. http://www.uvm.edu/vtvegandberry/factsheets/LateBlight2009.pdf

Hazzard, Ruth. 2009.University of Massachusetts Extension. Late blight management for fall, winter and spring -A Gardeners’ Checklist-.
http://www.umassvegetable.org/LateBlightAlertforTomatoandPotato.html

McGrath, Margaret Tuttle. 2009. Late blight: corrections of misinformation. Cornell University Extension. http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/lbmisinfo.pdf

McGrath, Margaret Tuttle. 2009. Late blight frequently asked questions by gardeners, growers & consumers. Cornell University Extension.
http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/lbmisin...

McGrath, Margaret Tuttle. Managing late blight organically produced potato. Cornell University Extension.
http://nofany.org/policies/hottopics/managing_late_blight_organically_pr...

McGrath, Margaret Tuttle. Managing late blight in organically produced tomato. Cornell University Extension.
http://www.nofany.org/policies/hottopics/managing_late_blight_organicall...

Mishanec, John. 2009. Cornell Extension. IPM Vegetable IPM Program. Pest Report 8/18/09

Stone, Alex. 2009. Organic management of late blight of potato and tomato (Phytophthora infestans). Oregon State University. E-Organic. http://www.extension.org/article/18361