Information About Late Blight
Phytophthora infestans affects both potato and tomato
plants. The strain in 2009 was the same as caused the infamous Irish
potato famine in the 1840s.
Late blight is a systemic infection. At maturity it releases wind-borne
spores that can travel 30 miles. Both its progression and rate of
infestation are weather-dependent, worsening rapidly in wet weather,
slowly in warm and dry.
Early blight shows up as drying and dying leaves at the bottom
of the plant. EB can be managed culturally, should not result in
significant loss of crop.
Late blight usually starts at the top of the plant or on the windward
side. As the earliest infection arrives on warm southerly winds,
monitor the south/ southeast side of the garden carefully. Later
in the season the wind can bring it from any direction. Early in
the season late blight infection shows as roundish lesions on leaves
that uniquely will cross the center vein of the leaf. Later, blotches
appear on stems. Still later, hard crusty lesions form on fruits.
PREVENTING LATE BLIGHT:
• Destroy, do not compost, any infected plants, fruits or
• If you compost any diseased material, do not use that compost
the next year.
• Late blight on tomatoes is not seed-borne. Saved tomato
seed is okay if you fermented it properly.
For Next Year:
• Grow your own tomato plants or buy locally grown seedlings.
Know your farmer!
• Do not use saved potatoes as seed stock.
• Purchase only new certified-disease-free seed potatoes.
• Destroy all potato volunteers!
Amy LeBlanc’s Successful
Here’s a distillation. For more information, contact Amy
at Tomato Lovers Paradise, email@example.com. We managed to
miss the early round of blight as we grow all our own seedlings.
However, it did arrive on the wind. Around July 18, I decided I
was not going to lose my tomato crop. That meant buying two new
hand-held sprayers and an NOP-approved copper fungicide formulation,
Available information was incomplete and conflicting. The most
prevalent advice was “It’s terminal. Kill all your plants.”
Instead I read all the references I could find about tomato diseases.
The best information came from The Compendium of Tomato Diseases
(Jones, Jones, Stall & Zitter, APS Press, ISBN 0-89054-120-5),
from Cornell’s information site (see sidebar below) and from
Dr. Bruce Watt at U. of Maine Plant Diagnostic Lab.
I monitored twice every day, and trained my workers to see the
difference between early and late blights and to attack LB immediately
and sytematically. We pruned extensively, clipping off every infected
leaf and stem below every stem lesion, making sure the remaining
pith was clear green. It’s important to take all infected
material to the dump—not the compost pile. We followed each
pruning with a copper spray (a total of five times at the recommended
5–10 day interval). The spray protected the fruit (which is
damaged by surface spores rather than systemically). We harvested
fruit as it began to ripen, washing with soap and water to remove
the copper and spores, and it continued to ripen. We stayed at least
one step ahead of the infection and we had a fabulous tomato crop.
Many of the protocols recommended by experts are farm-sized solutions.
Small-scale gardeners can attend our plants daily. Consistent observation
and action will see us through most problems.