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.
{loadnavigation}
