of my tomatoes died from the late blight that was so prevalent last
year. I removed the plants and cleaned up as well as possible, but
is there anything else I can do now to avoid a repeat of last year’s
disaster? Should I treat my soil with something?
A. Last year’s late blight
epidemic resulted from a perfect storm of environmental factors and
a high level of infectious late blight spores in the air.
Apparently, infected transplants were unknowingly sold by a
mega-grower in the South to big box stores and independent garden
centers throughout the Northeast. These transplants were sold and
planted in home gardens throughout, and favorable weather conditions
made the disease run rampant.
Even commercial growers who tend to
make preventative fungicide applications when conditions favor
late blight development may have been caught off guard since it
arrived so early in the growing season.
Ripened tomato affected by late blight
The fungus that causes late blight,
Phytophthora infestans, requires live tissue to overwinter in
our climate. The only way it survives winter in western Pennsylvania
is on infected potato tubers that have been overlooked at
harvest or in commercial potato cull piles. It does not survive on
dead tomato plants or related weeds common in area, such as climbing
nightshade (Solanum dulcamara) or American black nightshade (Solanum
americanum). The fungus does survive the winter in the southern
United States and gets blown to the north on summer winds. Spores
can easily move 40 miles in a day when weather conditions are
PLANT DISEASE TRIANGLE
Plant pathologists refer to device called the plant disease
triangle to explain disease development. Three things are needed
for disease to develop: a susceptible host (tomato or potato), the
pathogen (Phytophthora infestans) and a favorable environment
(moderately warm, humid, wet weather). Disease control requires
interfering with at least one leg of the triangle. Acting on the
susceptible plant would require a tomato resistant to late blight.
Although some varieties tout resistance to late blight, none are
considered reliably resistant, especially when conditions are as
favorable as they were last summer. Although you cannot stop the
rain or make the sun shine, you can provide optimum growing
conditions by testing the soil and amending it according to the
results, watering properly and controlling weeds to permit greater
air circulation. These are ways to change the environment to favor
the plants more than the disease to some degree. Acting on the
pathogen includes good garden sanitation because it removes a lot of
spores and reduces their number in the garden to cause disease in
the future. Applying a fungicide is another way to act on the
is good that you cleaned out your garden thoroughly last fall.
There are other common tomato diseases such as septoria leaf spot
and early blight that DO overwinter on diseased plant material left
in the garden. If you also grow potatoes, be sure to promptly
pull any overlooked tubers that sprout this spring or through the
summer. Destroy them or send them out with the trash, but do
not compost them.
BASICS FOR HEALTHY
Avoid getting the foliage wet when you water as much as
possible. Soaker hoses and drip irrigation are ideal because they
put the water on the soil - where you want it - and keep it off the
foliage – where you do not want it. If those are not an option, at
least use a watering wand and keep it as is close the soil as
possible without actually sticking the head into it. If you use a
watering wand, try to water early in the morning so the foliage
dries quickly when the sun comes up. That is less critical when
using drip or soaker irrigation since it does not wet the foliage to
start with. True, plants get wet when it rains or when there is
heavy dew, but you do not have to add to the problem by soaking the
foliage when you water. The longer tomato and potato foliage stays
wet, the greater the chance of disease, including but not limited to
Mulching around the base of plants reduces the number of
spores that can be splashed up onto plants by rainfall or watering.
While this is not important for late blight control, it can help
with early blight and septoria leaf spot. Organic mulches such as
clean oat straw or leaf mold have the added benefit of adding
organic matter to the soil as they break down. Plastic mulches –
especially red plastic mulch – have been shown to increase tomato
yields, and they also function to keep infectious spores from being
splashed onto susceptible foliage.
Foliar symptoms on tomatoes
There is nothing you can apply to
your soil that would keep your tomatoes from getting late blight
later in the summer. Unless there are some overlooked potatoes
there, the fungus is no longer present. It will be blown north on
wind currents later in the season. Since the fungicides available to
home gardeners are best applied preventatively – before symptoms
develop – it is wise to spray when weather conditions favor late
blight development. You can make preventative fungicide applications
when cloudy, wet, and moderately warm weather conditions favor
disease development. Follow label directions for mixing rates and
application frequency. You may be able to turn things around if you
catch late blight in the very earliest stage of development and
begin spraying, but you cannot “cure” a plant that is even
moderately infected. Even though commercial growers have more
fungicides to choose from, many of those products cannot save a
moderately infected crop.
Conventional gardeners can use Bonide Fung-onil, Ortho Garden
Disease Control, and other chlorothalonil products labeled to
control late blight on tomatoes and potatoes. Organic gardeners can
use fixed copper fungicides, but they are not considered as
effective against late blight as chlorothalonil. Always read and
follow label instructions.
HOW'S YOUR SOIL?
Penn State soil test kits are available for $12 each. If you
purchase more than one kit at a time, additional kits are $9 each.
Please make checks payable to Penn State Cooperative Extension (PSCE)
and indicate the number of kits you would like. Send your request to
Soil Test Kits, Penn State Cooperative Extension, 400 N. Lexington
Street, Pittsburgh, PA 15208.
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