August 12 Pest Report From MOFGA
Sorry for my absence from our virtual community garden. I was on vacation in…Egads!…a place without internet access. More on that later, but I wanted to pass along the latest Pest Report from MOFGA on Late Blight.
LATE BLIGHT
What to do with a field infected with late blight
[Reprinted and modified a bit from an article by Becky Grube Sideman, UNH
Cooperative Extension, Sustainable Horticulture Specialist]
Late blight is now in potato and tomato fields throughout the state. From
what I have heard, those using preventative fungicide programs have kept
the disease at bay thus far, but many who did not apply fungicides are
having fields go down. Once symptoms are widespread in the field, it is the
time to cut your losses and kill the plants to prevent the disease from
spreading into other fields or high tunnels. For tomatoes, this will mean a
crop loss. For potatoes, however, it may still be possible to get a decent
crop.
Killing potato vines before harvest will reduce the chance of infecting
tubers. Late blight spores do not survive on dead vegetation. Vines can be
killed by mowing, burning, or using dessicants (herbicides). Mowing can be a
challenge in a hilled potato field. After killing the vines, wait at least
two weeks before harvesting to give the vines time to die completely. This
helps avoid dragging the potatoes through spore laden foliage. The ground
is a good place to store potatoes while waiting to dig them; it is cool,
moist and dark, and the skins will harden on those tubers that are not
already infected. That said, ground storage poses some risks: wireworms,
grubs, and rodents may find and feed on potatoes, and the longer the tubers
stay in the ground, the higher the risk. Digging test hills in a few
locations to check for evidence of these pests may help you evaluate the
tradeoffs.
Harvest when soil is not excessively wet, and do not wash tubers. Tubers
should be dry when placed in storage. Remove any obviously infected tubers
before storage. Curing tubers (10-20 days at 50-60 F, 95% relative humidity)
generally heals wounds and help tubers store longer. These conditions will
also cause those tubers infected with late blight to decay rapidly, so that
you can cull rotted tubers. In storage, forced air ventilation through the
storage bin can help minimize spread from tuber to tuber. Storage of
potatoes with small amounts of late blight should be at 38 F to retard late
blight tuber rot. If you must wash tubers before sale, organic growers can
use Storox (Oxidate) or chlorine (dilute to 4ppm before discharge) at
labeled rates in wash water.
Some common questions…
How do I get rid of infected tubers? If you end up with infected tubers,
it’s important to dispose of them properly. Do not make cull piles. In a
cull pile, the late blight pathogen can survive the winter and be a source
of inoculum early next year. No matter how you dispose of tubers, carefully
monitor for (and destroy) and volunteer sprouting potatoes next spring.
Options for safe disposal include the following:
Burial. Trenches filled will culls should be covered with at least 18” of
clean soil to prevent tubers from sprouting.
Livestock feed. This can be done during late fall and winter, when freezing
temperatures will kill tubers that are not eaten within one day. During
summer, this is not recommended unless all culls are eaten within one day.
Composting. Compost piles must be turned and mixed routinely and may require
special equipment for large piles. The University of Maine fact sheet
‘Composting Cull Potatoes’ has more detailed information:
http://www.umaine.edu/umext/potatoprogram/Fact%20Sheets/Composting%20Cull%20
Potatoes.pdf
Spreading. Cull potatoes may be spread during late fall and early winter on
fields that are not destined to be potato fields the next year. Culls should
be left on the surface to ensure they are killed by freezing temperatures.
Can I save my potatoes for seed? If you have late blight symptoms in your
field, saving seed tubers from that field is very risky. Even when there’s
no evidence of tuber blight, up to 20% of the tubers in a seed lot can have
latent infections with the late blight pathogen. Tubers with latent
infections can cause foliar symptoms very early in the season, and set you
up for needing to apply foliar late blight fungicides. The best way to avoid
seedborne transmission of late blight is to purchase certified disease-free
seed next year and make sure to scout for and remove any volunteer potato
plants that come up next spring.
My tomato plants are going down, but fruit look OK. Can I harvest and sell
healthy fruit? Once plants are infected, symptoms continue to develop in
fruit even after they are harvested. In previous years, it has not been
uncommon to see infected fruits in markets. Fruits harvested from infected
plants are definitely at risk of developing symptoms during storage before
they are sold and consumed.
Information compiled from many sources including Universities of Vermont,
Maine, Massachusetts, and Idaho.
