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The 2005 potato challenge: late blight infected seed

Late blight, caused by the fungus Phytophthora infestans, is the most
serious disease of potatoes. It develops very quickly in wet weather and can
destroy potato fields in a few days if effective fungicides are not applied.

Unfortunately, 2004 was a 'late blight year' across North America. The wet
weather that prevailed for most of the growing season provided favourable conditions
for disease development. Potato growers had no alternative but to shorten spray
intervals in order to protect the crop. This resulted in a significant increase
in the cost of production.

The legacy of a 'late blight year' is a considerable increase in the sources
of infection: infected seed, cull piles and volunteer plants.

Although it is not recommended to plant infected seed, late blight is not a
quarantine disease. Tagged seed may have up to two percent of the tubers infected
with late blight. If tuber infection occurred shortly before harvest, infected
tubers may not show symptoms when dug. Late blight progresses slowly in storage
because seed is stored at approximately 40 degrees C. Infected seed tubers shipped
early in the spring may not show the symptoms of late blight but they may spread
the disease to healthy tubers when the seed is warmed up before planting.

It is always safest to assume that all seed lots have at least some tubers
infected with late blight and to take precautions.

As soon as the seed is received, store it at temperatures less than 45 degrees
F if possible. Then warm and cut the seed just before tubers begin to sprout.
At temperatures above 45 degrees F, the fungus will produce spores readily on
the surface of infected tubers. These spores will infect healthy seed tubers.
The spores produced on the surface of infected tubers are a major source of
inoculum.

Grade seed slowly and carefully
Grade out any suspicious tubers when cutting. Late blight causes a reddish brown,
granular dry rot in the tuber. Often, infected tubers are invaded by soft rot
bacteria. Soft rot turns the tubers into mush, making it impossible to determine
if the tuber was, in fact, infected with late blight. There is no quick test
currently available to identify tuber infection. AGDIA, an American company,
is developing an Inmuno Strip test to determine in a few minutes if tubers are
infected with late blight, but the kit is not likely to be available before
planting in 2005.

Do not mix seed lots during cutting and planting
Keeping seed lots separate will reduce the chance of spreading the disease over
a larger area. Keep a record of where each seed lot is planted particularly
if lots are planted in several fields or parts of fields. Clean and disinfect
equipment between seed lots. This will minimize the risk of late blight spread.

Dispose of infected tubers properly
Spring is a busy season, and a pile of infected tubers or seed pieces may be
forgotten in the rush. Once infected tubers warm up, they start to produce spores
that will disperse to healthy potatoes. Night temperatures in the spring are
usually not low enough to freeze tubers solid. Destroying culls and waste tubers
every day eliminates important sources of infection during the season.

Use seed treatments containing mancozeb
Seed treatments that contain mancozeb reduce the spread of late blight during
seed handling and cutting, but they do not cure infected seed. Also, mancozeb
does not move up the shoots to protect emerged plants. Plan to start spraying
early.

Try to match the seed lot to the field
If
possible, plant potentially-infected seed in fields with well-drained, sandy
soil. Avoid fields with heavier soil that may remain wet for extended periods
after heavy rainfall. There is nothing like humidity to speed the establishment
and spread of late blight.

After a 'late blight year' it is difficult to certify that a seed lot is free
of late blight. These management practices should help to reduce the risks associated
with planting infected seed.

Dr. Eugenia Banks is a potato specialist with the Ontario Ministry
of Agriculture and Food.