Late Blight of Potatoes in Ontario
Background
Late blight, caused by the fungus Phytophthora infestans, is a devastating
disease of tomatoes and potatoes. If the weather is wet and no effective
control methods are applied, late blight can completely destroy the canopy
of potato field in a week. Tubers may become infected if spores are washed
into cracks in the soil and come in contact with tubers.
Management Strategies
The best defense against late blight is to keep the disease from establishing
in the field. The fungus survives only in living tissue. The first late
blight outbreaks are usually caused by infected seed. Late blight may
also start from diseased tubers in cull piles or from volunteers in field
that grew potatoes the previous year.
Late blight can be spread long distances by wind, especially during rain
storms. This wind-borne infection pops up in scattered parts of fields.
Growers should start spraying fungicides early, when the plants are 10-15
cm tall, in order to cover the stems. Regular spraying is necessary to
protect the plants as they grow. Regular scouting is also necessary to
detect late blight early.
The 2009 Season
Last year, late blight developed in all seed production areas in Canada,
so the risk of planting infected seed this year was high. The growing
season has been cool and wet, great for the potato crop but also just
the weather that favors the development of late blight.
The first late blight outbreak was found on July 8 in a field near Grand
Bend. Stem lesions and radial spread indicated that the disease was seed-borne.
On July 9, a second outbreak was detected in the Alliston area that also
looked to be seed-borne. An intensive spray program has kept these two
outbreaks under control.
Near the end of July, a 30-acre infected field was found near Delhi.
The field had not been sprayed before a wet period, and late blight killed
the canopy in a week. The grower started harvesting the field this week
and runs the risk of infected tubers, but the crop is not a total loss.
By the beginning of August, wind-borne late blight was turning up in
small areas of potato and tomato fields in many areas of the province.
Intensifying the spray programs in commercial potato fields has kept these
outbreaks under control. .
The main concern is organic tomatoes and potatoes because there are no
effective late-blight fungicides allowed. Organic growers must pull and
compost infected plants, but the disease can spread quickly and overwhelm
growers. Several organic tomato and potato fields in Ontario are now infected.
Economic Losses
Due to the wet weather and the late blight risk, growers have had no
alternative but to follow a tight fungicide program. It is expensive to
spray every 5-7days and to use late-blight specific fungicides that are
more expensive than the regular protectant fungicides, but it is necessary.
The added costs will result in economic losses this year for many potato
growers.
Late blight lesion on underside of leaf
