Late blight is caused by a fungus that attacks all the
plants in the botanical family Solanaceae, which includes tomatoes,
potatoes, eggplant, peppers and tomatillos.
The fungus has two presentations: asexual zoo spores which
are parasitic and require living plant matter to survive and
reproduce; and sexual oo spores which can live in the soil without a host. Presently, only the zoo spores have been
identified in eastern Idaho but it is very likely that the oo spores will turn up here in the not too distant future.
The most effective action against the zoo spores is the
collection and burning of all plant parts including roots
and dropped fruit. See Maintenance Recommendations below for information to reduce effect of oo spores.
Maintenance recommendations to reduce re-introduction or spread of blight:
PLANT ROTATION: rotating the area of your garden in which
you grow plants susceptible to late blight
will decrease the likelihood that they will be re-infected by the soil dwelling oo spores. Be sure you plant in
an area where none of the several plants in the Solanaceae family were grown the previous year, i.e. move
your potatoes to an area where last year you grew beans not tomatoes or peppers.
PLANTING DISTANCE: because the fungus likes warm, moist
environments you can make it more
difficult for it to survive by leaving more space between your individual plants and by staking your tomatoes.
WATERING METHOD: avoid overhead sprinkling as much as
possible as this creates a moist and friendly
environment for the fungus. Drip irrigating is optimal.
PRUNING: Keep a careful watch for signs of blight on your
plants and immediately remove and destroy by
burning leaves, stems and fruit which appear infected.
TOOL SANITATION: clean tools (spades, clippers gloves,
etc.) by dipping them in a 9/1 water to bleach
solution after using them on infected plants to avoid contaminating other plants.
VOLUNTEER PLANTS: Remove and destroy all volunteer plants
( which may harbor the spores) in the
Tubers should be washed and thoroughly dried before storage
(this reduces the possibility of oo spores
wintering over in the soil.
Before storage tubers should be carefully examined for
any evidence of closed lesions - sunken wrinkled
spots on the potato surface which reveal when cut open a thin black layer just under the skin surface.
Destroy any that are infected.
Regular examination of stored tubers throughout the winter
to cull any that have developed the lesions.
Potatoes that are infected with late blight are significantly more vulnerable to other potato storage problems
like soft rot.
When using stored potatoes for spring planting great care
should be taken to ensure that no tubers are
planted which show any signs of lesions. Examine the cut seeds especially carefully and destroy any that are
even slightly suspicious. (Might be easier and less risky to just start with new seed potatoes for the season!)