Most of this information comes from Idaho's 1999 Potato Late
Blight Action Plan. The information given herein is supplied with the understanding
that no discrimination is intended and no endorsement by the University of Idaho
Cooperative Extension System is implied.
- Development and Use of this Action Plan
This action plan was developed in cooperation with the Idaho potato industry in
consultation with late blight experts in the Pacific Northwest, and other areas of the
United States. It is hoped this information will help the Idaho potato industry reduce
potential yield and quality losses caused by late blight. This action plan should also
help in making informed decisions about disease prevention, fungicide spray
programs, and minimizing disease management costs. Additional information about
late blight is contained in University of Idaho Extension publication CIS-1051 "Late
Blight of Potato and Tomato" available at local University of Idaho Extension Offices.
- Occurrence in Idaho
Late blight is the most important disease of potatoes on a worldwide basis. Prior to
1995, only one isolated case of late blight had been reported in Idaho. However, late
blight appeared in numerous fields throughout southwest Idaho in July 1995, and later
spread to south central Idaho. The late blight found in 1995 was the US-8 genotype,
which is very aggressive and resistant to the systemic fungicide mefenoxam (Ridomil).
The US-8 late blight genotype also infected more than 30 fields in south central and
southwest Idaho in 1996. Late blight was reported in eastern Idaho for the first time
in 1997 and was widespread in this area as well as the central production region.
Genotypes found in Idaho in 1997 included US-6, US-8, and US-11. The later two
are extremely aggressive and were found most frequently.
The two genotypes mainly found in Idaho in 1998 were US-8 and US-11. About 80
percent of the late blight found in Idaho in 1998 was US-8. Central and southeast
Idaho had numerous fields infected with late blight, whereas, western Idaho had a
limited number of infected fields.
- Spreading of Late Blight
Potatoes may be exposed to late blight during the growing season from inoculum
produced on infected cull piles, volunteer potato plants, or the disease may originate
from infected seed. Also, tomato transplants in home gardens may be infected with
late blight. Under the right conditions (see Avoid conditions that favor late blight),
spores from infected plants can be carried in moist air, such as thunderstorms, for
miles. These spores, under favorable conditions, infect healthy plants, thus
spreading the disease. Currently, no labeled chemicals will kill the late blight fungal
strains that have recently been found once they become established in a plant.
Because of this, it is important that everyone in the potato industry develops a "late
blight prevention attitude." An effective prevention program includes implementing
cultural and chemical management practices that reduce the potential for occurrence,
spread, and losses from late blight.