Cause:
The fungus-like microorganism, Phytophthora infestans, survives in tubers in cull piles or left in harvested fields, on seed potatoes, or is blown in as spores with rainstorms. Spores (sporangia) form on infected culls, volunteers, or crop plants. Rain or irrigation spread sporangia to healthy tissue. Sporangia can germinate on a wet leaf or stem and penetrate either directly through the cuticle or a stomate. Under cool, wet conditions, sporangia produce swimming spores (zoospores). The leaf or stem is colonized quickly, and large portions become necrotic as the microbe advances. At the lesion's margin, new sporangia form through the stomata to continue the disease cycle. With favorable conditions, the pathogen can complete one cycle in as little as 4 days. Many cycles can occur in the growing season. Sporangia and/or zoospores also can infect tubers throughout their development or at harvest or in storage. If potatoes with late blight are stored, the blight can directly or indirectly damage tubers. Indirect damage occurs when secondary infection by soft rot bacteria rot the infected tuber and then spread to other tubers below in the pile.
In the 1990s, a second mating type was discovered in many areas of the Pacific Northwest. A different survival structure, the oospore, possibly may form when different strains (mating types A1 and A2) of the fungus come in contact. The impact of this on management strategies is being hotly debated and researched. If mating occurs in the PNW, P. infestans could overwinter in soil rather than just in plant tissue.
The disease is found principally in the Willamette Valley of Oregon and in northwestern Washington. Since 1991, yearly outbreaks have occurred in the Columbia Basin of Oregon and Washington. Sporadic occurrences have been detected in central Oregon. Recent outbreaks also have been reported in Idaho.
Symptoms:
Water-soaked spots appear on leaves and stems, enlarge rapidly, and turn brown or black. Under humid conditions, a ring or surface of white mold may appear on the leaf underside at the lesion's edge. In cool, wet weather, blight may advance rapidly and ruin an entire field in a few days.
Infected tubers' skin at first is discolored brown to purple; later there is a brownish dry or wet rot. Blight infections on tubers usually become visible within the first month of storage, but symptoms may continue to appear throughout the storage season.
Stem hits are common.
Late blight symptoms on leaves.
Sporulation on the leaf undersurface.
Note the discoloration in the tissue beneath the skin.
This tuber has late blight symptoms.
Sporangia of Phytophthora infestans
Cultivar
Resistance:
The cultivars 'Norchip', Hilite', 'Russet Norkotah',
'Superior', 'Shepody', and 'Red LaSoda' are more susceptible than
'Russet Burbank'. These fields need to be scouted early and protected
with fungicides. 'White Rose' and 'Russet Ranger' are similar to
'Russet Burbank' in susceptibility. 'Norkotah' and 'Shepody' are
especially susceptible to late-blight tuber rot.
Cultural
control: Regular scouting for the disease is critical
for developing all types of management strategies.
Eliminate cull piles from field or from storage before spring plantings. Bury under at least 2 ft of soil or feed to livestock prior to current year plantings.
Use certified disease-free seed. Do not buy seed from
areas known to have had late blight the year the seed was grown.
Adequately hill plants to minimize the opportunity
for sporangia to wash from the foliage into cracks or shallow areas of a hill and then to the tubers.
Control volunteer plants where possible.
Use irrigation management to limit disease spread. Don't water during rain storms or soon thereafter to reduce time water in on foliage. Use of solid set or wheel line systems for more than 6 hours is not recommended.
To minimize tuber infection and facilitate decomposition of infected tubers, do not dig potatoes from late-blight-infected fields until 14 days after frost or herbicides kill vines completely (no green stems).
Do not harvest when soil is excessively wet.
Provide adequate air flow in storage to prevent free water or surface moisture from forming on tubers. Late blight spores can spread in storage if temperatures are above 40° F and will infect new potatoes if there is free moisture on tubers for 12 hours or more.
West of the Cascades, potatoes planted early in spring and harvested before wet weather in fall are more likely to escape severe infection.
Chemical
control: Consult your local county extension office for information on materials registered under Section 18's. In Western Oregon and Washington apply protectant fungicides every 7 days starting at 6 inches of shoot growth. In the Columbia Basin, applications of protectant fungicides should begin just prior to between row closeure.
Use of Ridomil products will not provide control due to resistance in currently found strains.
If weather, such as heavy rain, favors late blight infection, apply at 5- to 7-day intervals. Some formulations may be applied by solid-set and circle sprinkler systems. Treat vines with protectant fungicides, even after die-down, as long as vines are still green. Several forecasting programs are available that use local weather data to help time applications, contact your local extension office for more information.
Acrobat 50WP in combination with other fungicide(s) except do not mix with metalaxyl or mefenoxam. Disease pressure and environmental conditions determine application rate and spray intervals. Do not apply within 4 days of harvest. 12-hr reentry.
Agri-Fos at 1.25 quarts/A in 90 to 375 gal water on 5- to 14-day intervals. May be tank-mixed with mefenoxam-containing fungi-cide registered for this use pattern. 4-hr reentry.
Alude at 1.25 quart/A on 5- to 14-day intervals. 4-hr reentry.
Chlorothalonil products such as:
Bravo Ultrex at 0.7 to 1.36 lb/A or Bravo WeatherStik at 0.75 to 1.5 pints/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
Echo 720 at 0.75 to 1.5 pints/A on 5- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
Echo 90 DF at 0.625 to 1.25 lb/A on 5- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
Products containing copper offer limited control.
Using copper products alone to control late blight is not
recommended.
Bordeaux 8-8-100.
Champ Formula 2 Flowable at 0.66 to 2.66 pints/A on 3- to 5-day intervals, starting when plants are 6 inches high. Suppresses Colorado potato beetle populations. 24-hr reentry.
C-O-C-S WDG at 1.5 to 4 lb/A. 24-hr reentry.
Cuprofix Ultra 40D at 0.75 to 3 lb/A on 7- to 10-intervals. 12-hr reentry.
Kocide 2000 at 0.75 to 3 lb/A on 10- to 14-day intervals. 24-hr reentry.e. Kop-R-Spray at 1.5 to 2 teaspoons/gal wa-ter.
Nu Cop 50 WP at 1 to 4 lb/A on 7- to 10-day intervals. Do not apply within 1 day of harvest. 24-hr reentry.
Curzate 60DF at 3.2 oz/A. Use only with a protectant fungicide—do not use alone. Do not apply within 14 days of harvest.
12-hr reentry.
Evito 480 SC at 3.8 fl oz/A on 7-day intervals. Tank-mix or alternate with another fungicide that has a different mode of action (non-group 11). Do not make more than one (1) application of Evito 480 SC or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days. 12-hr reentry.
Forum at 4 to 6 fl oz/A on 5- to 10-day intervals. Must be applied as a tank-mix with another fungicide that has a different mode of action (non-group 15). Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Do not apply within 4 days of harvest.
12-hour reentry.
Gem at 8 oz/A in a tank-mixture on 7- to 10-day intervals. Do not make more than two (2) sequential applications of Gem or other Group 11 fungicide before alternating to at least an equal number of sequential applications of a labeled fungicide with a different mode of action. Do not apply within 7 days of harvest. 12-hr reentry.
Headline at 6 to 9 fl oz/A prior to disease development on 7- to 14-day intervals. Do not make more than two (2) application of Headline or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 3 days. 12-hr reentry.
Mancozeb products such as Manzate 75 DF or Dithane F-45 Rainshield; see label for rates and intervals. Do not apply within 14 days of harvest or more than 15 lb/A of Manzate 75 DF or 11.2 quarts/A of Dithane F-45 Rainshield per
season. 24-hr reentry.
Maneb 75 DF at 2 lb/A. Do not apply within 14 days of
harvest. 24-hr reentry.
Omega 500F at 5.5 fl oz/A on 7- to 10-day intervals. Do not apply within 14 days of harvest.
48-hr reentry.
Previcur Flex at no more than 1.2 pints/A on 7- to 10-day intervals in a tank-mixture with chlorothalonil, maneb, or mancozeb. Also controls early blight. Do not apply within 14 days of harvest. 12-hr reentry.
Polyram 80 DF at 2 lb/A. 24-hr reentry.
Quadris at 12 fl oz/A on 7-day intervals. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 4-hr reentry.
Quadris Opti at 1.6 pints/A on 7- to 14-day intervals. Do not make more than one (1) foliar application of Quadris Opti or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest
interval is 14 days. 12-hr reentry.
Ranman at 1.4 to 2.75 fl oz/A on 7- to 10-day intervals. Do not make more than three (3) applications of Ranman (Group 21 fungicide) before alternating for three intervals to a labeled fungicide with a different mode of action. Preharvest interval is 7 days. 12-hr reentry.
Reason 500 SC at 5.5 to 8.2 fl oz/A on 5- to 10-day intervals. Do not make more than one (1) application of Reason 500 SC or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Do not apply within
14 days of harvest. 12-hr reentry.
Revus Top at 5.5 to 7 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) foliar applications of Revus Top before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
Ridomil Gold Bravo SC at 2.5 pints/A. Do not use within 14 days of harvest. 48-hr reentry.
Ridomil Gold MZ at 2.5 lb/A on 14-day intervals. Do not apply within 14 days of harvest. 48-hr reentry.
Super Tin 80 WP at 1.87 to 3.75 oz/A. Use only in combination with chlorothalonil, mancozeb, or maneb products, or Polyram 80 DF. Do not apply within 7 days of harvest.
48-hr reentry. Restricted-use pesticide.
Tanos at 6 to 8 oz/A on 5- to 7-day intervals. Must be tank-mixed with an appropriate contact fungicide with a different mode of action (e.g. mancozeb, chlorothalonil etc.). Follow all tank-mix partner label restrictions and use at least minimum label rates of each. Do not apply within 14 days of harvest. 12-hr reentry.
Topaz at 1 to 3 quarts/A in 100 gal water on 2- to 3-week
intervals. 4-hr reentry.
Seed treatments can reduce the chance of spread of late blight from seed and prevent spread of late blight between seed pieces during cutting, treating, and handling operations.
Evolve (Tops-MZ+Curzate) Potato Seed-Piece Treatment at 0.75 lb/100 lb seed-pieces.
Tops MZ at 0.75 to 1 lb/100 lb seed-pieces. Tops MZ-Gaucho is registered for use by commercial seed
treaters only.
