Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Potato (Solanum tuberosum) -- Early Blight
 
Cause: Alternaria solani, a fungus that overwinters as viable mycelium and spores in infected crop residue. Heavy dews and frequent rains seem essential for severe outbreaks. The disease is found primarily under sprinkler irrigation.
Symptoms: First, oval or angular dark brown to black "target" spots appear on leaflets. Usually a narrow chlorotic zone is around the spot which fades into the normal green. Lowest, oldest leaves are infected first, and they droop and dry as the disease progresses. If spots are numerous, they kill leaves and consequently may reduce yield.

Tuber infection is less frequent than leaf infection. Tuber lesions are sunken spots, brown to black, from 0.12 inch to 0.37 inch in diameter, and normally circular. The margin between diseased and healthy tubers is usually quite well defined. Underlying tissue shows a brown, corky, dry rot more than 0.12 inch deep.

Note the small, brown, slightly angular lesions on the foliage.

'Ranger Russet' with severe early blight symptoms.

Classical early blight lesions on potato leaflets.

Both early blight and late blight lesions on can be seen on these potato leaves.

Early blight lesions on a potato tuber.

Early blight symptoms on a potato tuber.

 
Cultural control:

  1. Rotate out of potatoes for 2 years.
  2. Sprinkler irrigation should allow foliage to dry rapidly after irrigation. Avoid irrigation in cool, cloudy periods. Also, avoid open lenticels by timing the end of irrigation.
  3. High soil fertility reduces disease severity.
  4. Harvest only when tubers are fully mature.
  5. Handle tubers carefully to avoid bruising.
  6. Store tubers under conditions that promote rapid suberization.
Chemical control: Oregon--3 years of chemical-control experiments in Klamath Falls and central Oregon showed no increase in 'Russet Burbank' yield. Late season infection due to reduce soil fertility in the Columbia Basin does allow for extensive infection in Russet Norkotah. Use of Quadris 12.3 oz twice, mid and three weeks before water removal resulted in an increased yield of just over 6 tons/ac.

Idaho--research in eastern and southeastern Idaho indicates foliar sprays are effective when blight lesions appear above the lower third of the plant in non-perennial blight areas. In perennial areas, spray at 10- to 12-day intervals from mid-July to mid-August.

  1. Bordeaux 8-8-100 gives adequate control in the home garden if applied regularly. Frequent spraying stunts plants.
  2. Champ Formula 2 Flowable at 0.66 to 2.66 pints/A on 3- to 5-day intervals starting when the plants are 6 inches high. Suppresses Colorado potato beetle populations. 24-hr reentry.
  3. Chlorothalonil products such as:
    1. 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.
    2. Echo 720 at 0.75 to 1.5 pints/A on 5- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
    3. 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.
  4. Cuprofix Ultra 40D at 0.75 to 3 lb/A on 7- to 10-day intervals. 12-hr reentry.
  5. Distinguish 480 SC at 10 fl oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
  6. Endura at 2.5 to 4.5 oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications of Endura before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 30 days. 12-hr reentry.
  7. Evito 480 SC at 3.8 fl oz/A on 7- to 10-day intervals. 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.
  8. Gem at 6 to 8 oz/A 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.
  9. 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.
  10. Heritage at 3.2 to 10.5 oz/A on 7- to 14-day intervals in alternation with other appropriate fungicides. Preharvest interval is 14 days. 4-hr reentry.
  11. Inspire at 7 fl oz/A on 7- to 14-day intervals. Do not apply more than two (2) foliar applications of Inspire before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
  12. Kocide 2000 at 0.75 to 3 lb/A on 10- to 14-day intervals. 24-hr reentry.
  13. Kop-R-Spray at 1.5 to 2 teaspoons/gal water.
  14. Mancozeb products such as Dithane F-45 Rainshield, Gavel 75 DF, or Manzate 75 DF; see label for rates and intervals. Do not apply within 14 days of harvest. 24-hr reentry for Manzate 75 DF and Dithane F-45 Rainshield and 48-hr reentry for Gavel 75 DF.
  15. Nevado 4F at 1 to 2 pints/A on 10- to 14-day intervals. Do not apply within 14 days of harvest. 24-hr reentry.
  16. 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.
  17. Previcur Flex at no more than 1.2 pints/A on 7- to 10-day intervals in a tank-mixture with chlorothalonil or mancozeb. Also controls late blight. Do not apply within 14 days of harvest. 12-hr reentry.
  18. Quadris Flowable at 6.2 to 12 fl oz/A prior to disease development on 7- to 14-day intervals. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating with a different mode of action. Preharvest interval is 14 days. 4-hr re-entry.
  19. 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.
  20. 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.
  21. 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.
  22. Ridomil Gold Bravo SC at 2.5 pints/A, no later than row closure. Do not use within 14 days of harvest. 48-hr reentry.
  23. Ridomil Gold MZ at 2.5 lb/A on 14-day intervals. Do not apply within 14 days of harvest. 48-hr reentry.
  24. Rovral 4 Flowable at 1 to 2 pints/A on 7- to 14-day intervals. Do not apply within 14 days of harvest. Do not irrigate for 24 hr after application. 24-hr reentry.
  25. Scala SC at 7 fl oz/A on 7- to 14-day intervals. Use only in a tank-mix with another effective early blight fungicide. Do not apply within 7 days of harvest. 12-hr reentry.
  26. Super Tin 80 WP at 2.5 to 3.75 oz/A. Do not apply within 7 days of harvest. 48-hr reentry. Restricted-use pesticide.
  27. Tanos at 6 to 8 oz/A on 7- to 10-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.
Notes: A few forecasting systems exist to help time initial and subsequent applications.
Biological control: Efficacy in the Pacific Northwest is unknown.
  1. Ballad at 2 to 4 qt/A on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 4-hr reentry.
  2. Serenade WP at 4 to 6 lb/A on 5- to 7-day intervals. Applications can be made up to harvest. Can rotate with other fungicides. 4-hr reentry.
References:
Nolte, P., and Ojala, J. 1992. Control of Early Blight Disease of Potato. University of Idaho Current Information Series Publication 975.
Content edited by: Phil Hamm and Cynthia M. Ocamb on January 1, 2010
 
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