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Late blight lesions on leaves are brown to black with pale green margins.

Potato

Late Blight

Pathogen: Phytophthora infestans

(Reviewed 8/07, updated 5/08)

In this Guideline:


SYMPTOMS AND SIGNS

Late blight lesions can occur on all aboveground plant parts. On leaves, lesions typically first appear as small pale to dark green water-soaked spots that are irregular in shape and surrounded by a zone of yellowish tissue. Under conducive conditions, lesions expand rapidly and become brown to purplish black as tissue is killed. Under sufficient humidity, white sporulation of the fungus can be observed at the periphery of lesions, principally on the underside of leaves. On stems and petioles, lesions are brown to black and may also support sporulation of the fungus. Infected tubers develop a firm brown decay that starts on the outside and may later extend to include the outer 0.125 to 0.5 inch (3–12 mm) of tissues.

COMMENTS ON THE DISEASE

Late blight occurs commonly in coastal environments and in the southern San Joaquin Valley in California. In other California potato-growing areas its occurrence is sporadic, depending on the presence of the pathogen and cool, damp weather conditions. Inoculum of Phytophthora infestans can originate from seed tubers, cull piles, volunteers, closely related weed hosts such as nightshade, and adjacent plantings of potatoes or tomatoes that are affected. Late blight can develop and spread rapidly if inoculum is present and conditions are conducive. High humidity (above 90%) and average temperatures in the range of 50° to 78°F favor the disease.

MANAGEMENT

Late blight is controlled by eliminating cull piles and volunteer potatoes, using proper harvesting and storage practices, and applying fungicides when necessary. Air drainage to facilitate the drying of foliage each day is important. Under marginal conditions, overhead sprinkler irrigation can favor late blight; in Tule Lake under solid set sprinklers, conditions conducive to late blight development are enhanced by day time irrigation but not night time irrigation.

Plant certified seed tubers. When late blight has developed on foliage and tubers are at risk of infection, make sure that vines are completely dead for 2 to 3 weeks before harvest. Phytophthora infestans does not survive very long in dead foliage.

In districts that are commonly subjected to outbreaks of late blight, preventive applications of fungicides are advised when environmental conditions are favorable for the disease. Continue fungicide applications at 7- to 10-day intervals as conditions require. Shorter intervals may be needed under cool, rainy conditions. In interior districts where late blight occurs sporadically, watch for disease symptoms during routine field monitoring, and record your results (example form108 KB, PDF). Apply fungicides when late blight lesions appear in the field or in nearby plantings.

Strains of Phytophthora infestans have developed resistance to mefenoxam (Ridomil Gold).

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on environmental quality Not all registered pesticides are listed. Always read label of product being used.
 
A. AZOXYSTROBIN
  (Quadris) 6.2–15.4 fl oz 4 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
B. CYMOXANIL
  (Curzate) 60 DF 3.2 oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Cyanoacetamide (27)
  COMMENTS: Use only in combination with a protectant fungicide (e.g., mancozeb, chlorothalonil). For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i. Begin applications when conditions are conducive to disease development, repeat at 5- to 7-day intervals. Do not exceed 7 applications/growing season (see label for additional restrictions).
 
C. CHLOROTHALONIL
  (various products) Label rates 12 7
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
 
D. DIMETHOMORPH
  (Acrobat) 50WP 4–6.4 oz 12 4
  (Forum) 4–6 oz 12 4
  MODE OF ACTION GROUP NAME (NUMBER1): Cinnamic acid (40)
  COMMENTS: Begin applications when plants are 4–6 inches high or at onset of disease. See label for additional information.
 
E. FAMOXADONE/CYMOXANIL
  (Tanos) 6–8 oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/Cyanoacetamide (27)
 
F. FENAMIDONE
  (Reason) 500SC 5.5–8.2 lb 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
G. MANCOZEB
  (various products) Label rates 24 14
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
 
H. MANEB
  (Maneb) 75 DF 1.5–2 lb 12 14
  (Manex) 4L 0.8–1.6 qt 24 14
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
 
I. MEFENOXAM/CHLOROTHALONIL
  (Ridomil Gold Bravo) 2 lb 48 14
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Multi-site contact (M5)
  . . . or . . .
  MEFENOXAM/COPPER
  (Ridomil Gold Copper) 2 lb 48 14
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Multi-site contact (M1)
  . . . or . . .
  MEFENOXAM/MANCOZEB
  (Ridomil Gold MZ) 2.5 lb 48 14
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Multi-site contact (M3)
  COMMENTS: Apply Ridomil Gold Copper with 0.8 lb a.i./acre of maneb, mancozeb, or chlorothalonil. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i.
 
J. PROPAMOCARB HYDROCHLORIDE
  (Previcur Flex) 0.7–1.2 pt 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Carbamate (Group 28)
  COMMENTS: Apply as a tank mix with chlorothalonil, maneb, or mancozeb in a preventive program. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i. Do not apply more than 6 pt/growing season.
 
K. PYRACLOSTROBIN
  (Headline) 6–12 oz 12 3
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Diseases
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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