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How to Manage Pests

UC Pest Management Guidelines

Late blight fungus produces rings of mycelium and spore-forming structures on lower side of leaves.


Late Blight

Pathogen: Phytophthora infestans

(Reviewed 1/08, updated 1/08)

In this Guideline:


Leaf symptoms of late blight first appear as small, water-soaked areas that rapidly enlarge to form purple-brown, oily-appearing blotches. On the lower side of leaves, rings of grayish white mycelium and spore-forming structures may appear around the blotches. Entire leaves die and infections quickly spread to petioles and young stems. Infected fruit turn brown but remain firm unless infected by secondary decay organisms; symptoms usually begin on the shoulders of the fruit because spores land on fruit from above.


Late blight is found when humid conditions coincide with mild temperatures for prolonged periods. When humidity is above 90% and the average temperature is in the range of 60° to 78°F, infection occurs in about 10 hours. If conditions are ideal for disease development, disease development is rapid and losses can be severe. The fungus overwinters in potatoes, tomatoes, hairy nightshade, and possibly in the soil. Spores of the fungus are easily spread by wind to other plants.


Tomato varieties resistant to certain races of the late blight fungus are grown where the disease occurs regularly. Remove any nearby volunteer tomato and potato plants and nightshades. Check transplants to ensure they are free of late blight before planting. Fungicides are generally needed only if the disease appears during a time of year when rain is likely or overhead irrigation is practiced. Disc tomato fields in fall to eliminate a winter reservoir for the fungus. Avoid sprinkler irrigation, if possible, because it favors the development of late blight. Mefenoxam-resistant strains of the pathogen are widespread in California, and this fungicide is no longer effective.

Monitoring and Treatment Decisions
Apply a protectant fungicide before disease development begins; once an outbreak occurs in a field, it is important to apply additional applications at regular intervals. Coverage must be thorough for applications to be effective.

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

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
  (Tanos) 8 oz 12 3
  (Acrobat) 50 WP 6.4 oz 12 4
  MODE OF ACTION GROUP NAME (NUMBER1): Cinnamic acid (40)
  (Quadris) F 6 fl oz 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply at 5- to 7-day intervals. Make no more than 2 sequential applications before alternating with fungicides that have a different mode of action. Do not alternate or tank mix with fungicides to which resistance has developed in the pathogen population.
  (Bravo Weather Stik) 1.5–2 pt 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  (Dithane M-45) 80WP 1.5–2 lb 24 5
  (Dithane F-45) 37F 1.2–1.6 qt 24 5
  (Penncozeb) 75DF 1.5–2 lb 24 5
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Do not apply more than 6.4 lb a.i./acre/crop.
F. MANEB 75 DF 1.5–2 lb 24 5
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: For use on greenhouse and field tomatoes only. Do not exceed 8.5 lb/acre/season.
  (Cabrio) EG 8–16 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than 2 sequential applications or more than 96 oz/acre/season.
**  See label for dilution rate.
+ 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 until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
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. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470
R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano/Yolo counties
K. Subbarao, USDA Agricultural Research Station, Salinas
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgments for contributions to the disease section:
B. W. Falk, Plant Pathology, UC Davis

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