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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Early blight leaf lesions.

Potato

Early Blight

Pathogen: Alternaria solani

(Reviewed 8/07, updated 8/07)

In this Guideline:

SYMPTOMS

Early blight is primarily a disease of stressed or senescing plants. Symptoms appear first on the oldest foliage. Affected leaves develop circular to angular dark brown lesions 0.12 to 0.16 inch (3–4 mm) in diameter. Concentric rings often form in lesions to produce characteristic target-board effect. Severely infected leaves turn yellow and drop. Infected tubers show a brown, corky dry rot.

COMMENTS ON THE DISEASE

Between crops, the early blight fungus can overwinter on potato refuse in the field, in soil, on tubers, and on other solanaceous plants. Infection occurs when spores of the fungus come in contact with susceptible leaves and sufficient free moisture is present. Spore germination and infection are favored by warm weather and wet conditions from dew, rain, or sprinkler irrigation. Alternately, wet and dry periods with relatively dry, windy conditions favor spore dispersal and disease spread. Tubers can be infected as they are lifted through the soil at harvest. If sufficient moisture is present, spores germinate and infect the tubers.

MANAGEMENT

Early blight can be minimized by maintaining optimum growing conditions, including proper fertilization, irrigation, and management of other pests. Grow later maturing, longer season varieties. Fungicide application is justified only when the disease is initiated early enough to cause economic loss. Watch for disease symptoms during routine monitoring, and keep records of your results (example form—108 KB, PDF). When justified, apply fungicides as soon as symptoms appear; continued protection requires application at 7- to 10-day intervals.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider information relating to the impact on environmental quality.
 
A. AZOXYSTROBIN
  (Quadris, etc.) 6.2–15.4 oz 4 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than 2.88 qt/season.
 
B. BOSCALID
  (Endura) 2.5–4.5 oz 12 30
  MODE OF ACTION GROUP NAME (NUMBER1): Carboxamide (7)
 
C. CHLOROTHALONIL
  (various products) Label rates 12 7
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
 
D. FAMOXADONE/CYMOXANIL
  (Tanos) 6 oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/Cyanoacetamide (27)
 
E. FENAMIDONE
  (Reason) 500SC 5.5–8.2 fl oz 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
F. IPRODIONE
  (Rovral) 4F 2 pt 24 14
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
 
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. PYRACLOSTROBIN
  (Headline) 6–9 fl 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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