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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Spotting and discoloration of cucumber leaves caused by Pseudomonas.

Cucurbits

Angular Leaf Spot

Pathogen: Pseudomonas syringae pv. lachrymans

(Reviewed 11/05, updated 6/08)

In this Guideline:

SYMPTOMS

Angular leaf spot occurs most commonly on cucumber, but is also found on melons in California. Lesions on the foliage begin as water-soaked spots that later turn gray or tan. Spots may initially develop a yellow halo. As the affected tissue dries, the internal tissue may fall out, giving the leaf a tattered appearance. The lesions are delimited by veins, giving them an angular shape. Fruit lesions are usually superficial.

COMMENTS ON THE DISEASE

The bacterium survives on infected plant debris in the soil or on seed. Humid conditions favor development of the disease. Rain or overhead irrigation leads to rapid spread. The disease is also spread by workers picking fruit or machinery passing through the field.

MANAGEMENT

Resistant cucumber varieties are available. Limit the use of overhead irrigation. Pick fruit when the vines are dry to prevent spread in the field. Use pathogen-free seed and rotate out of cucurbits. Treat when symptoms first appear if the weather is predicted to be cool and rainy.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
A. COPPER HYDROXIDE 37.5% 1–1.33 pt 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
 
+ 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. 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445
Diseases
R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis

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