Thursday, August 27, 2015

Featured Diagnosis: Bacterial Leaf Spot of Cucurbits

A pumpkin leaf with a large brown spot surrounded by a yellow halo
Bacterial leaf spot on pumpkin leaf. Note the yellow halo
surrounding the dark lesion.  (Photo: NMSU - PDC)
Bacterial Leaf Spot of Cucurbits – Bacterial leaf spot of cucurbits is caused by the bacterium, Xanthomonas campestris pv. cucurbitae. This disease causes sporadic losses in cucurbit crops grown in temperate climates. In New Mexico, the disease is not common, but can occur when warm, humid conditions are persistent. The disease attacks a number of different hosts including pumpkin, cucumber, gourds, and summer and winter squash.

Symptoms may appear on both the foliage and the fruit. On the foliage, the disease causes small somewhat round water-soaked lesions on the underside of the leaf. A yellow spot appears on the upper leaf surface. In a few days, the spots turn brown with a distinct yellow halo. The appearance on fruit is variable and depends on rind maturity and how much moisture is present. Initial lesions are typically small, slightly sunken, mostly round spots with a tan to beige center. As the spots enlarge (reaching up to 15 mm in diameter), they become noticeably sunken and the rind may cracks. 

A pumpkin with scabby, water soaked spots caused by a bacteria
Sunken bacterial leaf spot lesion on pumpkin
(Photo: J. French, NMSU-PDC).

A pumpkin with a section removed to show the dark, watersoaked tissue  under the skin
Bacterial leaf spot lesion extends into the seed cavity
 (Photo: N. Goldberg, NMSU-PDC)

Small scabby lesions caused by bacteria on the outer skin of a white pumpkin
Bacterial leaf spot on a white pumpkin
(Photo: J. French, NMSU-PDC)


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