Pseudomonas syringae pv. Lachrymans
This disease can occur on most cucurbits but is of greatest importance on cucumbers. Foliar symptoms initially appear as small, water-soaked areas on the underside of the leaf, which develop an angular appearance due to restriction by the small leaf veins. Under humid conditions, a milky exudatte may appear from the water-soaked areas on the lower leaf surface. As this exudate dries, a white crust is left behind. Leaf spots turn brown and may develop yellow haloes. The centers of the spots may eventually disintegrate, giving leaves a tattered appearance. Infection on stems, petioles and fruit first appears as water-soaked spots, which may also produce the milky exudate under humid conditions and corresponding white crust upon drying. Infection of young fruit may result in deformation at maturity. Secondary soft rots often develop on infected fruit.
CONDITIONS FOR DISEASE DEVELOPMENT
This disease can originate with infested seed, infected transplants, or in the field from infested crop residue or infected volunteer plants. Infection occurs through stomata, hydathodes and wounds. On sandy soils, wind-blown sand can be particularly conducive to infection by abrasion of plant tissues. Humid conditions favor disease development. The bacterium can be spread from plant to plant by splashing water, insects, farm equipment and workers. Moisture on the leaves is especially conducive to spread by equipment and when workers contact plants.
Rotate out of cucurbits for at least two years. Avoid overhead irrigation and entry into the field when foliage is wet. Copper-based sprays may help limit spread. Resistant varieties of cucumber are available.