Angular leaf spot

Pseudomonas syringae pv. lachrymans

Spots squared off by the leaf veins that dry, crack and fall out, leaving cucumber leaves shot-holed. Never work among the plants while they are wet.

Bacterium Serious Seed-borneRain splashSpread in waterSpread on tools & hands Spreads to neighbors

What you'll see

Water-soaked spots with straight edges, boxed in by the small veins rather than round.
Milky droplets of bacterial ooze on the underside of a fresh spot in the early morning, drying to a white crust.
Centers turning tan, cracking and dropping out so the leaf ends up full of angular holes.
Small round water-soaked spots on the fruit, which crack open and let soft rot in.
Damage running down a row in the direction you last walked it — the giveaway that you carried it.

Why it happens

A bacterium carried on seed and surviving a season or two on crop debris. It moves entirely in water: rain splash, overhead irrigation, and above all the leaves of one plant brushing your wet sleeves and then the next plant. It enters through the leaf pores and through any wound.

Cool, wet weather early in the season, and any bed watered from overhead. Crowded plantings that stay wet through the morning, and picking cucumbers while the dew is still on them, are the two things that turn a spot or two into a row.

Treating it

Stay out of the patch until the leaves are dry — this does more than any spray.
Pick off and bin the worst leaves, and remove any fruit with a water-soaked spot before it rots.
Switch to base watering in the morning and mulch to stop soil splash.
Thin the planting or tie the vines up to open the canopy.
A copper spray protects healthy tissue in a wet spell, but it will not cure a spotted leaf.

Keeping it away

Buy certified disease-free seed and do not save seed from an affected crop.
Rotate cucurbits and beans two to three years and clear all debris.
Grow resistant cucumber cultivars, which are widely available.
Trellis cucumbers so the leaves are off the ground and dry fast.
Wash hands and tools between beds when working in wet weather.

Who gets it

Cucumbers above all, then melons, squash and pumpkins; the strawberry version is a different bacterium with the same look.