Give cucumbers the right conditions–warm, sunny weather and consistent water–and they will be one of your top producing garden crops.
To keep ahead of cucumber problems, pests and diseases, here is a troubleshooting list of possible cucumber problems with brief control suggestions.
Cucumber Problems and Solutions:
Plants are eaten or cut off near soil level. Cutworms are gray grubs ½- to ¾-inch long that can be found curled under the soil. They chew stems, roots, and leaves. Place a 3-inch paper collar around the stem of the plant. Keep the garden free of weeds; sprinkle wood ash around base of plants.
Small plants turn yellow and break off. Southern corn rootworm is the larvae of the spotted cucumber beetle (See below). Cultivate the soil before planting to expose larvae and interrupt the insect’s life cycle.
Leaves curl under and become deformed and yellowish. Aphids are tiny, oval, and yellowish to greenish pear-shaped insects that colonize on the undersides of leaves. They leave behind sticky excrement called honeydew which can turn into a black sooty mold. Use insecticidal soap.
Leaves turn pale green, yellow, or brown; dusty silver webs on undersides of leaves and between vines. Spider mites suck plant juices causing stippling. Spray with water or use insecticidal soap or rotenone. Ladybugs and lacewings eat mites.
Leaves yellow; tiny white winged insects around plants. Whiteflies will congregate on the undersides of leaves and fly up when disturbed. Remove infested leaves and the whole plant if infestation is serious. Introduce beneficial insects into the garden.
Coarse white speckling or stippling on upper surface of leaves; leaves may brown. Leafhoppers are green, brown, or yellow bugs with wedge-shaped wings. They suck the juices from leaves and stems. Use floating row covers to exclude bugs; spray with insecticidal soap.
Trails and tunnels in leaves. The leafminer larvae tunnel inside leaves. Destroy infected leaves and cultivate the garden to destroy larvae and keep adult flies from laying eggs. Cover crops with floating row covers.
Water-soaked blotches on leaves–not enlarging past leaf veins; water-soaked spot can appear on fruits Angular leaf spot or bacterial spot is a waterborne bacterium which causes irregular geometric patterns on leaves. Spots may turn yellow and crisp. Avoid wetting foliage with irrigation. Prune off infected leaves and stems. Clean up garden. Plant disease-resistant varieties. Rotate crops up to 2 years.
Holes chewed in leaves, leaves skeletonized; runners and young fruit scarred. Spotted cucumber beetle is greenish, yellowish, ¼ inch (7mm) long with black spots and black head. Striped cucumber beetle has wide black stripes on wing covers. Hand pick; mulch around plants; plant resistant varieties; dust with wood ashes.
Leaves have yellow specks that turn brown, then black; vines wilt from point of attack. Squash bug is a flat, shield-shaped black or brownish bug with a triangle on its back; it sucks juices from plants. Trap adults beneath boards in spring, hand pick and destroy.
Round white powdery spots and coating on leaves. Powdery mildew is caused by fungal spores. Spores germinate on dry leaf surfaces when the humidity is high; spores do not germinate on wet leaves. Common in late summer or fall but does not result in loss of plant. Avoid water stress. Pick off infected leaves.
Irregular yellowish to brownish spots on upper leaf surfaces; grayish powder or mold on undersides. Downy mildew is caused by a fungus. Improve air circulation. Plant resistant varieties. Rotate crops. Keep garden free of plant debris.
Mottled, distorted leaves. Mosaic virus causes leaves to become thickened, brittle, easily broken from plant; plants are stunted and yields are poor. The virus is spread from plant to plant by aphids and leafhoppers. Remove diseased plants. Remove broadleaf weeds that serve as virus reservoir.
Knots, galls, or swollen beads on roots; plants wilt; poor yield. Nematodes are microscopic worm-like animals that live in the film of water that coats soil particles; some are pests, some are not. Root-knot nematodes feed in the roots and stunt plant growth. Most common in sandy soils. Rotate crops. Solarize the soil with clear plastic in mid-summer.
Plants wilt and die beginning with crown or older topmost leaves. Verticillium wilt is a soilborne fungus. Light brown streaks can be seen in stem split lengthwise. Rotate crops. Avoid soil previously planted in cucumbers and family members, potatoes, peppers, eggplant, and tomatoes.
Vines wilt suddenly and die starting with one or two leaves. Bacterial wilt clogs the circulatory system of plants. It is caused by bacteria that live in cucumber beetles and is seen often where the soil stays moist. Remove and destroy infected plants before the disease spreads. Control cucumber beetles with rotenone or sabadilla. Wash hands and clean tools with a bleach solution.
Plants are stunted and yellow; runners gradually die. Fusarium wilt is a fungal disease which infects plant vascular tissues. Fungal spores live in the soil and can be carried by cucumber beetles. Plant disease-resistant varieties. Rotate crops. Remove and destroy infected plants. Fungicides are not effective
Water-soaked spots–sunken, brown or black–on fruit. Belly rot or bacterial spot or blight. Remove and destroy infected fruits. Remove all plants and plant debris at the end of the season. Promote good drainage adding organic materials to planting beds. Avoid over-head watering. Rotate crops. Stake or cage plants to keep fruit off ground.
Water-soaked or pale green spot on leaves that turn white; fruit cracks. Scab is caused by soilborne bacterium. Disease can be cosmetic. Plant resistant varieties. If scab occurs, change varieties next year. Sulfur may be worked into soil to make it slightly acid and reduce disease.
Early flowers don’t set fruit. A couple of possible reasons: (1) the first flowers to appear are male; female flower appear next. Fruit is produced by female flowers. Wait until female flowers appear and are pollinated. Plant all-female (gynoecious) cucumber hybrids–a few male plants will be added to the seed mix. (2) There may not be enough pollinators, mostly bees, to carry the pollen from male to female flowers. Pick off male flowers and dust the pollen into the female flowers.
Plants produce few fruits, mostly foliage. Plants are likely spaced too close together. Space plants at recommended distances, 8 to 12 inches apart. Plant spaced too close or too far apart yield fewer fruits as a result of poor pollination.
Plant fruit but then stop fruiting. Pick fruit as soon as it is the right size to use. This will allow the plant to put energy into additional fruit production. If you fail to pick mature fruit, the plant will quit producing.
Fruit tastes bitter. Uneven watering will cause plants to produce bitter compounds that affect taste. Uneven temperatures–swings of temperature by 20° or more–will do the same. Keep the soil evenly moist and mulch to conserve soil moisture. Grow varieties that do not turn bitter: Marketmore 70 is a bitter-free cucumber.
More tips: How to Grow Cucumbers.