Pests
Symptoms Small yellow beetles with stripes or spots chew the leaves and flowers, especially on young plants. Worse, they spread bacterial wilt, a disease that makes cucumber vines suddenly wilt and die.Small yellow beetles, striped (three black stripes, Acalymma vittatum) or spotted (eleven black spots, Diabrotica undecimpunctata howardi), chew young leaves, stems, flowers, and the skin of slicing fruit. The bigger threat is indirect: both kinds transmit bacterial wilt, to which cucumbers are highly susceptible, so beetle feeding on seedlings can be followed weeks later by sudden, runaway vine wilt and collapse. Cause Cucumber beetles feed on the plants and carry the germ that causes bacterial wilt, infecting cucumbers as they chew. Young seedlings are most at risk, and cucumbers catch the wilt easily.Striped and spotted cucumber beetles feed on cucurbits and carry the wilt bacterium in their gut, infecting plants as they feed; they can even enter the soil to feed on seedlings below the surface. Slicing cucumber is more susceptible to the wilt than squash or pumpkin, so even light beetle pressure on seedlings can be costly, and bitter-fruited plants attract more beetles. Organic treatment Cover young plants with row cover (take it off when flowers open so bees can get in), pick the beetles off in the morning when they are slow, and use yellow sticky traps. Keeping the beetles down is the best way to avoid the wilt disease they spread.Protect seedlings with row cover removed at flowering, hand-pick beetles in the cool morning, and set yellow sticky traps to monitor and thin numbers. Where bacterial wilt is a known problem, controlling the beetles is the only effective control of the disease, since there is no cure once a plant is infected. Prevention Cover seedlings until they flower, keep them growing strong, clear away weeds and old plants, and do not plant where squash-family crops grew last year.Exclude beetles from seedlings with row cover, keep young plants vigorous so they grow past the vulnerable stage, choose non-bitter varieties that attract fewer beetles, clear weeds and debris, and rotate away from where cucurbits grew. Aim control at the early-season colonizers that vector the wilt.
Symptoms Leaves get yellow spots, then brown and wilt, and you find clusters of shield-shaped gray-brown bugs and their bronze eggs on the undersides of leaves near the base of the plant.Leaves develop yellow specks that brown and wilt, and heavily fed vines wilt fast and turn brittle. Flat bronze to brick-red egg clusters appear on leaf undersides along the veins, and gray-brown adults (Anasa tristis) and pale nymphs cluster near the crown and under debris. Cause Squash bugs pierce the leaves and stems and suck out the juices, which makes the vines wilt. They breed through the summer and hide in plant debris over winter.The squash bug feeds with piercing-sucking mouthparts on cucurbit foliage and stems, injecting saliva that wilts and browns the vines. It is primarily a squash pest but will feed on cucumber; populations build through the season and are hardest to control once nymphs are large, and the adults overwinter in garden debris. Organic treatment Pick the bugs off by hand and scrape off the egg clusters under the leaves. A board laid on the ground overnight draws them underneath; lift it in the morning and destroy them. Treat the young bugs with insecticidal soap.Hand-pick adults, crush the egg masses on leaf undersides, and trap bugs by laying a board near the plants overnight, then lifting it in the morning and destroying the cluster underneath. Target young nymphs with insecticidal soap, since large nymphs and adults are tough to kill. Prevention Cover young plants until they flower, check under the leaves for eggs early in the season, and clean up old vines after harvest so the bugs have nowhere to overwinter.Use row cover on young plants until flowering, clean up vines and debris at season's end to cut overwintering sites, and rotate cucurbits. Scout leaf undersides for the bronze egg clusters from early in the season and act before nymphs mature.
Symptoms Tiny soft insects gather on the undersides of leaves and on new growth, making leaves curl and turn yellow, often with a sticky film. They can also spread viruses to your cucumbers.Colonies of small soft-bodied insects, often melon aphid (Aphis gossypii), cluster on leaf undersides and growing tips, curling, puckering, and yellowing leaves, with sticky honeydew and sooty mold following. Beyond direct feeding, melon aphids transmit cucumber mosaic and other viruses that mottle and distort foliage and fruit and cut fruit quality. Cause Aphids are small insects that suck the plant's juices and multiply fast, especially on soft, over-fed growth. The sticky honeydew they leave behind grows black mold.Melon and other aphids suck sap and reproduce rapidly in warm weather, building fast on stressed or over-fertilized plants. The honeydew they excrete fosters sooty mold, and several aphid species spread cucumber mosaic virus. Organic treatment Blast them off with a strong spray of water and treat the undersides of the leaves with insecticidal soap, repeating every few days. Ladybugs and other helpful insects eat aphids, so welcome them.Knock colonies off with a strong jet of water and spray insecticidal soap or horticultural oil with thorough coverage of leaf undersides, repeating every few days. Conserve ladybugs, lacewings, and parasitic wasps, which usually bring aphids under control; reflective or aluminum-foil mulch deters them from young plants. Prevention Do not over-fertilize with nitrogen, check the undersides of leaves regularly, and let helpful insects do their work instead of spraying everything.Avoid excess nitrogen, which drives the soft growth aphids favor, scout leaf undersides and growing tips, and support natural enemies rather than spraying broadly. Reflective mulch deters aphids from young plants.
Symptoms Tiny specks and a dusty, stippled look on the leaves, sometimes with fine webbing underneath, mostly in hot, dry weather. The leaves can dry out and die fast.Fine pale yellow and reddish-brown stippling on the upper leaf surface, often with faint webbing beneath, in hot, dry weather. Damage develops very fast: leaves bronze, dry, and drop, and a heavy infestation of two-spotted spider mite (Tetranychus urticae) can stunt or kill a plant. Cause Spider mites are tiny relatives of spiders that suck the juice from leaf cells. They love hot, dry, dusty conditions and breed very fast.Two-spotted spider mites feed on the contents of individual leaf cells and multiply explosively in hot, dry, dusty conditions, especially on water-stressed plants. They are not insects, so many insecticides do not touch them. Organic treatment Spray the undersides of the leaves with a strong jet of water, and treat with insecticidal soap, repeating in about five days. Keeping the plants watered and the area free of dust helps a lot.Knock mites off with a strong spray of water to the leaf undersides, and treat with insecticidal soap or horticultural oil twice, about five days apart, covering the undersides well. Conserve predatory mites, ladybugs, and minute pirate bugs, the main natural controls. Prevention Keep cucumbers well watered, since dry, dusty plants attract mites. Rinse the leaves in hot weather and avoid spraying broad insecticides that kill the helpful bugs.Keep plants well watered and avoid drought stress, since mites flare on dry, dusty plants. Hose down foliage in hot spells, avoid broad-spectrum insecticides that kill the mites' natural enemies, and scout leaf undersides in hot weather.
Diseases
Symptoms A white, powdery coating spreads over the leaves until they turn yellow and brown and die. The plant fades early and the cucumbers may not size up well.White, powdery patches spread over upper and lower leaf surfaces and stems until leaves yellow, brown, and die back ahead of schedule. Early defoliation shortens the season, cuts yield, and leaves slicing fruit exposed to sunscald. Cause A fungus that spreads on warm days with high humidity, especially where plants are crowded and air does not move well. It does not need wet leaves to take hold.Powdery mildew fungi thrive between about 50 and 90°F in dry weather with high humidity, and unlike most leaf diseases they do not need leaf wetness to infect. Crowded plantings, poor airflow, and shade worsen it, and it is one of the most common late-season cucumber diseases. Organic treatment Treat early, at the first white spots, with a labeled fungicide like sulfur or horticultural oil, and remove badly affected leaves. Water the soil, not the leaves. Once it covers the plant it is hard to stop.Treat at the first spots, before it spreads, with a labeled fungicide such as sulfur, potassium bicarbonate, neem, or horticultural oil, covering both leaf surfaces, and remove the worst leaves. Avoid overhead watering. Once it is widespread, control is mostly damage limitation. Prevention Pick resistant varieties, give plants plenty of room so air moves between them, plant in full sun, and water at the base. Start checking in midsummer and act at the first white spots.Choose resistant varieties, space plants generously for airflow, plant in full sun, and avoid excess nitrogen and overhead watering. Scout from midsummer and treat at the first sign, since prevention and early action beat rescue.
Symptoms Yellow patches show on top of the leaves, often boxed in by the veins, with fuzzy gray growth underneath in damp weather. The spots brown and spread until the leaves die. Cucumbers catch it easily.Angular yellow blotches, bounded by the leaf veins, appear on the upper surface, with grayish or purplish fuzzy growth on the underside in humid weather. Spots brown and run together, and severe infection rapidly defoliates the vines. Cucumber is one of the most susceptible cucurbits. Cause A mold-like disease that spreads in wet, humid weather and needs wet leaves to infect. It can move through a planting very quickly, and cucumbers are especially prone to it.A fungus-like water mold (Pseudoperonospora cubensis) spread on wind-borne spores that needs leaf wetness and humidity to infect, so it flares in wet, humid, dewy spells. It moves fast once established and is favored by crowding and prolonged leaf wetness. Organic treatment Open up the planting so leaves dry faster, remove the worst leaves, and use a labeled fungicide when wet weather sets in. Do not water over the leaves.Improve airflow, let foliage dry, remove badly affected leaves, and apply a labeled fungicide such as copper or chlorothalonil, covering leaf undersides, when conditions favor the disease. Avoid overhead irrigation, which keeps leaves wet and feeds the infection. Prevention Give plants room so air moves between them, water at the base, and watch closely in humid weather. Choose resistant varieties if downy mildew is common where you live.Space for airflow, water at the soil line rather than overhead, choose resistant varieties where available, and avoid working among wet vines. Scout closely in humid weather, when the disease can appear and spread quickly.
Symptoms Vines wilt and die even though the soil is wet. If you cut a wilted stem and pull the two ends apart slowly, infected sap stretches into sticky white strings. Cucumbers get this disease easily.Individual leaves, then whole vines, wilt and collapse even though the soil is moist, often until the plant dies. A field test helps confirm it: cut a wilted stem and slowly draw the cut ends apart, and infected sap pulls into thin, sticky white strands. Slicing cucumber is especially susceptible. Cause A germ spread by cucumber beetles as they feed. It blocks the plant's water pipes, so the vine wilts even when watered. Cucumbers are very prone to it.A bacterium (Erwinia tracheiphila) spread by striped and spotted cucumber beetles, which carry it in their gut and infect plants as they feed. It clogs the water-conducting tissue, so the plant wilts despite adequate water. Cucumber and muskmelon are far more susceptible than squash and pumpkin. Organic treatment There is no cure once a plant has it, so pull and destroy infected plants. The real fix is keeping cucumber beetles down, since they spread it.There is no cure once a plant is infected; remove and destroy wilting plants to limit spread. Control is really beetle control, so management aims at keeping cucumber beetle numbers down, especially early in the season. Prevention Keep cucumber beetles under control, since they spread the disease. Cover young plants until they flower, and pull any plant that wilts and tests positive with the sap-string check.Manage cucumber beetles from the seedling stage with row cover removed at flowering, keep young plants vigorous, choose less-bitter varieties that attract fewer beetles, and remove infected plants promptly. Controlling the beetle vector is the only effective control of the disease.
Symptoms Small water-soaked spots on the leaves, bounded by the veins so they look angular. In damp weather a white crust forms underneath, and the dried spots tear out, leaving ragged holes in the leaves.Small, water-soaked spots on the leaves expand until they are boxed in by the larger veins, giving them an angular shape. In humid weather a milky bacterial ooze dries to a thin white crust on the underside, and as the spots dry they tear away, leaving ragged holes. Fruit can develop small, sunken, water-soaked spots. Cause A bacterial disease carried on seed and spread by splashing water and wet hands. It is worst in warm, rainy weather and survives the winter in old plant debris.A seedborne bacterium (Pseudomonas syringae pv. lachrymans) spread by splashing water, rain, and handling wet plants, and it overwinters in crop residue. It is most damaging in warm, wet weather, roughly 75 to 82°F with frequent rain. Organic treatment There is no cure, so remove the worst leaves, stay out of the patch when the plants are wet, and use a labeled copper spray early if it is spreading. Water the soil, not the leaves.There is no rescue spray that cures it; remove badly affected leaves, avoid working among wet plants, and apply a labeled copper bactericide at the first sign to slow spread. Water at the base to keep foliage dry. Prevention Buy clean seed and resistant varieties, water at the base, stay out of the patch when it is wet, and clean up old vines at the end of the season. Move cucumbers to a new spot each year.Plant certified disease-free seed and resistant varieties, do not work among wet vines, water at the soil line rather than overhead, rotate cucurbits on a three to four year cycle, and remove crop debris after harvest.
Symptoms Brown or black spots on the leaves that can fall out and leave holes, and dark, sunken, circular spots on the cucumbers themselves that rot in wet weather.Yellowish or water-soaked leaf spots that turn brown to black and may drop out, giving a shot-hole look, plus elongated tan lesions on stems and dark, circular, sunken cankers on the fruit. In wet weather the fruit cankers ooze a pinkish spore mass and rot. Cause A fungus that spreads in warm, rainy weather and splashing water, and survives on old plant debris and infected seed.A fungus (Colletotrichum orbiculare) that needs several days of rainy, warm weather to infect and spreads in splashing water. It survives on old cucurbit residue and on infected seed. Organic treatment Remove infected leaves and fruit, water at the base, and use a labeled fungicide in long wet spells. Do not save seed from sick plants.Remove infected leaves and fruit, avoid overhead watering, and apply a labeled fungicide such as chlorothalonil preventatively in wet spells. Do not save seed from infected plants. Prevention Start with clean seed, rotate cucumbers to a new spot for a few years, clean up old vines after harvest, and water the soil rather than the leaves.Use disease-free seed and resistant varieties, rotate out of cucurbits for three years, remove and destroy old vines and residue after harvest, and water at the soil line to keep foliage dry.
