Pests
Symptoms Tiny specks and a dusty, stippled look on the leaves, sometimes with fine webbing underneath, spreading fast in warm, dry, still air. The leaves can dry out and die quickly. Mites are the most common pest under cover.Fine pale yellow and reddish-brown stippling on the upper leaf surface, with faint webbing on the undersides, developing fast in the warm, dry, still air under cover. Leaves bronze, dry, and drop, and a heavy infestation can strip and kill a plant. Mites are the signature protected-culture cucumber pest. Cause Spider mites are tiny relatives of spiders that suck the juice from leaf cells. They love the warm, dry, still air inside a greenhouse or tunnel and breed very fast.Two-spotted spider mites feed on the contents of individual leaf cells and multiply explosively in hot, dry, dusty conditions, exactly the environment of a warm greenhouse or high tunnel. They are not insects, so many insecticides do not touch them, and they build unseen on leaf undersides. 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 air a little more humid and the plants well watered helps a lot, and predatory mites work well under cover.Raise humidity and hose the leaf undersides with a strong spray of water, and treat with insecticidal soap or horticultural oil twice about five days apart with thorough underside coverage. Predatory mites (Phytoseiulus persimilis) are the standard and effective biological control under cover; conserve them and avoid broad-spectrum sprays that wipe them out. Prevention Keep cucumbers well watered, since dry, dusty plants attract mites, and do not let the air get too hot and stale under cover. Check the undersides of the leaves in warm weather and avoid broad insecticides that kill helpful bugs.Keep plants well watered and avoid drought and dust stress, since mites flare on dry plants. Manage humidity and airflow under cover, scout leaf undersides in warm weather, release predatory mites early where mite history is bad, and avoid broad-spectrum insecticides that kill the mites' natural enemies.
Symptoms Tiny soft insects gather on the undersides of leaves and on new growth, making leaves curl and yellow, often with a sticky film. They can also spread viruses to your cucumbers.Colonies of small soft-bodied insects, often melon aphids, cluster on leaf undersides and growing tips, curling and yellowing leaves, with sticky honeydew and sooty mold following. Under cover they build quickly on lush, well-fed growth, and melon aphids transmit cucumber mosaic and other viruses. Cause Aphids are small insects that suck the plant's juices and multiply fast, especially on soft, over-fed growth and in the warmth under cover. The sticky honeydew they leave grows black mold.Melon and other aphids suck sap and reproduce rapidly in warm weather, building up fast on stressed or over-fertilized plants and in the sheltered warmth under cover. 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, especially under cover.Knock colonies off with a strong jet of water and spray insecticidal soap or horticultural oil with thorough underside coverage, repeating every few days. Under cover, conserve or release ladybugs, lacewings, and parasitic wasps, the standard biological controls. Reflective mulch deters aphids from young outdoor 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 regularly, and support natural enemies rather than spraying broadly, since aphids are prime virus vectors on a mosaic-susceptible crop.
Symptoms Tiny white flies scatter up in a little cloud when you brush the plant, and their young feed under the leaves, turning them yellow and sticky. They thrive in the warmth of a greenhouse or tunnel.Clouds of tiny white flies lift from the plant when disturbed, and their nymphs feed on leaf undersides, yellowing and weakening leaves and coating them in honeydew and sooty mold. Whiteflies are a classic protected-culture pest that builds in the sheltered warmth of a greenhouse or tunnel and can transmit viruses. Cause Whiteflies are small sap-sucking insects whose numbers explode in the warm, protected air of a greenhouse or tunnel, where nothing knocks them back.Greenhouse and other whiteflies feed with piercing-sucking mouthparts on leaf undersides and reproduce continuously in warm, sheltered conditions, so populations can build fast under cover without the weather checks they face outdoors. Organic treatment Hang yellow sticky traps to catch the adults and treat the undersides of the leaves with insecticidal soap, repeating often. Under cover, the tiny helper wasp Encarsia formosa controls them well.Use yellow sticky traps to monitor and reduce adults, spray insecticidal soap or horticultural oil to leaf undersides repeatedly, and release the parasitic wasp Encarsia formosa, the standard biological control for whitefly under cover. Remove heavily infested lower leaves. Prevention Check new plants before bringing them under cover, keep yellow sticky traps up to catch the first ones, and do not over-feed with nitrogen. Catching them early is the key.Screen vents and doorways, inspect new transplants before bringing them in, keep sticky traps up to catch the first arrivals, and avoid the lush over-fed growth whiteflies favor. Early detection is everything under cover.
Symptoms Small yellow beetles with stripes or spots chew the leaves and flowers of outdoor plants, and worse, they spread a wilt disease that kills cucumber vines. Growing under insect screening keeps them out.Small yellow beetles, striped or spotted, chew leaves, flowers, and fruit on outdoor or open-sided plantings. Their real threat is bacterial wilt, which they transmit and to which cucumber is highly susceptible, so beetle feeding on young plants can be followed by sudden vine collapse. Under full insect exclusion this pest, and the wilt it carries, is largely kept out. Cause Cucumber beetles feed on the plants and carry the germ that causes bacterial wilt. They mostly bother outdoor cucumbers; the same screening used to keep bees off English cucumbers also keeps beetles out.Striped and spotted cucumber beetles feed on cucurbits and carry the bacterium that causes bacterial wilt, infecting plants as they feed. They are chiefly a problem for outdoor or vented plantings; the insect exclusion many growers use to keep pollinators off parthenocarpic types also keeps beetles and the wilt they vector out. Organic treatment Outdoors, cover or screen young plants, pick the beetles off in the morning when they are slow, and use yellow sticky traps. Keeping them down is the best way to avoid the wilt they spread. Under cover you rarely see them.On open plantings, protect young plants with row cover or screening, hand-pick beetles in the cool morning, and use yellow sticky traps to monitor. Since there is no cure for the wilt they spread, keeping beetle numbers down early is the only real control; under full exclusion little action is needed. Prevention Screening the plants keeps beetles out and keeps bees off your seedless cucumbers at the same time. Outdoors, cover young plants, keep the area weeded, and do not plant where squash-family crops grew last year.Exclude beetles with screening or row cover, which doubles as the pollinator exclusion parthenocarpic types want, keep young plants vigorous, clear weeds and debris, and rotate away from where cucurbits grew. Aim any control at early-season colonizers that vector wilt.
Diseases
Symptoms A white, powdery coating spreads over the leaves until they turn yellow and brown and die, and the plant fades early. It is the most common problem for cucumbers grown under cover.White, powdery patches spread over the upper and lower leaf surfaces and stems until leaves yellow, brown, and die back early. It is the most common disease of cucumbers under cover, where crowding, shade, and still humid air favor it, and early defoliation cuts the crop short and exposes fruit to sunscald. Cause A fungus that spreads in warm, humid air, especially where plants are crowded and the air does not move, which is common under cover. It does not need wet leaves to take hold. Some English varieties resist it better than others.Powdery mildew fungi thrive between about 50 and 90°F in humid air, and unlike most leaf diseases they do not need leaf wetness to infect, so they flourish in the warm, humid, still conditions of a greenhouse or tunnel. Crowded single-stem plantings with poor airflow worsen it. Organic treatment Treat early, at the first white spots, with a labeled fungicide like sulfur or horticultural oil, and remove badly affected leaves. Open vents to move the air and lower humidity. Once it covers the plant it is hard to stop.Treat at the first spots with a labeled fungicide such as sulfur, potassium bicarbonate, neem, or horticultural oil, covering both leaf surfaces, and remove the worst leaves. Improve airflow and lower humidity under cover with venting. Once it is widespread, control is mostly damage limitation. Prevention Pick resistant varieties like Tyria, give plants room and good airflow, vent under cover to keep the air moving, and water at the base. Start checking early and act at the first white spots.Choose tolerant varieties, keep plants well spaced and single-stemmed for airflow, vent to manage humidity, give full light, and avoid excess nitrogen. Scout from early in the crop and treat at the first sign, since prevention beats rescue.
Symptoms Yellow patches show on top of the leaves, often boxed in by the veins, with fuzzy gray growth underneath in damp air. 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 a grayish or purplish fuzzy growth beneath in humid weather. Spots brown and run together, and severe infection rapidly defoliates the vines. Downy mildew is one of the most important leaf diseases of cucurbits, and the high humidity under cover can drive it hard. Cause A mold-like disease that spreads in wet, humid conditions 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) that spreads on wind-borne spores and needs leaf wetness and humidity to infect, so it flares in wet, humid spells and wherever foliage stays wet, including a poorly vented tunnel. It moves fast once established. Organic treatment Open up the planting and vent so leaves dry faster, remove the worst leaves, and use a labeled fungicide when damp weather sets in. Do not water over the leaves.Improve airflow, vent to let foliage dry, remove badly affected leaves, and apply a labeled fungicide such as copper when conditions favor the disease, covering leaf undersides. Water at the soil line, never overhead, so leaves stay dry. Prevention Give plants room so air moves, water at the base, keep the air from staying damp under cover, and watch closely in humid weather. Choose resistant varieties if downy mildew is common where you live.Space and single-stem for airflow, water at the soil line, keep humidity down under cover, choose resistant varieties where available, and avoid working among wet vines. Scout closely in humid weather.
Symptoms Tan spots on the leaves and, worse, dark sunken cracks on the stems that ooze a gummy sap and can kill the vine. The cucumbers can rot black at the ends. It is a common problem for cucumbers grown under cover.Water-soaked to tan leaf spots and, more seriously, dark, sunken, gummy cankers on the stems that ooze an amber gum and can girdle and collapse a vine. Fruit can develop black rot. It is a persistent, damaging disease of greenhouse and tunnel cucumbers favored by high humidity, and it survives on residue and seed. Cause A fungus that attacks in warm, damp air and gets in through cuts and wounds, so it can spread as you prune. It survives on old plant debris and infected seed.A fungus (Didymella bryoniae, also called black rot) that infects in warm, humid conditions and enters through wounds and pruning cuts, so single-stem training under cover can spread it if tools and hands carry it between plants. It persists on crop debris and can be seedborne. Organic treatment Cut out and throw away infected parts, vent to lower the humidity, and do not water over the leaves. Clean your pruning tools between plants and prune when the plants are dry.Remove and destroy infected stems, leaves, and fruit, lower humidity with venting, and avoid overhead watering. Disinfect pruning tools between plants and prune in dry conditions so cuts heal fast. Apply a labeled fungicide preventatively where the disease has a history. Prevention Start with clean seed, keep the air from staying damp under cover, water at the base, keep your tools clean, and clean up old plants. Move cucumbers to a fresh spot each year.Use clean seed and resistant varieties, keep humidity down under cover, water at the soil line, disinfect tools, remove debris, and rotate cucurbits. Prune only dry plants so cuts do not stay open to infection.
Symptoms A fuzzy gray mold on flowers, fruit, and dying leaves, often starting where a flower or wound is, then rotting the cucumber. It shows up in cool, damp, still air under cover.A fuzzy gray-brown mold on flowers, fruit, and dying tissue, often starting at the blossom end of a fruit or on a wounded or pruned surface, then rotting it and spreading in cool, humid, still air. It is a common protected-culture disease wherever humidity is high and airflow is poor. Cause A mold (Botrytis) that starts on dead flowers, wounds, and dying leaves, then spreads to the fruit in cool, damp, still air, the kind of air a closed-up greenhouse or tunnel gets.The fungus Botrytis cinerea, which colonizes dead and dying tissue, spent flowers, and pruning wounds, then spreads to healthy fruit in cool, humid, poorly vented conditions, exactly what a closed greenhouse or tunnel can become. Organic treatment Take off dead flowers, dying leaves, and any moldy fruit, open vents to lower the humidity and move the air, and do not water over the leaves. Clean up trimmings instead of leaving them lying around.Remove spent flowers, dying leaves, and infected fruit promptly, vent to lower humidity and keep air moving, and avoid overhead watering. Prune and pick in dry conditions, and dispose of trimmings rather than leaving them under the plants. Prevention Keep the air moving and not too damp under cover, and keep the planting clean of dead leaves and flowers. Give plants room, water at the base, and clear away what you prune off.Manage humidity and airflow above all, since gray mold is a high-humidity disease, and keep the planting clean of dead tissue. Space and single-stem for airflow, water at the base, and remove trimmings after pruning.
Symptoms Leaves turn mottled light and dark green, get puckered and distorted, and the plant is stunted, with warty, misshapen cucumbers. Aphids spread it, and there is no cure once a plant has it.Mottled light and dark green, puckered, and distorted leaves, stunted growth, and warty, misshapen, mottled fruit. The virus is spread by aphids, so it tends to follow aphid buildups, and there is no cure once a plant is infected. It is one of several mosaic viruses that affect cucumbers. Cause A virus spread by aphids as they feed, and carried on some weeds. Since aphids move it around, keeping aphids down and pulling infected plants is the way to manage it. The virus itself cannot be cured.Cucumber mosaic virus, transmitted by aphids as they probe and feed and also carried on some weeds and infected plants. Because aphids move it, controlling aphids and removing infected plants promptly is the practical defense; the virus itself cannot be treated. Organic treatment There is no cure, so pull and destroy infected plants right away. Keep aphids under control and weeds down, and wash your hands and tools after touching a sick plant.There is no cure; remove and destroy infected plants promptly to cut the source. Manage aphids, control nearby weeds that harbor the virus, and disinfect hands and tools after handling suspect plants. Prevention Grow resistant varieties if you can, keep aphids and weeds down, and pull out any plant with mottled, distorted leaves before it spreads to the others.Choose resistant varieties where available, manage aphids from early in the crop, control weeds around the planting, and rogue out infected plants at the first clear symptoms to protect the rest.
