Cucumbers aren’t the fussiest plant in your vegetable garden, but they aren’t the easiest to grow either. If you’re aware of the pests and diseases that can attack these garden favorites, you’ll have a better chance of a bountiful harvest.
Cucumber Plant Diseases
Angular Leaf Spot
(Pseudomonas syringae pv. Lachrymans)
Symptoms: Small angular-shaped spots on leaves, often sunken and brown; yellow leaves; circular spots on fruit that can crack and turn white.
Causes: Bacteria that thrive in hot, humid conditions; spreads through the handling of plants, wind-blown rain.
Control: Choose (from the many) disease-resistant cultivars; avoid watering from above or working the plants when they are wet; no effective natural controls but some copper-based chemical treatments can work if applied early.
Bacterial Wilt
(Erwinia tracheiphila)
Symptoms: Wilting first in infected branches and leaves, spreading eventually to the entire plant.
Causes: Bacteria transmitted by cucumber beetles as they feed.
Control: Use floating row covers to keep beetles from feeding, removing them when flowers appear (they must be uncovered for bees to pollinate them and fruits to form); remove and destroy wilted plants; consider planting late season cucumbers (cucumber beetles are done feeding by midsummer); while cucumber beetle traps get mixed reviews, chemical insecticides targeting cucumber beetles are available but must be applied regularly starting when plants are one week old or when beetles first appear in the spring.
Anthracnose
(Colletotrichum orbiculare)
Symptoms: Large yellow to brown spots on leaves, eventually forming ragged holes; sunken lesions on fruits, sometimes colored pinkish by spore masses.
Causes: Infection that thrives in rainy and humid conditions; spreads through the handling of plants, wind, wind-blown rain.
Control: Plant disease-resistant cultivars; treat with chemical fungicides as soon as vines start to run or earlier if symptoms appear; the pathogen can overwinter for several years in infected soil, so practice strict crop rotation and remove all plant debris, dispose of it far from your vegetable beds.
Downy Mildew
(Pseudoperonospora cubensis)
Symptoms: Yellowish brown irregular spots which quickly spread over the top of the leaves; occasionally gray fungus develops on the leaves’ undersides.
Causes: Spores that spread rapidly in wet weather when the nights are cool through air currents, rain and handling.
Control: Plant (one of the many excellent) disease-resistant cultivars; treat with chemical fungicides as soon as vines start to run, though it is rare for the disease to appear before mid-summer.
Powdery Mildew
(Podosphaera xanthii or Erysiphe cichoracearum)
Symptoms: White powdery mass covering leaves and stems, starting in mid-July through the end of the season and eventually leading to the leaves turning yellow and brown, shriveling and dying. Fruits are unaffected by the spores themselves but can have secondary damage (sunscald, malformed fruits) caused by the leaves’ death.
Causes: Spores thrive in humid, shady conditions among crowded, fertile plants.
Control: Plant (one of the many excellent) disease-resistant cultivars; treat with chemical fungicides as soon as vines start to run, though it is rare for the disease to appear before mid-summer; quell excessive leaf growth by avoiding excessive nitrogen fertilization, and don’t crowd plants; practice strict crop rotation and remove all plant debris, dispose of it far from your vegetable beds to keep the spores from overwintering.
Cucumber Mosaic Virus
(Cucumovirus)
Symptoms: Distinct green and yellow mosaic pattern on leaves, eventually leading them to curl; stunted growth and little fruiting.
Causes: Infected seeds or transplants, insect feeding (primarily aphids), cuttings.
Control: Plant (one of the many excellent) virus-resistant cultivars; keep weeds, which can harbor the virus over winter, cut and cleared from around your beds; no chemical controls exist though insecticidal soaps may help control the aphids that often carry the virus. Plant cucurbit crops in beds as far from each other as space allows to keep aphids from transferring the virus from one infected plot to another healthy one.
Cucumber Insect Pests
Aphids
(Myzus persicae)
Identification: 1/16 to 1/8 inch long and of almost any color; winged or wingless with two tubes that protrude from their belly and one ling slender antenna, though these features can be hard to see with the naked eye.
Damage: Aphids feed on plant sap, which can keep the plant from thriving and setting fruit. They also excrete a substance that causes black, spotty mold, further reducing plant yields. And they can be vectors for viruses, particularly the cucumber mosaic virus.
Control: Introduce natural predators such as lady beetles in adult and larvae form, lacewing larvae and parasitic wasps; avoid planting downwind from hedgerows or forested areas that decrease the wind, which can disperse aphids; as a last resort, use chemical insecticides formulated for aphids and apply according to the label, treat the underside of the leaves, as that is where the aphids will often clump.
Cucumber Beetles
(Striped – Acalymma vittata, Spotted – Diabrotica undecimpunctata, Banded – Diabrotica balteata)
Identification: 1/5 to 1/2 inch long; striped beetles have three black stripes the length of their bodies; the spotted ones, twelve black spots on the back; and the banded ones, three transverse bands of green atop a yellow outer shell.
Damage: Cucumber beetles migrate in the spring from wooded areas and fields to cucurbit crops, feeding voraciously on leaves, stems and flowers, all negatively impact the quality and quantity of the fruit and the health of the plant. The larval stage of the beetles, known as the rindworm, feeds directly on the cucumbers’ roots. The beetles are also the primary vector of bacterial wilt.
Control: To control rindworm, eliminate the adult beetles before they have a chance to lay their eggs. Control adults by eliminating surrounding weeds in the fall, which are overwintering sites for beetles, plant wilt resistant cultivars, and use insecticides approved in your state for cucumber beetles.
Thrips
(Western – Frankliniella occidentalis, Onion – Thrips tabaci)
Identification: 1/25-inch yellowish bugs with fringed wings.
Damage: Thrips suck the contents of the cucumber’s cells, weakening the plant and leaving leaves and fruits damaged, malformed and sometimes covered in silver spots.
Control: Scatter diatomaceous earth on the ground around the plants and plant leaves to destroy larvae and feeding bugs; use reflective mulches or ground covers; Introduce Hypoaspis and cucumeris mites or pirate bugs when the plants are small and an infestation is not yet established; spray leaves regularly with an insecticidal soap such as Neem (it’s necessary to cover both the top and bottoms of all leaves, a process that may make this impractical as a method of control for the home gardener).
Spider Mites
(Tetranychus urticae)
Identification: Miniscule mites practically invisible to the naked eye; when spider mites are present, leaves develop yellowing between the veins and their bottoms turn brown and crusty.
Damage: The mites feed on the underside of the leaves, causing them to die and fall off, eventually killing the plants.
Control: Spider mites thrive in arid, hot weather and tend to migrate in from surrounding grassy areas. Biological controls through natural predators are not realistic given the area (including not just the cucumber beds but also any surrounding weeds or grassy patches) that must be treated; avoiding cutting fields or grasses nearby until mid-summer or dry times can help keep the mites from migrating to your crop; applying insecticidal soap or chemical insecticides must be done on both sides of every leaf to work and hence is impractical.
Tips for Controlling Diseases and Pests in Cucumbers
- Practice crop rotation, planting your cucumbers in beds where no cucurbits have been for at least two years.
- Plant disease-resistant cultivars.
- Introduce and attract natural predators (beneficial insects) to your garden.
- Plant cucumbers in fertile soil amended with rich compost. Provide adequate food and water because healthy plants are more likely to fight off diseases and pest attacks.
- Don’t overcrowd your plants, so that disease has a harder time spreading.
- Clear nearby weeds that can harbor pests and diseases.
- In the fall, clean the beds of all plant material and dispose of it away from your garden.
Tomatoes are another popular choice for home gardeners that can develop problems with pests and diseases. Keep your plants healthy by monitoring them for these Top 10 Tomato Pests and Diseases.
