Key to Common Problems of Cucumbers

• Symptoms: small yellow-green or white spots on leaves (called “stippling”); leaves may eventually appear tattered, yellow; plant growth and yield can be reduced significantly.
• This symptom is caused by squash bugs (Anasa tristis), which use piercing-sucking mouthparts to sip on plant sap. The eggs of this insect are golden and oval-shaped, usually laid in a cluster on the underside of leaves. 
• Use row cover over plants until they begin to bloom. Remove egg clusters by hand.

More about squash bugs

• Symptoms: Fine yellow or white spots (stippling). Spots may coalesce to form yellow patches on the upper side of the leaf. Leaf undersides may appear dirty. 
• Spider mites can be seen on leaf undersides with the naked eye or by using a hand lens. They are about the size of a period at the end of a sentence.
• Spider mites suck the green pigment (chlorophyll) out of the leaves, resulting in yellowing. Heavy feeding may cause leaf browning and leaf death. 

More about spider mites

• Symptoms: Circular or irregular, angular spots on leaves, may be yellow, light brown, or gray.
• Several different fungal and bacterial pathogens cause these symptoms and these diseases may occur simultaneously. It is challenging to tell them apart without a laboratory diagnosis.
• Management practices for these diseases in a home garden are similar:
  • Plant disease-resistant varieties of cucumber;
  • Avoid crowding plants too closely; allow for good air circulation so leaves dry faster after rain; avoid watering leaves and stems;
  • Clean up and dispose of symptomatic leaves and stems; and
  • Plant seeds or transplants multiple times during the growing season.

• Symptoms: Yellow or tan angular spots between the leaf veins are visible on the upper surface of older leaves in early-late summer.
• The undersides of the leaves have fuzzy gray spots.
• Downy mildew is caused by a water mold (fungus-like) pathogen, Pseudoperonospora cubensis. 
• Moisture, high humidity, and cool temperatures (~60F) favor development of this disease, but it can infect plants and spread with higher temperatures

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• Symptoms: White powdery growth on leaves, may be on upper and lower leaf surfaces and stems. 
• Powdery mildew on cucumbers is caused by two primary fungal pathogens, Podosphaera xanthii and Erysiphe cichoracearum. Fungal spores spread by wind.
• Infection can occur at temperatures ranging from 50F-90F, in dry to moist conditions. This is a very common disease.
• Infected leaves may wither and die, leading to premature plant death.

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• Symptoms: Yellow or green mosaic pattern or mottling of the leaves; leaves may be distorted, twisted, crinkled, deformed, or stunted; they may curl upward or downward.
• There are many types of viruses including cucumber mosaic virus, squash mosaic virus, and watermelon mosaic virus that can infect cucumbers and other plants in the cucurbit family (squash, melon).
• Viruses are transmitted by insects and can survive in perennial weeds.
• Pull out and dispose of infected plants. Control weeds, aphids, and cucumber beetles.
• Plant disease-resistant varieties.

More about viruses

• Symptoms: Chewed leaves, irregular, jagged holes. Similar damage may be found also on flower petals and stems.
• Yellow-green beetles with black stripes or spots may be visible. Adult cucumber beetles are about ¼ inch long. 
• Striped cucumber beetles are more likely than spotted cucumber beetles to carry the bacteria that causes bacterial wilt.
• Striped and spotted cucumber beetles both can carry and transmit other plant diseases such as Fusarium wilt and viruses.

More about cucumber beetles

• Symptoms: Chewing damage on the surface of leaves, in between the leaf veins.
• This damage is caused by adult squash beetles (Epilachna borealis), which resemble ladybugs/ladybeetles and Mexican bean beetles. They have seven large black spots on each of the wings.

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• Symptoms: Initially individual leaves wilt and dry out; wilting progresses to individual stems and entire plants. 
• This is caused by a bacterial pathogen, Erwinia tracheiphila, which blocks water movement inside the plant. The bacteria is transmitted mostly by striped cucumber beetles feeding on the leaves.
• Plant varieties with some purported resistance to cucumber beetles, and control cucumber beetles early in the growing season. ‘County Fair F1’ and ‘Little Leaf H-19 ' are cucumber varieties with resistance to bacterial wilt. 
• Bacterial wilt is fatal to plants and cannot be cured with pesticides. Remove infected plants from the garden and dispose of them.

More about bacterial wilt

• Symptoms: Cucumber fruits appear deformed or underdeveloped on one end; small fruits fall off the plant.
• This symptom is typically the result of poor pollination between male and female flowers. This can be caused by weather conditions (too rainy, too cool for bees to pollinate), excessively high temperatures (injuring flowers, reducing or killing pollen), and/or pesticide use (killing bees).

More about pollination problems