misshapen cucumber due to poor pollination and downy mildew disease

Poor pollination resulting in an increase in crooked and nubbed fruit. Photo: Gerald Holmes, Cal Poly San Luis Obispo, Bugwood.org

Updated: January 3, 2024

Cucumbers (Cucumis sativus) are warm-season annual plants that are popular to grow in home gardens. They are a member of the gourd family (Cucurbitaceae or “cucurbit”) and share characteristics with related vegetable plants: squash, zucchini, melons, and pumpkins. All are vulnerable to similar diseases and pests, while some problems are more prevalent among cucumbers in particular.

Use this picture guide (or the table below) to identify the most common problems of cucumbers in Maryland gardens and how to address them. To help prevent some of the most common problems, we recommend:

  • Growing disease-resistant cucumber varieties. Numerous options are available for fresh eating (slicing) or pickling cucumbers.
  • Giving plants adequate space in a sunny location to allow for good air circulation and minimize leaf wetness.
  • Using a row cover or micromesh to protect plants from pests prior to flowering. When flowers open, remove covers to allow bees to transfer pollen from male to female flowers (except for parthenocarpic varieties that do not require pollination). 
  • Grow a variety of flowering plants around your garden to support beneficial insects; predators and parasitoids help provide natural pest control.
  • Practice good garden sanitation; remove diseased or infested plant material promptly.

Symptoms on leaves - yellowing & spots

Squash bugs

tiny yellow and white spots on a cucumber leaf
Small yellow spots are a symptom of squash bug feeding. Photo: UME/HGIC
  • 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

Spider mites

yellow spots on a cucumber leaf
Spider mite feeding injury on cucumber. Symptoms can mimic downy mildew symptoms. The angular lesion on the right was caused by downy mildew. Check leaf undersides for spider mites. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
  • 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 on leaves - spots & blotches

Fungal & bacterial leaf spot diseases

tan and gray spots on a cucumber leaf
Cucumber leaf with anthracnose lesions (Colletotrichum orbiculare). Lesion centers will fall out, leaving a "shot hole" appearance on the leaf. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
yellow and brown irregular spots on cucumber leaf
Cucumber leaf with symptoms of angular leaf spot (Pseudomonas syringae pv. lachrymans). Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
yellow and tan spots on a cucumber - disease symptoms
Cucumber leaf with symptoms of target leaf spot (Corynespora cassiicola). Photo: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org
  • 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.

Downy mildew

yellow angular spots on a cucumber leaf - symptom of disease
Symptoms of downy mildew on cucumber: tan to dull yellow angular lesions on the top of a leaf. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
  • 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

More about downy mildew

 Powdery mildew

gray or white powdery substance on cucumber leaves
Powdery mildew on cucumber leaves. Photo: S. A. Johnston, Rutgers University
  • 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.

More about powdery mildew

Symptoms on leaves - yellow & green patches, distortion

Virus diseases

unusual yellow pattern on cucumber leaves - virus symptom
A mosaic pattern or mottling is a symptom of cucumber mosaic virus. Photo: Dr. Parthasarathy Seethapathy, Amrita School of Agricultural Sciences, Amrita Vishwa Vidyapeetham, Bugwood.org
  • 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 on leaves - holes or chewing damage

Cucumber beetles

chewed leaves of a cucumber plant - insect pest symptom
Feeding injury by cucumber beetles. Photo: J. Traunfeld, UME
  • 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

Squash beetles

orange insects with black spots - squash beetle
Squash beetles etch the surface of cucumber leaves. They first chew a semi-circle of leaf area in which they are going to feed. Photo: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org
  • 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.

More about squash beetles

Symptoms on leaves - wilting

vines and leaves of a cucumber plant wilting
Bacterial wilt symptoms on cucumber. Photo: UME
leaves of cucumber plant are wilting
Symptoms of bacterial wilt. Photo: J. Traunfeld, UME
  • 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 on fruits - misshapen, deformed, or undersized

Poor pollination

misshapen cucumbers with smaller part on one end
Cucumber cross-section showing incomplete development of seed cavity due to poor pollination. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
  • 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

Table: Problems of garden cucumbers

This table includes the most common problems of cucumbers described above, as well as minor, occasional problems of lesser significance.

Symptoms on Leaves  Details Possible Causes Frequency 
Small yellow-green or white spots on leaves (stippling) Leaves may eventually appear tattered, yellow Squash bugs Common
Fine yellow or white spots (stippling) Leaf undersides may appear dirty. Mites can be seen on leaf undersides with the naked eye or hand lens

Spider mites

Common during extended hot, dry weather
Pale yellow, tan, or gray spots on leaf surfaces Spots may be round or irregular, angular-shaped. Centers of the spots may drop out and leave ragged holes Fungal and Bacterial Diseases
Angular leaf spot
Target spot
Very common
Yellow, angular spots on upper leaf surface, with gray fuzzy spots on the undersides Starts on older leaves in early to late summer Downy mildew Common
White powdery growth on leaves May be on upper and lower surfaces; starts mid-summer on older leaves. Can spread rapidly, causing plants to yellow and die Powdery mildew Very Common
Yellow or green patches (mottling) on the leaves Leaves may be distorted, deformed, or puckered. Leaves may curl upward or downward Viruses diseases Common
Brown leaf edges, tan blotches Leaves may have a scorched appearance Fertilizer or
pesticide burn
Small holes in leaves; chewing damage Chewing damage may also be on flowers, stems, and fruit; beetles with stripes or spots may be present Cucumber beetles Very Common
Chewing/etching of leaves Chewed leaf tissue is between the leaf veins; visible on the underside

Squash beetles

Leaves curling/cupping Leaves curl downward, turn brown, and die. Aphids transmit viral diseases. Aphids Common
Leaves wilting Progresses from single leaves to whole stems and whole plant Bacterial wilt Very Common
Symptoms on Fruits Details Possible Causes Frequency 
Deformed, misshapen, or undersized fruits Small fruits fall off the plant Poor pollination Common
Holes in fruits Caterpillars burrow into developing fruits, buds, and flowers Pickleworm Infrequent; more likely in Southern MD and Lower Eastern Shore
Poor flavor Stressed plants Bitterness Occasional

Compiled by Christa Carignan, Coordinator, Home & Garden Information Center, University of Maryland Extension. Reviewed by Jon Traunfeld, Extension Specialist, Vegetables/Fruits, University of Maryland Extension, 12/2022

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