Please refer to the cucumber disease page at Plantvillage.org. I listed a few below

Diseases
Alternaria leaf blight FUNGUS
Alternaria cucumerina
SYMPTOMS

Small, yellow-brown spots with a yellow or green halo which first appear on the oldest leaves; as the disease progresses, lesions expand and becone large necrotic patches, often with concentric patternation; lesions coalesce, leaves begin to curl and eventually die

COMMENTS

Disease is prevalent in growing areas where temperatures are high and rainfall is frequent

MANAGEMENT

Cucurbits should be rotated with another crop every 2 years to reduce levels of inoculum; crop debris should be removed from the field as quickly as possible after harvest or plowed deeply into the soil; applications of appropriate protective fungicides can help to slow the development of the disease; water plants from the base rather than from above to reduce periods of leaf wetness which are conducive to the development and spread of disease

Angular leaf spot BACTERIA, BACTERIUM
Pseudomonas syringae
Symptoms of angular leaf spot on cucumber leaves
Angular leaf spot symptoms on cucumber foliage
Angular leaf spot symptoms on cucumber foliage
Angular leaf spot symptoms
Angular leaf spot symptoms on cucumber leaf
Angular leaf spot symptoms on cucumber leaf
SYMPTOMS

Small water-soaked lesions on leaves which expand between leaf veins and become angular in shape; in humid conditions, lesions exude a milky substance which dries to form a white crust on or beside lesions; as the disease progresses, lesions turn tan and may have yellow/green edges; the centers of the lesions dry and may drop out leaving a hole in the leaf

COMMENTS

Spread through infected seed, splashing rain, insects and movement of people between plants; bacterium overwinters in crop debris and can survive for 2.5 years

MANAGEMENT

Use disease-free seed; do not grow plants in field where cucurbits have been grown in the previous 2 years; protective copper spray may help reduce incidence of disease in warm, humid climates; plant resistant varieties

Anthracnose FUNGUS
Colletotrichum orbiculare
Anthracnose symptoms on cucumber foliage
Post-harvest anthracnose symptoms on cucumber fruit
Anthracnose lesions on cucumber foliage
Anthracnose lesions on cucumber foliage
Anthracnose lesions on cucumber foliage
Anthracnose symptoms on cucumber foliage
SYMPTOMS

Brown roughly circular lesions with yellow edges on leaves on leaves, petioles, stems and/or fruit; lesions on resistant varieties appear tan with green edges; lesions dry out and drop out of leaves

COMMENTS

Disease favors warm temperatures

MANAGEMENT

Plant resistant varieties; use only certified seed; apply appropriate protective fungicides; rotate crops every year

Aphids (Peach aphid, Melon aphid) INSECTS
Myzus persicae
Aphis gossypii
SYMPTOMS

Small soft bodied insects on underside of leaves and/or stems of plant; usually green or yellow in color, but may be pink, brown, red or black depending on species and host plant; if aphid infestation is heavy it may cause leaves to yellow and/or distorted, necrotic spots on leaves and/or stunted shoots; aphids secrete a sticky, sugary substance called honeydew which encourages the growth of sooty mold on the plants

COMMENTS

Distinguishing features include the presence of cornicles (tubular structures) which project backwards from the body of the aphid; will generally not move very quickly when disturbed

MANAGEMENT

If aphid population is limited to just a few leaves or shoots then the infestation can be pruned out to provide control; check transplants for aphids before planting; use tolerant varieties if available; reflective mulches such as silver colored plastic can deter aphids from feeding on plants; sturdy plants can be sprayed with a strong jet of water to knock aphids from leaves; insecticides are generally only required to treat aphids if the infestation is very high - plants generally tolerate low and medium level infestation; insecticidal soaps or oils such as neem or canola oil are usually the best method of control; always check the labels of the products for specific usage guidelines prior to use

Aster yellows PHYTOPLASMA
Aster yellows phytoplasma
SYMPTOMS

Foliage turning yellow; secondary shoots begin growing prolifically; stems take on a rigid, upright growth habit; leaves are often small in size and distorted, may appear thickened; flowers are often disfigured and possess conspicuous leafy bracts; fruits are small and pale in color

COMMENTS

Disease is transmitted by leafhoppers and can cause huge losses in cucurbit crops

MANAGEMENT

Remove any infected plants from the field to reduce spread; control weeds in and around the field that may act as a reservoir for the phytoplasma; protect plants from leaf hopper vectors with row covers

Aster yellows PHYTOPLASMA
Aster yellows phytoplasma
SYMPTOMS

Foliage turning yellow; secondary shoots begin growing prolifically; stems take on a rigid, upright growth habit; leaves are often small in size and distorted, may appear thickened; flowers are often disfigured and possess conspicuous leafy bracts; fruits are small and pale in color

COMMENTS

Disease is transmitted by leafhoppers and can cause huge losses in cucurbit crops

MANAGEMENT

Remove any infected plants from the field to reduce spread; control weeds in and around the field that may act as a reservoir for the phytoplasma; protect plants from leaf hopper vectors with row covers

Bacterial leaf spot BACTERIUM
Xanthomonas campestris
SYMPTOMS

Initial symptoms of the disease are the appearance of small water-soaked lesions on the undersides of the leaves which lead to the development of yellow patches on the upper leaf surface; the lesions become round and angular and may be mistaken for angular leaf spot; the centers of the lesions become thin and translucent and lesions become surrounded with a wide yellow halo

COMMENTS

Bacteria is spread via infected seeds

MANAGEMENT

Use disease-free seed; do not grow plants in field where cucurbits have been grown in the previous 2 years; avoid overhead irrigation, water plants from the base instead to reduce the spread of bacteria

Bacterial wilt BACTERIUM
Erwinia tracheiphila
Wilting cucumber vines caused by bacterial wilt
Cucumber plant infected with bacterial wilt
Cucumber plant infected with bacterial wilt
Symptoms of bacterial wilt of cucumber
SYMPTOMS

Individual runners or whole plant begins to wilt and rapidly die; infected runners appear dark green in color but rapidly become necrotic as the disease progresses

COMMENTS

Can result in crop losses of of 75%; spread by striped or spotted cucumber beetles; disease can be confirmed by cutting the stem and slowly pulling the two ends apart - infected plants will ooze strings of bacterial exudate

MANAGEMENT

Control cucumber beetle populations on plants; hand pick adult beetles and destroy; soil and foliar application of appropriate insecticides may help to control populations

Belly rot (Fruit rot, Damping-off) FUNGUS
Rhizoctonia solani
Symptoms of belly rot on cucumber fruit
Symptoms of belly rot on cucumber fruit
Symptoms of belly rot on cucumber fruit
Symptoms of belly rot on cucumber fruit
Water-soaked lesion on cucumber fruit caused by belly rot
SYMPTOMS

Yellow/brown discoloration on fruit; water soaked spots on side of fruit in contact with soil; brown mold growing on rotting areas; collapse of seedlings

COMMENTS

Disease favors warm, humid condtions

MANAGEMENT

Till soil deeply prior to planting; use plastic mulch to create a barrier between fruit and soil; plant in sites with good drainage to avoid wet soils; apply appropriate protective fungicides when plants begin to vine