Depending on the pathogen or pest infecting your plant, the symptom and treatment will vary. Some cucumber diseases are curable, while some are not. For the latter, it is important to let the threat pass or be completely removed before planting again.
VIRAL INFECTION
Viruses can cause your plant to wilt and develop white, yellow, or brown spots on the cucumber leaves.
Cucumber Mosaic – Caused by Cucumber Mosaic Virus (CMV). This can be spread through contact with contaminated tools. However, it is primarily transmitted by aphids that carry the virus once it feeds on the plant.
- SYMPTOMS – Parts of an infected plant are covered with distinct mosaic (pattern of yellow or green spots and lines) and are curling downwards. The growth is severely stunted, with small leaf sizes and deformed flowers with green petals. Fruits are also discolored and small in size while distorted in shape.
How to prevent mosaic disease
There are no treatments for a CMV infection, but prevention and management can be done.
- Remove all infected ones immediately and sanitize all your gardening materials thoroughly after every use to avoid spreading the virus.
- Control the aphid vectors that may infect your plant. If there is an aphid outbreak, treat it with mineral oils or insecticidal soap. Use insecticides with carbaryl and methoxychlor to control pests.
- Avoid grafting plants. Some may not show symptoms but are still infected by the virus.
- Grow varieties that are resistant to CMV instead.
cucumber showing CMV infection symptoms
Image source: Scot Nelson on Flickr
cucumber fruits showing CMV infection symptoms
Image source: William Brown on Bugwood.org
FUNGAL INFECTIONS
Fungi can cause your cucumber to wilt and develop white or yellow spots.
Alternaria Blight/Spot is caused by Alternaria cucumerina and is common in areas with warm temperatures and frequent rainfall. This can be spread via fungal spores carried by wind and soil. They can also be carried by water. The fungal spores can survive winter and infect plants once again during spring.
- SYMPTOMS – Irregular yellow- or brown-colored spots with yellow or green halo appear in older leaves first. These spots expand to become large lesions. As the disease progresses, they will wilt and die.
How to treat and prevent Alternaria blight
- Fungicides must be used to treat fungal infections. There are a lot of available fungicides, but not all can be applied that produce edible fruits. Consult with your local gardener at a nursery or store which ones can be used so you can still eat the fruits.
- Remove the infected parts to prevent the spread of the disease. If the whole plant is infected, you must remove it. Treat the soil with fungicide or replace it. Plant a new transplant.
- Water at the base and avoid getting the top wet – only the soil should be getting wet. Moist and warm areas support the growth of fungi.
symptoms of Alternaria blight
Image source: University of Minnesota Extension
Fusarium Wilt / Cucumber Wilt / Foot-rot– Fusarium oxysporum causes this. It thrives in areas that are warm and moist. This disease is more common in tomato and potato plants, but cucumbers can also be infected.
The fungus targets the root system, restricting its water translocation. It spreads through insects and soil. Water or other contaminated tools may also be the issue. Watering consistently will not save it from dehydration.
- SYMPTOMS – Stems rot at the base, near the soil line. The leaves turn yellow and develop brown lesions. It will wilt due to the restricted water supply.
How to treat and prevent Fusarium wilt
- Fungicides must be used to treat fungal infections, such as Mycostop. Use the fungicide according to its instructions, ensuring it reaches the root system.
- Remove the infected parts to prevent the spread of the disease. If the whole plant is infected, you must remove it. Replace the soil.
- Use a fungicide-treated seed instead of normal ones.
Image source: Garden.eco
Powdery mildew – This can be caused by Podosphaera xanthii or Erysiphe cichoracearum. This is a common disease among cucurbits. Warm, wet areas favor the growth of the causative fungi. P. xanthii is a more prevalent and destructive cause of powdery mildew.
The spores of these fungi can be spread via insects and wind. The water or soil are places where you wouldn’t expect it, but they can still spread through there. Your tools could be contaminated.
- SYMPTOMS – Both on the upper and lower surface of the leaves, white powdery spots are formed. As the disease progresses, it will expand into large blotches that can cover the whole leaf and stem.
How to treat and prevent powdery mildew
- Fungicides must be used to kill fungi. Apply these immediately once you see the symptom manifest. There are other organic treatments that you can use to treat this disease, such as sulfur products. You can also use potassium bicarbonate and baking soda. You can also use vinegar.
- Remove all infected ones to avoid spreading the disease.
- Water it at the base and avoid getting the leaves wet. Moist areas favor the growth of the pathogen.
- Use resistant varieties instead of normal ones.
- Space them appropriately and use sanitized tools when planting to avoid easy transmission.
Powdery mildew
Image source: University of Minnesota Extension
Downy mildew – This is caused by the fungus-like organism, Pseudoperonospora cubensis.This oomycete was once classified as a fungus, and it thrives in areas that are cool and humid. This can infect other plants via its airborne spores.
- SYMPTOMS – Leaves have yellow or light green spots on the upper side. The underside has purplish mildew or fuzzy spots. When the disease progresses, lesions will appear, and the leaf will die.
How to treat and prevent downy mildew
- Fungicides can also be used for downy mildew. Use the fungicide according to its instructions.
- Remove all infected plants to avoid spreading the disease.
- Water at the base and avoid getting the leaves wet. Moist areas favor the growth of the pathogen.
- Space them appropriately and use sanitized tools when planting to avoid easy transmission.
Upperside infected
Image source: University of Minnesota Extension
The underside of an infected leaf
Image source: Integrated Pest Management – University of Missouri
BACTERIAL INFECTIONS
Bacteria can cause your cucumber to wilt and develop white and yellow spots. They can also be brown.
Bacterial / Angular Leaf Spots are caused by Pseudomonas syringae (angular leaf spot) or Xanthomonas campestris. This disease is common in areas that are cool and moist. The disease can be spread through:
- contaminated insects,
- water,
- soil,
- seeds,
- or tools.
- SYMPTOMS:
- Angular leaf spot caused by P. syringae – Small water-soaked lesions that are angular in shape appear. The bacteria may produce a milky substance that dries into a white crust. Lesions turn reddish-brown with yellow/green edges. As the disease progresses, the lesions will dry and form a hole in the leaf.
- Bacterial leaf spot caused by X. campestris – Small water-soaked lesions that are circular appear on the underside. Yellow patches or brown spots with yellow edges form.
How to prevent bacterial leaf spots
As of now, there is no effective treatment for this disease. Doing preventive measures will be your key to preventing them from getting infected by bacterial spots.
- Avoid planting in areas that have been used to grow other cucurbits in the last two years.
- Water at the base to avoid spreading the bacteria.
- Space your plants appropriately and use sanitized tools when planting to avoid easy transmission.
cucumber plant suffering from angular leaf spot
Image source: Schwartz, H.F. – Colorado State University
