How to Identify and Treat Common Cucumber Diseases
Why Are My Cucumber Plants Wilting?
Are the leaves wilting despite consistent watering? Do you notice brown, white, or yellow spots on the leaves or cucumbers? Your cucumber plant may be infected by bacterial, viral, and/or fungal pathogens. Diseased plants may not produce as much fruit (if any), and any fruit that is produced may not be palatable or edible. This article will discuss the most common cucumber diseases, what causes them, and how to treat and prevent them.
10 Common Cucumber Diseases
- Alternaria Leaf Blight (Alternaria cucumerina)
- Anthracnose (Colletotrichum orbiculare)
- Bacterial Leaf Spots (Pseudomonas syringae, Septoria cucurbitacearum, or Xanthomonas campestris)
- Bacterial Wilt (Erwinia tracheiphila)
- Cucumber Mosaic (Cucumber mosaic virus)
- Downy Mildew (Pseudoperonospora cubensis)
- Fusarium Wilt (Fusarium oxysporum)
- Powdery Mildew (Podosphaera xanthii or Erysiphe cichoracearum)
- Rhizoctonia Belly Rot (Rhizoctonia solani)
- Verticillium Wilt (Verticillium dahliae)
1. Alternaria Leaf Blight
Fungus (Alternaria cucumerina)
This disease more commonly affects melons but can also affect cucumbers. The fungal spores can be carried in by wind or spread through contaminated soil and water. Wet and warm conditions favor the disease. The fungus can survive over winter in plant debris to attack the plants again in the spring.
Irregular brown spots on the leaves—sometimes with yellow edges.
This disease usually affects mature leaves. You may see small, brown spots at first. These brown spots can grow into irregular shapes with a yellow halo. Soon after, the leaves turn brown, wilt, and die, exposing the fruits to direct sunlight that can scald them.
Fungicides—either commercial or homemade.
- The only way to treat fungal diseases is with fungicides. There are many brands available at your local nursery, but note that fungicides may make the fruits inedible.
- You can also try spraying homemade fungicides made of a mixture of soapy water, baking soda, and vinegar. Some organic gardeners also opt for copper fungicides.
- If only a few parts of the plant are affected, cut and remove those parts to prevent the fungus from spreading.
- If the plant is severely infected, you may have to remove the whole plant, treat or replace the soil, and start over.
Fungus (Colletotrichum orbiculare)
Another fungal disease, anthracnose is most prevalent in wet and warm conditions. Regions with high humidity like southern and mid-Atlantic states are highly susceptible to fungal plant diseases.
The fungus overwinters in fallen vines, leaves, and other plant debris, releasing spores in the spring to infect new plants. The spores (conidia) need moisture and mild temperatures to germinate.
Yellow, water-soaked, circular spots on the leaves and fruit with dark brown to black edges.
You may see circular, yellow spots on the leaves that are small at first but enlarge as the disease progresses. Spots on the fruits may appear black and sunken with a pink- or salmon-colored gelatinous substance in the center. This gelatinous substance is a cluster of fungal spores—a characteristic sign of anthracnose.
Fungicides (Bravo or Benlate)
Again, treated fruits are not edible. If a large part of your plant is infected, you may need to remove the entire plant and start over with sanitized soil.
- Spray fungicides according to the instructions on the product label. In rainy seasons, more frequent applications may be required.
- Chlorothalonil (Bravo) and benomyl (Benlate) are popular fungicides used to treat anthracnose.
3. Bacterial Leaf Spots
Bacteria (Pseudomonas syringae, Septoria cucurbitacearum, or Xanthomonas campestris)
The bacteria that cause leaf spots are opportunistic, spreading through lesions created by insect bites and through seeds that spread the bacteria through contact with water. However, they are not as hardy as other pathogens. Although they can overwinter in plant debris, they need wet and cool conditions to thrive.
Small, brownish, angular or circular spots—sometimes with yellow edges—or black spots on the leaves.
Depending on the genus of bacteria infecting your plant, the symptoms may differ slightly—especially in the early stages. Xanthomonas infection usually causes brown spots with yellow edges, while Pseudomonas infection usually causes reddish-brown spots. In both cases, spots will quickly turn black.
Spots on the leaves and fruit can give other pathogens—especially fungi—the opportunity to infect. This disease often causes leaves to fall off, making fruits vulnerable to sunscalding.
There is currently no effective treatment for bacterial leaf spots.
Prevention is still considered the best control method.
- Look for cucumber beetles starting early in the spring, when the weather starts to warm up. They usually come out in early evening. Remove them as you see them.
- Use plant covers or row covers to prevent beetles and other pests from accessing your plants.
- Clear your garden of weeds, which cucumber beetles also feed on.
- Plant tansy, catnip, and/or radish plants nearby, which can repel the cucumber beetles.
- Spray insecticides to control the beetle population.
- Sanitize gardening tools, including boots, gloves, spades, and shears.
- Provide plenty of air circulation by spacing plants apart and propping them up above the soil.
4. Bacterial Wilt
Bacteria (Erwinia tracheiphila)
Bacterial wilt is spread mainly through cucumber beetles (striped and spotted). The cucumber beetle is a small insect—yellow in color with either black stripes or spots on its back. It picks up the bacteria while feeding on infected cucumbers and transfers them to other plants in the same manner. The bacteria survive winters by living in their beetles' guts.
Yellow, wilting leaves that appear to be drying out.
Initially, you'll only see a few yellowing, drying, and wilting leaves. Within a few days, the stems and vines will also wilt, turning yellow and brown. One way to tell if it's bacterial wilt is to cut a section of the affected stem and place it in a glass of water. The presence of a milky sap oozing from the cut signifies bacterial wilt.
There is currently no effective treatment for bacterial wilt.
If a large part of your plant is affected, it's best to remove the entire plant from your garden. Again, prevention is still the best treatment. Make sure you use sanitary cultural practices and avoid exposing stems, leaves, and fruit to water and soil contact.
5. Cucumber Mosaic
Virus (Cucumber mosaic virus)
There are many variants of the mosaic virus that are named according to the fruit they specialize in. The virus spreads through aphids feeding on the plant, grafting of an infected plant onto a healthy plant, and through contact with unsanitary tools, boots, and gloves.
White, yellow, or green lines and patterns on leaves and fruit.
The characteristic sign of cucumber mosaic is the pattern of spots and/or lines on the leaves or fruit. The patterns may be white, yellow, or light to dark green. The veins of the leaves may also become very distinct and yellow.
Plants may not produce as much fruit, and if they do, the fruits may be small and malformed. The leaves may also be malformed.
Like many viral infections, symptoms may not always be noticeable, making control really difficult. High temperatures, undernourishment, insect predation, and other stressors can trigger symptoms.
There are currently no treatments that can cure or prevent a cucumber mosaic virus infection.
Prevention is the only way to prevent or get rid of a viral plant infection.
- Remove any infected plants and plant materials.
- Sanitize gardening tools and clothes frequently.
- Plant virus-free seeds.
- Avoid grafting—plants can still be infected and not show symptoms.
- Remove aphids and beetles as you see them.
- Spray insecticides containing carbaryl and methoxychlor to help control the pest population.
- Use a homemade spray of crushed garlic and soapy water to kill aphids.
- Remove weeds and plant debris often since both can be breeding grounds for many plant diseases.
6. Downy Mildew
Fungus (Pseudoperonospora cubensis)
There are a variety of fungal species that cause this disease; some specialize in one type of plant, while others can infect multiple plant types. Downy mildew favors shade and moisture. The fungi cannot survive extremely cold winters (like those in the Northeastern U.S.), but in temperate regions, they can overwinter in plant debris.
Light green or yellow spots on the leaves that appear angular.
Downy mildew causes light green to yellow, angular spots on the leaves. You will also find fuzzy, dark gray spots with a purplish tint (spores) on the underside of the leaves—a tell-tale sign of downy mildew. As the disease progresses, leaves will dry out, become brown, and fall off. However, visible symptoms are not always consistent.
Spray fungicides and remove infected plant parts
Treatment includes a combination of clearing affected plant parts and spraying fungicides like Orondis, Ranman, Curzate, Zing!, Zampro. Use fungicides only as directed on the labels. Severe infections may require you to remove most or all of the affected plant to prevent further spreading.
7. Fusarium Wilt
Fungus (Fusarium oxysporum)
Fusarium wilt is commonly seen in tomato and potato plants but can also be seen in cucumber plants. The fungus infects plants through the root system and gradually restricts water supply to the rest of the plant. Unlike the other fungal diseases, fusarium fungus favors hot, dry weather. It spreads via insects, and contaminated water and gardening equipment.
Leaves suddenly turn yellow or brown and wilt
As the plant’s water conduction system fails, the stems, vines, and leaves turn yellow/brown and wilt. Older leaves are usually affected first, followed by younger leaves. The appearance of dehydration is a symptom of fusarium wilt. If you are watering consistently but you notice sudden yellowing of leaves, your plant may have fusarium wilt.
Spray fungicide near the root system and remove affected plant parts
Fungicides like Mycostop can be effective against fusarium wilt. The key is to make sure the treatment reaches the root system. You should also remove dead or dying parts to prevent the disease from spreading.
8. Powdery Mildew
Fungus (Podosphaera xanthii or Erysiphe cichoracearum)
Powdery mildew commonly affects cucurbits, including cucumbers. It is mainly caused by Podosphaera xanthii and Erysiphe cichoracearum, with P. xanthii being more prevalent and more destructive. The fungi favor warm, wet conditions, which is why it is commonly found in greenhouse-grown plants. The fungus can overwinter in plant debris. The spores don’t need moisture to germinate, and the can spread to other plants by wind, insect, and contaminated water and gardening equipment.
White, powdery spots or layers on the leaves and stems
As the name suggests, affected plants show white, powdery fungal growths on the stems and leaves—especially on the underside of leaves. Fuzzy white spots or layers of white powder on the stem leaves are a characteristic sign of powdery mildew. Fruits can also be affected, although this is rare.
Spray chemical or organic fungicides and remove affected plant parts.
Powdery mildew can be easier to treat than the other fungal diseases. The usual method of spraying fungicide and removing affected plant parts still applies, but there are many organic treatment options that are just as effective.
You can try some of these organic powdery mildew treatments:
- Potassium bicarbonate
- Neem oil
- Baking soda
- Copper fungicides
9. Rhizoctonia Belly Rot
Fungus (Rhizoctonia solani)
The fungus infects the fruits directly, causing lesions and giving other pathogens the opportunity to infect. The fungus survives in the soil and favors warm, humid conditions.
Brown spots or lesions on the fruit
Fruits with prolonged contact with the soil may have brown, water-soaked spots. These spots can grow and spread deep into the fruit, becoming firm and crusted as the disease progresses.
There are currently no effective treatments for Rhizoctonia belly rot
Prevention is the best form of treatment for this disease. Remove affected plant parts and sanitize your gardening tools. In severe cases, you may have to remove the entire plant, treat the soil, and start over. Planting vertically to avoid contact between the fruit and the soil may help prevent future infections.
10. Verticillium Wilt
Fungus (Verticillium dahliae)
Like fusarium wilt, the fungi that cause verticillium wilt infect plants through the root system, clogging the plant' vasculature. They need moisture and warm temperatures to infect plants. They can survive in the soil, weeds, plant debris over the winter. The spores spread through water, insects, and garden tools.
Irregular, yellow spots on the leaves that can turn brown
Although the name suggests that the foliage wilts, the first signs of verticillium wilt are actually irregular, yellow or brown spots on the leaves. Verticillium wilt is often confused with alternaria leaf blight, but the spots don’t have halos around them. Unlike fusarium wilt, verticillium wilt symptoms start at the bottom of the stem and slowly work their way up the plant, and the symptoms rarely reach the ends of the plant.
There are no effective treatments for verticillium wilt.
If you suspect that your plant is infected, it is best to remove the affected parts or the entire plant to prevent the fungus from spreading. Sanitize the soil and gardening equipment.
How to Prevent Cucumber Plant Diseases
Using fungicides and pruning infected or dying plant parts can prevent diseases from spreading. The application of pesticides and home remedies can control pests and reduce the chances of infection. However, preventative measures should also be used, such as companion planting, controlling irrigation, and using sanitary cultural practices.
- Clean your tools, boots, and gloves after each use.
- Sanitize pots with bleach before reusing them.
- Obtain seeds from a reputable dealer.
- Regularly remove plant debris.
- Solarize the soil before planting.
- Irrigate underground if possible.
- Create a barrier between fruit and soil using plastic mulch.
- Rotate your crops in a three-year cycle.
- Grow companion plants.
- Control pests like aphids and striped cucumber beetles.
Questions & Answers
What is a home remedy to cure yellow spots on cucumber leaves?
Yellow spots on cucumber leaves may be caused by iron deficiency, in which case you could add fertilizer, and that should clear it up. Another cause is overwatering. Cease watering for a couple of days or so and see if it bounces back.Helpful 46