Cucumbers (Cucumis sativus) make their ways into home gardens because of their variety of uses in the kitchen. These vining cucurbits may succumb to several pests and diseases, including fungal and bacterial problems. These diseases leave different symptoms, including spots, wilting, rot and powdery deposits on leaves. As the term implies, fungal diseases are caused by different types of fungi. Most of these can be treated and prevented through cultural practices.
Powdery and Downy Mildews
Powdery mildew attacks a wide range of plants and leaves similar symptoms to all victims: white, powdery growth on leaves. Powdery mildew becomes most prevalent during dry weather when the relative humidity is very high. Powdery mildew rarely affects the fruit.
Ensure proper air circulation among your cucumbers by thinning plants to the recommended distance on the back of the seed packet or plant tag. This ranges from 3 to 5 feet apart depending on variety when planted in the ground; if you're using a trellis for vining cucumbers, space them at least 1 foot apart.
If the disease becomes apparent, thin out the leaves of your cucumbers as well. Pinch out leaves in the interior of your plant to allow more air circulation. Spraying the entire plant with a ready-to-use fermented salmon spray may help control powdery mildew while fertilizing your garden.
Downy mildew is characterized by yellow spots on the leaves that often start small and grow into larger, irregularly shaped brown lesions. The leaves will sometimes exhibit extreme upward curling; rarely, pests and nutrient deficiencies can cause leaf curling, as well. This fungal disease also thrives in moist conditions. Downy mildew doesn't often affect the fruit. Purchase only cultivars that are resistant to this disease.
Gummy Stem Blight
Gummy stem blight starts as brown or tan spots on most cucurbit leaves; however, leaf symptoms may or may not be present on cucumbers specifically. The spots may grow larger into wedge-shaped necrotic areas and many may have a distinctive, necrotic ring around them. A thick, gummy goo may appear around the spots. Gummy stem blights may kill entire young vines as the fungus spreads throughout the plant, leading to reduced production and death of the entire plant.
The best way to prevent this disease from spreading is to remove and destroy all affected plants. Leaving infected plants increases the chance of this fungal disease spreading to your other cucumbers. After harvest, remove all leftover plants -- vines, leaves and root systems -- as this disease may live on in these parts.
Leaf Spots and Anthracnose
Alternaria and cercospora leaf spots, and anthracnose are three additional fungal diseases of cucumbers that cause spots. Alternaria begins as tan spots that enlarge to 1 1/2 inches or more; cercospora starts with dark spots that become yellowish with dark margins. Anthracnose starts with yellowish spots that turn blackish and have the appearance of tiny "shot holes" in the leaves.
Overhead irrigation is a major contributor to these diseases; avoid watering from overhead and instead opt for placing your hose near the base of the plant. If you must water from overhead, do so in the early morning. Humidity can be another issue: Reduce humidity by picking weeds and clearing out foliage to increase air circulation.
For each of these fungal diseases, remove and destroy all affected plants at the first sign of symptoms. Remove any cucurbit vines and residues left over in your garden after harvest.
Disinfect all tools that may have come in contact with diseased plants with full-strength household antibacterial cleaner. Soak for five minutes, and rinse with water or allow to air dry.
Cucumber Mosaic Virus
Cucumber mosaic virus is a pronounced viral disease that affects cucurbits and more than 1,200 species of plants worldwide. CMV can cause distorted leaves and damages fruit, as well.
Symptoms also include a mosaic pattern of yellowish green and darker green on foliage, yellow streaks, yellow spots and distinctive yellow veins. Note: These symptoms are common of several other problems that are not as severe.
This disease spreads heavily through aphid feeding, but can also be spread by garden tools; disinfect garden tools after each cut.
Unfortunately, this and other viruses come to their host plants by way of various insect pests. There are no control methods beside controlling insect pests and maintaining healthy plants.
Several varieties of cucumbers are resistant to downy and powdery mildews, as well as cucumber mosaic virus. The plant tag should indicate this; your nursery salesperson may also be able to guide you in the right direction.
As the name implies, the first symptoms of bacterial wilt are wilting of leaves and then stems. The leaves begin to wilt and dry out; stems may quickly succumb following leaf damage. The disease can be tricky to identify as the leaves and stems may wilt during the hottest part of the day and during water stress but seemingly recover at night.
This disease is spread by the cucumber beetle, a prominent pest of cucurbits. Place insect netting over your cucumber garden to prevent the pests from feeding on and transmitting disease to your cucumbers.
Remove infected cucumber plants immediately at signs of the disease, as feeding beetles will transmit the disease to unaffected plants.