What Causes Rust Spots on Cucumber Plants?

Anthracnose Disease

• Anthracnose infects the stems, leaves and fruit of a number of plants, including cucumbers. Caused by the fungus Glomerella lagenarium, initial symptoms of an anthracnose infection on the leaves of cucumbers appear as water soaked areas that expand to form 1/4-inch- to 1/2-inch-wide light-brown to tan spots. The fungal spores spread during high humidity and rain. Anthracnose also causes lesions on fruit and stems.

Insects

• Aphids are tiny pests that cause rustlike spots on cucumber leaves. Measuring less than 1/8 inch, the tiny insects live on the underside of the leaves from where they suck juices with their piercing mouthparts, causing tiny, brown spots to appear. Left untreated, the leaf gradually turns brown and gains a lacy appearance. Although small infestations of aphids are easily controlled through a jet of spray from a hose, use a low-toxicity insecticide to control larger populations.

Alternaria Blight

• Caused by the fungus Alternaria cucumerina, alternaria blight is a disease that infects cucumbers, melons, squash and pumpkins. Initial symptoms include tiny, circular, water-soaked spots on the foliage that gradually enlarge and turn brown as the disease progresses. Wet weather and a temperature range between 60 and 90 degrees Fahrenheit favor the disease and cause rapid defoliation on the cucumber plant.

Stem Blight

• Gummy stem blight is a common disease of cucumbers that damages various parts of the plant. Initial symptoms of the disease appear as tiny, brown, angular spots on the leaves that progressively grow in size as the infection spreads. The holes the disease causes on the leaves cause a yellow halo around the brown surrounding area. Older spots appear dry and cracked.

Prevention

• Adopting preventive measures keeps the disease from occurring in the first place, thereby limiting the use of fungicides. Grow cucumber plants in well-draining soil and rotate the crop every three years. Planting corn on one side of the cucumbers helps prevent aphid infestations. Avoid picking cucumbers when the plants are wet, since moisture favors the spread of fungal diseases. Remove and destroy damaged or diseased plants and clean up plant debris in fall. Always use sterilized tools when pruning the plants.