Cucumber Fungus

Powdery Mildew

• Powdery mildew looks like it sounds. This fungus, Erysiphe cichoracearum, infects cucumber plant leaves, particularly older leaves. The fungus looks like a white powdery substance on the leaves. Powdery mildew thrives in the hot, dry weather of late summer and early fall. According to North Dakota State University, powdery mildew can damage the leaves but does not affect the cucumbers themselves. Treat infected plants with fungicidal spray as soon as you notice the first signs of powdery mildew. Use fungicides as indicated on the package label.

Downy Mildew

• Downy mildew thrives in damp, humid conditions and on wet leaf surfaces. The disease is caused by Pseudoperonospora cubensis. Look for yellow or pale green spots on the upper and underside of the cucumber plant leaves. You can treat downy mildew with fungicidal spray if you catch it early. Take out heavily infected cucumber plants. Downy mildew jumps from plant to plant in water droplets or on tools or clothes. Minimize spreading this disease by watering only the soil rather than the plant's foliage.

Alternaria Leaf Blight

• Alternaria leaf blight is caused by the fungus Alternaria cucumerina. This fungus spreads as the spores get blown on the wind and in water drops. It thrives in wet weather and on wet leaf surfaces. Healthy cucumber plants, growing in nutritiously rich soil conditions, tend to be more resistant to this fungal disease. Look for brown circular spots approximately 1/2 inch around developing on the older leaves. Remove infected cucumber plants to stop the disease from spreading.

Anthracnose

• Anthracnose, caused by the fungus Colletotrichum lagenarium, damages cucumber plants, affecting not only the leaves and stems, but the fruit as well. Look for brown spots on the leaves that turn into holes with brown edges. As with other fungal diseases, anthracnose spores transfer between plants through water droplets. Water plants close to the ground, and avoid weeding during damp weather. Remove cucumber plants infected with anthracnose before the disease spreads to other areas of the garden.