How to Treat Cucumber Blight
Blights include a range of plant diseases that share one common symptom: a slow death. Blights first cause leaves or branches to quickly wilt and stop growing, and later other plant parts may or may not rot. Blights that affect cucurbit vegetables, such as cucumbers (Cucumis sativus), often cause rotting of the vines and fruits. Because blights affect so many different plants, you have to narrow down the potential disorders by identifying the symptoms and matching them with the blights that commonly affect cucumbers, which include gummy stem (Didymella bryoniae), phoma (Phoma cucubitacearum), early or alternaria (Alternaria cucumerina) and phytophthora (Phytophthora capsici).
Check the cucumber leaves for brown, irregularly shaped round spots, and check the fruits for mushy, sunken depressions of varying size that ooze a black, viscous liquid, to determine if your plants have gummy stem blight or phoba blight.
Examine the cucumbers for a thin layer of white, cottony mold that covers the skin and eventually overtakes the fruit, a strong indication of phytophthora blight. The vines of cucumber plants infected with phytophthora blight rot at or just above the soil line.
Look at the leaves for small brown spots with yellow rings around them that grow to entirely brown spots about 3/4 inch wide, a sign of alternaria leaf blight. Other signs of alternaria leaf blight include brown, upward-curling leaves, and brown rings with several concentric yellow circles. Alternaria leaf blight often starts with the mature leaves around the plant’s crown.
Work the area you will plant the cucumbers in to a depth of 12 inches as soon as the temperature stays above 70 degrees Fahrenheit during the day in spring. Remove and rocks or debris from the area.
Water the soil 12 inches deep using a sprinkler or a soaker hose. Cover the planting area with a piece of 2-mil clear plastic sheeting and tuck the edges into the soil to secure it. Place rocks around the edges if you live in a windy area.
Leave the plastic on the soil for eight to 10 weeks if the temperature stays between 70 and 80 degrees Fahrenheit, or for six to eight weeks if the temperature stays between 80 and 95 F. Sterilizing the soil kills the soil-borne fungus that causes gummy stem blight and phoma blight.
Water the cucumbers using a soaker hose or other drip-irrigation system to minimize the conditions that favor blight diseases.
Prune and care for the cucumber plants when dry. Working with wet plants can spread blight fungus around to other plants and into the soil.
Remove plants affected by alternia leaf blight at the end of the season and give them to your sanitation service on garden waste pickup day.
Grow another plant other than a curcurbit every two or three years in the soil normally used to grow cucumbers. Rotating out the cucumbers for anything other than a curcurbit for one year eliminates the fungus that causes alternaria leaf blight, as it survives in cucurbit plant debris.
Mix together 3/4 teaspoon of a fungicide containing 29.6 percent chlorothalonil with 1/2 gallon of water for every 100 square feet of growing area in the tank of a pump-type sprayer if the cucumbers have gummy stem blight or alternaria leaf blight. Spray the cucumber vines and foliage as soon as the first true leaves emerge or at the start of the rainy season, whichever comes first. Repeat every seven to 10 days throughout the growing season.
Mix together 1 tablespoon of 7 percent liquid copper concentrate with 1 gallon of water in a pump-type sprayer at the first sign of phoma blight or alternaria leaf blight. Spray the all parts of the cucumber plants until covered on all sides, and repeat every seven to 10 days, as needed. You can also spray copper on the cucumber as a preventative measure when the rainy season starts.
Mix together 1 tablespoon of 50 percent captan with 1 gallon of water in the tank of a pump-type sprayer if the plants have phytophthora blight. Spray the foliage until covered on both sides, and repeat every 10 to 14 days as needed.
Buy seeds from companies that treat their cucumber seeds with fungicides or produce their seeds in arid regions, such as California's inland valleys, to help prevent gummy stem blight and phoba blight.
Spray pesticides on a calm day to minimize drift.
Always wear chemical-proof gloves, a respirator and safety goggles when mixing and applying garden chemicals.
If you're pregnant or nursing, wear heavy-duty garden gloves when working with soil to prevent skin contact with soil-borne pathogens.
Keep garden chemicals out of reach of children.
Don't allow children or pets in an area treated with fungicides until they dry.
Don't compost plants affected by blight, but give them to your sanitation service on garden waste pickup day.
Stop using fungicides 30 days before harvest.
- University of Delaware Cooperative Extension: Phytophthora Blight Management in Cucumber
- NC State University Cooperative Extension Service: Gummy Stem Blight and Phoma Blight on Cucurbits
- University of Minnesota Extension: Alternaria Leaf Blight
- Sunset: Let the Sun Clean Your Soil
- University of Illinois Integrated Pest Management: Phytophthora in Peppers and Cucurbits
- The Old Farmer's Almanac: Cucumbers