Why are my cucumber leaves turning yellow?

Look at where the yellow is before you treat anything. Yellowing only on the oldest, lowest leaves while new growth stays green usually means nitrogen is running low or those leaves are aging out. Pale new leaves on a young plant often mean cold soil or roots sitting in wet ground. Yellow patches boxed in by the leaf veins, with gray fuzz underneath, point to downy mildew. Fine yellow speckling with a dirty-looking underside suggests spider mites. A yellow-and-green mosaic with puckering points to a virus. Leaves that wilt first and yellow afterward, vine by vine, suggest bacterial wilt.

Old lower leaves are the most common and least worrying case. Nitrogen moves within the plant, so when supply runs short, a cucumber pulls it from its oldest leaves to feed new growth, and those leaves fade to a uniform pale yellow. A few lower leaves yellowing late in the season on a vine heavy with fruit is normal. If it climbs the vine quickly, side-dress with compost or a nitrogen fertilizer; a fast source such as fish emulsion can show results in 5 to 7 days. Yellow between green veins on older leaves points to magnesium instead.

Downy mildew is the cause to act on fastest. University of Maryland Extension describes yellow or tan angular spots between the veins on the upper side of older leaves, with fuzzy gray growth on the undersides. In humid weather, infected leaves can brown and collapse within days. It is a water mold that travels on the wind, so it often shows up across a whole area at once. Pick off and bag infected leaves, water at the base in the morning, and trellis vines for airflow. Copper sprays can protect healthy leaves but don't cure infected ones; copper is toxic to fish, and the label is the law.

Spider mites leave fine pale stippling that merges into yellow patches, mostly in hot, dry weather. Turn a leaf over and look with a hand lens for tiny moving dots and fine webbing; spraying the undersides hard with water every 2 to 3 days knocks them back. Squash bugs cause similar speckling, along with copper-colored egg clusters under the leaves. A yellow-and-green mosaic with crinkled, cupped or stunted leaves points to a mosaic virus spread by aphids or cucumber beetles. No spray cures it, so pull infected plants and manage aphids, beetles and nearby weeds.

When whole vines droop and then yellow, check for bacterial wilt. Leaves first look dull and wilt during the day, then the wilt spreads stem by stem until the plant dies. Cut a wilted stem near the base, press the cut ends together and slowly pull them apart; sticky threads of ooze point to bacterial wilt, though the test can miss some infections. Cucumber beetles spread it, and infected plants can't be cured, so pull them. Next season, keep row cover on young plants until flowering, then hand-pick beetles. University of Maryland lists 'County Fair' and 'Little Leaf H-19' as resistant cucumbers.

Before using any product, read the whole label. It must list the crop you’re treating, and its directions on amount, timing, protective gear, bees and days before harvest are legal requirements. If this page and the label disagree, follow the label.