Leaves on zucchini plant turning yellow?
Zucchini leaves turning yellow usually means water stress or hungry soil, though one or two oldest lower leaves can yellow normally as the plant ages.
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The causes in full
Drought and heat stressMOST COMMON
How to tell
The soil 2 inches down feels dry and crumbly or baked hard and cracked, the plant wilts in sun, and yellowing or scorched patches are worst on the hottest or afternoon-sun side.
What to do
If the soil 2 inches down feels dry and crumbly or harder, water slowly at the base until the whole root area is moist; on baked soil, water in two passes so it can soak in instead of running off. Add mulch, and use temporary afternoon shade only during extreme heatwaves.
Waterlogged roots after heavy rainCOMMON
How to tell
The soil 2 inches down feels waterlogged or stays damp, the plant looks limp even though it is wet, and yellowing starts on lower leaves or spreads evenly after storms.
What to do
Do not water while the soil 2 inches down feels damp or waterlogged; resume only when it feels dry and crumbly or harder. Pull mulch back for a few days, keep water from pooling around the crown, and improve drainage before the next planting if this spot stays soggy.
Normal lower-leaf aging and shadeCOMMON
How to tell
Only one or two of the oldest, lowest leaves are yellow, the rest of the plant is growing strongly, and there are no spots, sticky insects, webbing, or twisted new leaves.
What to do
Clip off the fully yellow lower leaf at the stem so it does not sit on damp soil. Do not change watering unless the soil 2 inches down feels dry and crumbly or harder.
Nitrogen shortage or nutrients leached by rainCOMMON
How to tell
Older leaves turn pale yellow first, growth slows, and the yellowing is fairly even rather than spotted or chewed, especially after heavy rain or in poor soil.
What to do
Apply compost plus a balanced vegetable fertilizer according to the label, keeping it off the leaves. Water it in only if the soil 2 inches down feels dry and crumbly or harder; if it feels damp, wait until it reaches that same dry 2-inch test.
Aphids, spider mites, or mosaic virusLESS COMMON
How to tell
Check leaf undersides and new tips for clusters of aphids, fine webbing or tiny speckling from mites, or puckered new leaves with patchy yellow-and-green mosaic patterns.
What to do
For aphids or mites, spray leaf undersides with labeled insecticidal soap or horticultural oil in the cool evening; dish soap is not insecticidal soap. If the plant has strong mosaic patterning with distorted new growth, remove that plant because virus-infected growth will not recover.
Cold snap, late frost, or herbicide spray driftLESS COMMON
How to tell
Yellowing appeared right after a cold night, or the newest leaves are twisted, strapped, or bleached on the side facing a sprayed area or drift path.
What to do
After cold damage, wait for new growth before removing leaves, and cover plants on future cold nights. For spray drift, stop the exposure and remove badly damaged leaves only after healthy new growth appears; do not water extra unless the soil 2 inches down feels dry and crumbly or harder.
Will it recover?
Yellow zucchini leaves usually will not turn green again, so the goal is to stop the cause and get healthy new growth.
Stopping it happening again
Grow zucchini in full sun, feed regularly, mulch the soil, inspect leaf undersides weekly, and water deeply when the soil 2 inches down feels dry and crumbly or harder, not while it feels damp or waterlogged.
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