Leaves on squash plant turning yellow?
On squash, one or two lowest leaves yellowing late in the season can be normal, but wider or fast yellowing usually means water stress, hungry soil, pests, disease, weather injury, or spray damage.
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The causes in full
Dry-soil stressMOST COMMON
How to tell
The plant wilts in the heat, the soil is dry 2 inches down, and the older leaves fade yellow before turning crisp at the edges.
What to do
If the soil is dry 2 inches down, water deeply at the base until the root zone is moist; if it is still damp at that depth, wait. Add mulch after watering to slow drying, but keep mulch a few inches away from the stem.
Waterlogged soil or poor drainageCOMMON
How to tell
The soil stays wet or sour-smelling, the plant may wilt even though the bed is wet, and the lowest leaves yellow after rain or frequent watering.
What to do
If the soil is waterlogged, stop irrigation and let the bed drain; resume watering only when the soil is dry 2 inches down. Clear blocked drainage paths and, for future plantings, build squash on compost-improved raised rows or mounds.
Nitrogen deficiency or hungry soilCOMMON
How to tell
The oldest leaves turn evenly pale yellow first while the growing tips stay mostly green, and the plant looks weak or slow without spots, powder, or insects.
What to do
Side-dress with compost or a measured vegetable-garden fertilizer, following the label so you do not overdo it. If the soil is dry 2 inches down, water it in at the base; otherwise wait to irrigate until the soil is dry 2 inches down.
Powdery mildew diseaseCOMMON
How to tell
Look for white or gray dusty patches on leaf surfaces, often starting on older leaves, with yellowing around the infected areas.
What to do
Remove the worst infected leaves if the plant still has plenty of healthy leaves, and use a fungicide labeled for powdery mildew on squash if it is spreading. Water only at the base when the soil is dry 2 inches down, because wet leaves and crowded vines help the disease move faster.
Squash bugs or spider mitesLESS COMMON
How to tell
Check leaf undersides for bronze speckling, fine webbing, clusters of bronze eggs, gray squash bugs, or leaves that yellow in patches while nearby leaves still look normal.
What to do
Handpick squash bugs and egg clusters, and use insecticidal soap, not dish soap, on mites or soft-bodied pests while covering leaf undersides and following the label. If the soil is dry 2 inches down, water at the base because dry-stressed squash is hit harder by pests.
Heat scorch, cold snap, or herbicide/fertilizer spray damageLESS COMMON
How to tell
Damage shows up fast: sun-facing leaves look bleached or scorched after severe heat, frost-hit leaves look limp then yellow or brown, or new tips are twisted, cupped, or oddly yellow after weedkiller, fertilizer, or other spray drift.
What to do
Do not remove lightly damaged leaves until new growth is taking over, but clip leaves that are fully dead. If fresh fertilizer or spray residue is on the leaves, rinse it off right away; for irrigation, water only when the soil is dry 2 inches down.
Will it recover?
Yellow, scorched, or mildewed leaves usually will not turn green again, so the goal is to stop the cause and see healthy new growth.
Stopping it happening again
Keep squash evenly moist by watering at soil level when the soil is dry 2 inches down, mulch after the soil is moist, feed moderately during active growth, give vines full sun and good spacing, inspect leaf undersides weekly, and avoid spraying herbicides or fertilizers near squash during wind or extreme heat.
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