Basil leaves turning yellow
There are 7 common causes, and they need completely different responses — so working out which one you have matters more than treating quickly.
3 of them can kill the plant. Rule those out first.
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.
The 7 causes, and how to tell them apart
1.Downy Mildew
Act nowFungal disease
Yellow patches on the upper leaf surface with straight edges, boxed in by the leaf veins so they look angular rather than round. Turn the leaf over and there is grey, purple or brown fuzz directly underneath each yellow patch.
How to know it is this one
Always check the underside. Fuzzy growth beneath a yellow patch is definitive.
2.Fusarium Wilt
Act nowFungal disease
Yellowing and wilting that often affects one side of the plant, or one branch, while the rest looks fine. Slice the main stem lengthwise near the base and the tissue just under the skin is stained chocolate brown instead of pale green.
How to know it is this one
Cut the stem lengthwise: brown staining in the ring just under the skin confirms it.
3.Spider Mites
Act nowPest
Fine pale stippling across the leaf, as though it had been sprayed with a very fine dust of pinpricks. Leaves go bronze or grey, and fine webbing appears between leaves and along stems in bad infestations. The mites themselves are just visible as moving specks.
How to know it is this one
Fine even stippling rather than discrete spots or holes.
4.Aphids
TreatablePest
Clusters of small soft-bodied insects, usually green but also black, grey, pink or white, packed along new shoots and the undersides of young leaves. Leaves curl and pucker, and everything below becomes sticky with honeydew.
How to know it is this one
Dense clusters on new shoots and flower buds rather than scattered individuals.
5.Nitrogen Deficiency
TreatableNutrient problem
Uniform pale yellowing of the whole leaf, including the veins, starting with the oldest lowest leaves and working upward. Growth is slow and the plant looks generally thin and pale.
How to know it is this one
Whole leaf yellows evenly, veins included.
6.Underwatering
TreatableGrowing conditions
Drooping leaves that feel dry and papery, crisp brown edges and tips, and soil that has shrunk away from the sides of the pot. The whole plant looks deflated, and lower leaves go crisp and yellow before dropping.
How to know it is this one
Leaves dry and crisp rather than soft and limp.
7.Powdery Mildew
Mostly cosmeticFungal disease
White or pale grey powder on the upper leaf surface, looking exactly as though the plant has been dusted with flour. It starts as small round patches that merge. Affected leaves yellow, then go brown and papery.
How to know it is this one
White powder on the top of the leaf that rubs off with a finger.
Still not sure?
The diagnostic key adds two more questions — which part is affected and where the trouble started — and those usually settle it.
Run the diagnostic key