Diagnose: Yellow Leaves on Sweet Basil

1. Turn a yellow leaf over and look at the underside
2. A dense gray to dusty purple fuzz on the underside, sitting in angular patches that stop at the leaf veins, is downy mildew. Nothing about your watering caused it and nothing about your watering will fix it, so stop adjusting and read that card. If the underside is clean, slice the main stem open lengthwise near soil level. Dark brown streaking running up inside a stem that still looks green outside is a soil-borne wilt, and that plant is finished. A clean underside and a clean cut means the cause is care or environment, and every one of those is fixable. Sort them by which leaves went yellow and what the soil felt like when it happened. Bottom leaves first over wet soil points one way, an even pallor across the whole plant points another, and yellowing that followed a hard wilt or a cold night points somewhere else again.

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