Alternaria fungus attacks a plethora of food, fiber, and ornamental species including Cannabis. It is important that cannabis growers recognize the symptoms early for prompt and effective control efforts. It is often misdiagnosed as a nutrient, lighting or wind burn which distract from an early and effective fungicide program which can actually save your crop.
The disease usually starts with yellowing leaves low on the plant near the crown. In short order, the disease spreads upward through the canopy in a rather random pattern. In later stages large brown spots appear near the edges of the leaves.
alternaria on cannabis – Google Search
The above photo offered on the Growers Network forum by Growmanwilk18 are very characteristic of the disease’s presentation!