What the spots do
Cause
Septoria cannabis and S. neocannabina, two cannabis-specific fungi
Look
yellow, white, or gray-brown spots, often with reddish-brown edges, starting on lower leaves
Progress
tissue dies, dries, and drops out, leaving holes; severe cases defoliate the bottom
Yield
not usually fatal, but photosynthesis loss can cut the harvest
Weather and spread
Trigger
Aromawarm rain releases spores
Start
Aromainfections often begin in the 60s F (15 to 20 C) and get worse toward just under 80 F (25 C)
Carryover
Aromaspores overwinter on fallen cannabis leaves and debris
Vectors
Sanitation
Neighbors
risk rises if cannabis or hops is growing nearby
Same patch
last year’s garden is the usual outdoor source
Cleanup
remove residue, then treat the area; move the plot if you can
Tools
clean and decontaminate anything that touched infected plants
Leaf-surface treatments listed
Copper
a long-used fungicide with many garden brands
Bicarb
potassium bicarbonate (about a 1 percent solution in the book) beats baking soda because sodium can build up
pH-up
an alkaline leaf surface around pH 9 is listed as harder for spores to germinate on
Also listed
compost tea, neem, sesame or fish oil, sulfur, Trichoderma, and commercial herbal-oil fungicides