This picture is often misdiagnosed as Leafhoppers but it is cannabis leaf damage of a four-lined plant bug (Poecilocapsus lineatus), which make spots in clusters.
Plant Problems Symptoms: Bronze or brown patches
Planthoppers can attack and infest cannabis, leaving a unique cotton-like growth while they suck the life out of your plants! Planthoppers also spread plant diseases, so you want them away from your plants as soon as possible. Example of leaf damage from planthoppers. (Thanks to George4green for several of the awesome planthopper pics) Melanoliarus planthopper
Manganese deficiency can cause cannabis leaves to become yellow in between the veins, with mottled brown spots on the affected leaves. Learn how to treat it!
Cannabis leaf edges and tips turning yellow? Copper deficiency can cause dark, shiny leaves with purple or blue undertones. Learn how to spot and treat it!
Discover how to finally beat root rot (brown or slimy roots) for good! The secret to curing and prevention is a supplement with a particular type of bacteria!
Leaves with a potassium deficiency have yellow, brown, or burnt edges and tips. It may look like nutrient burn, but the leaf margins also turn yellow.
Boron deficiencies are rare and appear as brown blotches in the center of leaves. See what a cannabis boron deficiency looks like and learn how to treat it!
Too hot for your cannabis in the grow room? Learn how to save your plants from heat stress with a variety of techniques as well as certain supplements!
Phosphorus deficiencies cause dark splotches on leaves and can appear at the bottom of the plant on some of the oldest leaves. Learn to spot & fix the issue.
Overwatering and underwatering cannabis plants are easy to do and can cause multiple symptoms including slow growth. Learn to recognize and fix these issues.