Picture this: You check your cannabis plants and see some weird spots—not round like alternaria, not the yellow of septoria. Instead, you have leaf sections that look like they’re drying out from the inside, the tissue goes gray or brown (that’s necrosis), and sometimes the petiole or main leaf vein is damaged—almost like something is eating them from the inside. That’s classic anthracnose.
How to Recognize Anthracnose on Cannabis (So You Don’t Get Fooled)
Drying, necrotic sections of leaf: Usually brown or gray, irregularly shaped, often look “burned out.”
Petiole & vein damage: You might see dark streaks or lesions on the leaf stem (petiole) or veins—sometimes the stem looks limp or hollow.
Leaf wilts “from the inside out”: Tissue between the veins dries up, even as the rest of the leaf is still green for a bit.
Advanced stage: The whole leaf crisps up and drops, leaving dried stubs on the stem.
Where Does Anthracnose Start and When Is It Most Dangerous?
This fungus—Colletotrichum or Gloeosporium—loves moisture, cool nights, and spots where leaves rub or touch (like against the tent wall or in a too-dense canopy). It often uses wounds—bug bites, cracks, or broken spots—to get started, usually on older or lower leaves, especially where things are overcrowded and damp.
How to Keep Anthracnose from Wrecking Your Grow
Inspect your plants regularly, especially petioles and leaf veins. The sooner you spot “dead patches,” the less damage the fungus can do.
Avoid packed, touching leaves—give every plant space!
Always water at the roots, NEVER on the leaves—anthracnose loves wet leaf surfaces!
Cut out any infected leaves or stems immediately: Use clean, sharp scissors—no tearing—then disinfect!
Boost air circulation and vent your space—moving air is your best friend.
What If Anthracnose Hits Anyway?
Remove all infected leaves and entire petioles ASAP, before it spreads.
Spray with natural treatments: diluted alcohol (1:1 with water), baking soda spray (2–3 tsp per liter), garlic extract.
Use sulfur, copper fungicides (like copper sulfate or Bordeaux mix), or chlorothalonil: Apply every 7–10 days, ALWAYS as directed.
Watch the pre-harvest interval! Don’t spray close to flower.
Strengthen plant defenses: Use horsetail teas, aloe or propolis extracts for foliar sprays.
Clean up the soil base regularly, remove fallen leaves—this is where the fungus hides!
If anthracnose gets into stems—cut back hard and compost worst cases.
Most Common Mistakes and Misdiagnosis
Mixing up with alternaria or septoria: Those produce rounder, more defined spots; anthracnose eats irregular patches, often following the vein.
Thinking it’s a deficiency: If just a part of the leaf dies suddenly while the rest is green, it’s almost never a regular nutrient issue.
Ignoring the petioles: Anthracnose often starts on the leaf stem/vein. If you don’t check them, you’ll miss early symptoms!
Reacting too late: If you wait until half a leaf is dead, you’re already losing the fight.
FAQ – Anthracnose in Cannabis
Does anthracnose only affect leaves?
No—if it advances, it can hit petioles, stems, and even flowers.
What conditions does it like best?
Humidity above 60%, lots of old leaves, dense canopies, and poor ventilation.
Can you save a badly infected plant?
If you catch it early—absolutely! Quick pruning and better airflow are game changers.
What’s the best spray for anthracnose?
Sulfur, copper-based fungicides, baking soda, and consistent hygiene. Always follow the label!
What about slightly infected leaves?
Better to cut them off—fungus often hides in the edges of damaged spots.
Wrap-Up
Anthracnose is a silent predator—it doesn’t make a fuss, but can devastate your grow almost overnight. Regular monitoring, knowing when to snip, and fast disinfection are far better than scrambling to save dying plants. And a set of healthy, juicy leaves? That’s every real grower’s happy ending.
