Seeing anything odd on your cannabis fan leaves, such as black spots, can be cause for alarm. With so many little things to remember it can still get overwhelming. Especially, if you notice your leaves curl discolor. Cannabis is an excellent silent communicator that will tell you exactly what’s happening to it.
Noticing black spots on your cannabis leaves can be a cause for alarm. That’s why it’s important to address the situation quickly before it turns into something that could ruin your crop.
But what causes black spots to appear on the fan leaves? It can actually be several different problems, either severe or easily rectified. In fact, it could be the progression of certain deficiencies or diseases that if left unchecked, could be disastrous.
Luckily, if you don’t leave it too long, cannabis is prone to bouncing back quickly as long as you address the situation. First, let’s take a look at some possible causes of black spots appearing on your cannabis fan leaves.
What Can Cause Black Spots On Fan Leaves?
If you have black spots on your cannabis plant it can catch you by surprise. Here are some possibilities to look into with possible solutions.
Calcium is the nutrient responsible for fighting disease. It helps promote strong cell wall structure and supports healthy growth patterns. When this nutrient is deficient in your plant it can start to show up as black/brown spots on your cannabis fan leaves.
Calcium and magnesium tend to show a deficiency together which is why a lot of growers use a cal-mag solution in their regimen.
A calcium deficiency will show up on both the older and newer cannabis fan leaves. That makes it easier to diagnose than other nutrient problems. However, it’ll generally start with the newer growth and since it is a semi-mobile nutrient, will work its way to the older growth.
Older leaves damaged in this way cannot recover and will need to be taken off.
Growers who use tap water to water their plants will seldom see a calcium deficiency as it’s generally added to city water. The same goes for hard water which is usually loaded with calcium. Growers who use reverse osmosis water will likely be more prone to developing a calcium deficiency in your plant.
If you leave a calcium deficiency unchecked it can lead to things such as weak stems, stunted root growth, and eventually root death. Luckily, the fix is generally an easy one. Just supplementing your plant with a little calcium-magnesium mixture will generally bring your plant back to normal.
Manganese deficiency can be tricky to diagnose as it looks like other nutrient issues (like calcium, for example). This immobile nutrient is needed for photosynthesis and helps the plant absorb nitrogen.
Since this is an immobile nutrient you will notice it affecting the new growth on your plant and generally starts with dark spots around the perimeter of the fan leaves.
If left unchecked, plants will start to show chlorosis and then progressively get necrotic, paler, and eventually fall off. When your fan leaves fall off they won’t be able to absorb light and photosynthesis comes to a stand-still.
As the deficiency progresses it will move to the adult leaves as your cannabis tries to supplement the newer growth that is dying.
Causes of manganese deficiency are directly linked to the PH of your water. If you have a high PH (above 6.5) your cannabis plant experiences nutrient lockout and can’t utilize the manganese in the soil. If you’re growing outdoors the cause could also be a lack of manganese in the soil.
To fix a manganese issue means to fix the PH of your water source, as well as the soil your plant lives in. This means that you need to flush your cannabis plant with PH corrected water to get your EC and PH back in balance. Ensure to check the drainage of your pot to make sure that water is not getting stuck in the pot, this could cause root rot.
If you don’t see any positive results within a week of adjusting the PH, you can add something like a manganese chelate. This will help to bump up the levels in your growing medium.
Having an infestation of spider mites or other insects is rare but it is possible and oddly enough, can cause black spots on your cannabis fan leaves. Believe it or not, the black spots can actually be feces from these insects.
Fortunately, it’s very easy to identify if you hate a mite issue with your plant. The leaves will more than likely be withering and you’ll see spider-like webs between the fan leaves. The best way to eliminate spider mites from your cannabis plants would be to introduce predatory bugs or ladybugs.
Cannabis plants are subject to fungal infection and can be quite susceptible to them if they aren’t taken care of properly. Leaf Septoria is an aggressive fungal disease that halts the growth of your cannabis plant by destroying foliage and crippling photosynthesis.
Initially, Leaf Septoria will start during the early part of flowering and will appear as yellow and black spots on your leaves. Eventually, it’ll progress throughout the entire leaf which will wither and fall off. This will travel throughout the plant until it is too weak to continue. Eventually, it’ll have to be harvested early or given a proper burial.
Leaf Septoria, like most fungus, thrive in constant humid, damp environments. Ensuring you have adequate airflow in your grow room, proper humidity control, and control over how much you water your plant will mitigate any kind of fungal growth.
If you do come across this particular fungus it’s best to remove all the affected leaves and track your humidity, air circulation, and water schedules to dry out the area as much as possible to inhibit any more fungal growth.
Keep Your Cannabis Plants Environment Balanced
Growing cannabis can be a balancing act; meaning you have to make sure to keep all of the functions of your growing plant within a certain range.
Making sure your PH and nutrient strength is within a specific range keeps the plant healthy and growing. Also, maintaining the environment around it stops any external forces from killing your plant from the outside in. As long as you pay attention to what your plant is telling you it’ll grow huge, healthy buds with minimal maintenance.
Learn what the red tiny hairs on your cannabis buds mean and other interesting cannabis-growing facts on our website!