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Home » Black Spots On Cannabis Leaves: Causes And Treatments

Black Spots On Cannabis Leaves: Causes And Treatments

Blacks and brown spots can appear on cannabis leaves for various reasons, most of which have simple solutions. However, growers who notice discoloured patches on their marijuana plants would do well not to hang around, as rapid treatment is essential. In many cases, diagnosing and correcting the problem early makes the difference between healthy plants and a ruined crop.

Nutrient Deficiencies That Cause Black Spots On Cannabis Leaves

The majority of brown or black spots on the leaves of cannabis plants are caused by some sort of nutrient deficiency. However, rather than rushing out to the garden centre to buy more fertiliser, growers should first test the pH of their soil. In most cases, the problem has nothing to do with a lack of minerals but is instead caused by pH levels that are either too high or too low.

Cannabis plants thrive in soil with pH between 6.0 and 7.0. Values outside this optimal range can lead to nutrient lockout, whereby the roots cannot absorb the nutes in their environment.

Only after ascertaining that no lockout has occurred should growers consider supplementing their growing medium with extra nutrients. Figuring out which ingredients to add requires some extra diagnosis, although calcium is often the answer.

Lack of Calcium

Calcium deficiency shows up as spots on new leaves before spreading to older ones. Some beginners might mistake these signs for the symptoms of pest infestation, although more experienced growers can easily tell the difference. For instance, bugs tend to appear all over a plant, while calcium deficiency causes spots to manifest on some leaves but not others.

black spots cannabis

Usually, leaves that are directly exposed to light are the first to become discoloured, while the lower leaves, which are more shaded, retain their verdant splendour until the problem becomes more severe. Growers who notice this tell-tale sign can quickly remedy the situation by feeding their plants some calcium-magnesium solution.

Potassium Deficiency

Potassium, meanwhile, is a vital nutrient that becomes particularly important during the flowering stage. Plants suffering from potassium deficiency at this point in their life cycle often develop black and brown spots on their leaves and may produce disappointing yields. Assuming nutrient lockout has been ruled out, the problem can usually be fixed by introducing a source of potassium like kelp meal or wood ash.

black spots cannabis

Phosphorous or Nitrogen

Though less common, phosphorus deficiency can also lead to leaf discolouration, while manganese deficiency can be resolved by adding manganese chelate to the soil. Nitrogen deficiency, meanwhile, results in yellow leaves that later turn brown and die due to an inability to photosynthesise. Because cannabis plants absorb nitrogen reasonably well even when the pH is suboptimal, this problem is rarely caused by nutrient lockout.

Growers who suspect a nitrogen deficiency should therefore supplement their cannabis plants with nutrient-rich ingredients like bat guano or worm castings.

black spots cannabis

Fungal Infections That Cause Black Spots On Cannabis Leaves

Leaf septoria – also known as yellow leaf spot – is a fungus that can devastate a cannabis plant if left untreated. The disease typically manifests as yellow and brown spots appearing on the lower leaves as a plant enters the flowering stage before spreading to the higher leaves.

Affected leaves eventually wither and die, so failing to deal with an infection early can have damaging consequences.

To avoid this destructive fungus, cultivators need to ensure that the environment in their grow room is not conducive to the spread of such organisms. Temperature and humidity levels should therefore be controlled at all times, while airflow can be improved by pruning bushy plants. Such measures can help keep all kinds of mould at bay, including powdery mildew.

Manipulating the environment is obviously less feasible when growing outdoors, so growers should pay particular attention to their plants during warm, humid weather. It’s also a good idea to avoid planting in a spot that has previously been affected by leaf Septoria, as spores could be lurking in the soil.

Once the disease sets in, it’s best to remove all affected leaves. Some growers also use fungicides while others prefer natural antifungals like neem oil. Acting fast can save a plant from disaster, although the grow medium should be disposed of and the growing space sterilised before any more seeds are planted.

Insects To Look Out For

From aphids to leafhoppers, there are loads of insects that love a good bit of weed. Spider mites, for instance, are a particularly despised enemy of cannabis growers everywhere.

Though tiny, these horrible little creepies can attack in large numbers and are difficult to get rid of once an infestation takes hold. With a voracious appetite, spider mites suck the nutrients out of marijuana leaves and stems, causing them to become pale before later turning brown and dying. Sometimes the poop of spider mites shows up as small black spots on cannabis plants, although this isn’t harmful per se.

Though the insects themselves are too small to see, the webs they spin between the fan leaves of a plant indicate that your grow space has been visited by these uninvited pests. By the time the webbing becomes noticeable, however, it’s often too late to save a plant, so it’s essential to act fast if you think you’ve got spider mites.

Introducing predators such as ladybirds is the most effective way to keep spider mites under control, although even these helpful little bugs are unlikely to eradicate a large infestation.

Thrips are another pain in the backside for cannabis growers. As with spider mites, these little yobs poop all over your cannabis, leaving black spots on the leaves and stems. Once again, this excrement isn’t harmful but is indicative of a much larger problem as thrips suck the sap out of cannabis plants.

Growers usually notice fan leaves turning yellow with a few brown spots once thrips get to work. Over time, the situation deteriorates and leaves eventually die off.

Getting rid of thrips can be tricky if you’ve got lots of them, though neem oil and other pesticides are quite effective. Just remember never to get any of these substances on your buds, as they can wreck the taste of your weed. Placing sticky yellow strips around your plants also lures the bugs in and traps them and is an easy way to deal with a thrips infestation.

Environmental Factors

Brown and black spots on cannabis leaves often signify an environmental imbalance of some sort. We’ve already mentioned how suboptimal pH levels can cause nutrient lockout while high humidity encourages fungal infections, although growers also need to keep an eye on light and heat within their grow space.

Heat stress can occur if plants are suddenly subjected to a dramatic increase in temperature which often results in funky discolourations, which is why it’s important to prevent your equipment from overheating the room. Fans and air conditioning provide an obvious solution to this problem while switching from HPS to LED lights can also prevent temperature increases.

Speaking of illumination, light burn is another common cause of black and brown spots on cannabis leaves. Ironically, LED lights present the most significant danger, as their lack of heat lulls many growers into placing them too close to their cannabis plants. All those lumens can be too much for some leaves to handle, so it’s wise to raise the lights by a few inches if you notice some ugly leaf spots.

black spots cannabis

Finally, giving plants the wrong amount of water can cause all sorts of visible problems, including spots. Overwatering raises the risk of root rot as plants get “wet feet”, often leading to saggy, discoloured leaves. Wet soil also attracts pests like fungus gnats, which can then attack the roots of your plants.

On the other hand, underwatering causes roots to dry out and prevents plants from carrying out basic biological functions. If this goes on for long enough, chlorophyll starts to disappear, and veg turns from green to yellow to brown.

Fortunately, most of the causes of black spots on cannabis plants are relatively easy to solve, provided you act quickly and don’t wait until the problem has become too severe. Often, the solution can be found in the basics of cannabis growing, so ensuring you’ve got decent air circulation, the right lighting and a suitable watering regime is the first step towards restoring the green glory of your weed plants.

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

Ben Taub