Do your cannabis plants look unwell? Do you notice some cannabis leaf symptoms and are afraid there could be something worse? Here is a list of some of the common leaf symptoms in cannabis plants and their treatments. 

Some Important Cannabis Leaf Symptoms, Diagnosis, and Treatments

Boron Deficiency

This is a rare deficiency that causes your plants to grow slowly and sometimes grow abnormal plant parts. Leaf symptoms are

• Pale color of leaves
• Purple or darker leaves
• The presence of dark brown spots
• Leaf edges are browning or look brunt

Avoid the use of severely-filtered water. Use boron supplements or fertilizers rich in boron.

Copper Deficiency

Copper deficiency can lead to poor plant growth, poor ripening of the buds and may affect the younger as well as older leaves. Cannabis leaf symptoms are:

• Dark and twisted leaves
• Purple or bluish undertones
• Leaves that shine like metal
• Curling under of leaves
• Leaves tips are white or yellow

There are also weak stems, limp leaves, immature leaf growth, and lack of buds. Treat copper deficiencies by improving pH in soil and using a fertilizer that has good copper amounts.

Calcium deficiency

Calcium deficiency can lead to very poor growth; weak plant parts sand poor flower and bud growth, and development. Calcium is a very important mineral, and lack or reduced amounts in plants can lead to root problems and very weak plants that won’t survive the heat.

• Darkening leaves
• Necrotic spots on leaves, huge light brown spots
• Presence of molting and spotting
• There are tiny and distorted leaves
• Curling in of the tips of the leaves
• Dying leaves

Other symptoms include stunted growth,  crinkling and discoloration of young growths, very weak branches, hollowing or decaying stems and weak and poorly developed roots. To treat calcium deficiencies, use CAL MAG, a popular and effective treatment that can stop calcium and magnesium deficiencies.  Correcting soil pH is also important since poor pH can affect the way calcium and other minerals are absorbed.

Iron Deficiencies

Iron is important to plants despite being a trace nutrient. When a plant lacks iron, it cannot produce its food. It cannot produce new leaves. 

• Chlorosis – young leaves turn to pale yellow while the veins are green. The symptoms affect the newest growth.
• Late symptoms include young leaves turning brighter 
• The yellowing will occur in all leaves in late stages

To treat iron deficiencies, check the pH of the soil and roots. Fix pH problems and add iron supplements or use fertilizers that have higher amounts of iron.

Manganese Deficiency

Manganese is needed by plants to create enzymes necessary for producing food and for growth. Manganese deficiencies are very rare, and mostly, it’s related to deficiencies in iron and zinc. The following leaf symptoms are expected:

• Pale leaves
• New leaves start to become yellow
• Older leaves or leaves at the lower parts are also yellowing 
• Veins of leaves remain green
• Presence of brown spots
• There is a patchy pattern or speckles
• New leaves are damaged, soon, lower leaves become damaged
• Less growth and sometimes unusual growth

Plants will have very slow growth. Treatment includes correcting pH problems and using iron supplements or fertilizers rich in iron.

Molybdenum Deficiencies

Molybdenum is a rare element but is needed in trace amounts to support plant growth and development. Lack of molybdenum can result in:

• Yellowing of older leaves
• Spotted leaves 
• Leaf edges turning orange, pink or red
• Changes in color may move towards the middle of the leaves

Correct pH problems as soon as you notice molybdenum problems. Also, look for fertilizers and supplements that contain more molybdenum to replace the product you were using before. 

Nitrogen Deficiencies

Nitrogen is one of the essential nutrients needed for proper plant growth and development. It is the main macronutrient needed during the vegetative stage, and lack or reduced nitrogen intake can lead to poor growth, weak plants, and poor flower production. Some symptoms include:

• Pale or yellow leaves
• Veins are green yellowing in between the veins
• Older leaves are affected/damaged
• Old leaves may wilt and fall off

Cure nitrogen deficiency by improving soil pH and using nutrients with high nitrogen amounts. Correct this deficiency ASAP to avoid growing and flowering problems. 

Phosphorus Deficiency

In phosphorus deficiency, plants will have poor flowering performance. Your plants won’t be able to grow well and may affect your yields. 

• Dark green, gray and blue leaves affecting older leaves
• Spots and thickening of the lower leaves
• Yellowing of leaves where there are no spots
• Leaves curling or twisting
• Leaf tips burning or dying 
• Old leaves fall off

Other symptoms of phosphorus deficiency include discoloration of the stems, stunted plant growth, slower growth, poor bud development, the presence of root rot, mold, root disease, and underdeveloped seeds in female plants. Treat phosphorus deficiencies with correct fertilizers used in the flowering stage. Use organic fertilizers that contain high amounts of phosphorus. Correct root or soil pH to allow the absorption of phosphorus and other minerals.

Potassium Deficiencies

Potassium is necessary for the flowering stage as this helps plants form large and dense buds. It’s also needed during the vegetative phase but in smaller amounts only. Potassium deficiency leaf symptoms are the following:

• Leaf edges and tips turning brown or looking burnt
• Leaves pale
• Older leaves yellowing
• Leaves with dark and brown spots

Lack of potassium also results in slowed or stunted growth, twisted or abnormal plant growth, curling over or under of the leaves, very tall plants or too stretchy plants, and poorly formed buds. Correct potassium deficiency by using fertilizers that are made for the flowering stage. Also, check and correct soil and root pH to promote the absorption of nutrients. Use organic fertilizers with high amounts of potassium.

Sulfur Deficiency

Sulfur is abundant in soil, and thus, deficiencies occur in plants grown in hydro. This element can help promote plant health and overall development. Lack of sulfur will result in these leaf symptoms:

• Pale leaves, yellow new leaves
• Red or color pink on leaves
• Upper leaves and new leaves are abnormal or damaged
• Older leaves are also damaged
• Reduced growth

Sulfur deficiency also results in fluffy or airy buds, poor growth, and slow growth. Cure sulfur deficiencies with fertilizers and supplements high in sulfur. Correct pH ASAP to avoid complications.

Zinc Deficiencies

Zinc is needed in the growth of new leaves, in proper plant development and the formation of large and dense buds. Zinc deficiencies go hand in hand with iron and magnesium deficiencies. The following can be seen in plants that lack zinc:

• Yellowing of leaves
• Specks and irregular patterns on leaves
• Upper and new leaves are damaged
• Burnt tips, dying tips
• Curling under of leaves

You will also notice poor growth, non-vertical growth, and other abnormalities in bud formation in plants that lack zinc. To treat zinc deficiencies, use zinc supplements. Correct iron and magnesium deficiencies as well because these affect zinc absorption.

Leaf Septoria

These are yellow spots on leaves caused by the fungus Septoria cannabis and the S. neocannabina species. These opportunistic creatures attack during rain and warm weather and are common in marijuana grown outdoors. 

• White or grayish-white, yellow marks or spots on leaves
• Presence of circular lesions on the lower leaves
• A pimple-like bump on leaves
• Leaves may dry and just fall off 

It can be hard to treat septoria, but you can use helpful plant-bacteria such as Bacillus pumilis or Trichoderma that can remove the fungus in the soil. Prevent this from happening by prepping the soil before you grow your plants.  

Powdery-White Mildew 

This is a very common fungus that can affect your plants, especially during the flowering stage. It’s hard to treat PWM, and thus, most growers harvest early to save their buds. 

• Presence of white material on leaves
• Will spread on all the parts of the plant, including the buds in just a short amount of time. 

Harvest early, but if you can’t use homemade treatments like spraying a solution of water and liquid soap or water and baking soda plus vegetable oil. Spray these on your plants, even parts that are not affected. Use tissue paper to lift the mold from the leaves. Discard the tissue right after to prevent the spread.  

It’s better to prevent PWM than treating it. Be mindful of the humidity inside your growing area. Make sure to reduce humidity and control moisture inside the room to prevent mold and mildew growth. Grow only a few plants and avoid plants touching each other inside the room. 

Fusarium 

Fusarium targets the leaves and the roots of cannabis plants. This is a type of fungus that lays dormant in tropical soil and can be activated in the right conditions. This is often spread through the use of fusarium-infested tools and running water. Signs are

• Leaf wilting; color from green – yellow-brown in just a few days.
• Tips of wilted leaves are curling upwards
• Leaves appear to drop off but are still on the plant

Use fungicides that can kill fusarium. However, this pesky fungus is very difficult to kill and will usually persist even after you have applied fungicides. You may also apply hydrogen peroxide to the soil to clean and disinfect it. The best way to deal with fusarium is prevention.

Prevent fusarium by using clean and sterile garden tools and equipment. Use good quality soil. Never reuse soil with fusarium.

Verticillium Wilt

This is another fungus problem that is common in cannabis plants. This happens in improperly drained, rich soil. Signs of Verticillium wilt are:

• Yellowing older leaves
• Yellowing is seen in between the veins and along the outside edges of the leaves
• Leaves turn grayish-brown especially leaves found on the base or near the soil

There is no effective way to treat verticillium wilt. The only way to save your plants is through prevention. Use clean and healthy soil or grow your plants in hydro to completely prevent this condition. Use clean and sterilized garden tools and equipment. 

Damping Off

When seedlings and seeds are attacked by fungi in the soil, the young plants may simply topple over and will soon die. This is a condition known as damping off. Symptoms include

• Death of the plant
• Lesions in the stems
• Cankers between the nodes
• Plant falling over or wilting
• Lesions on the upper portion of the plant

There is nothing you can do with damping-off, but you can prevent this by growing in healthy soil.

 You may also germinate seeds in peat cubes and transfer to a better growing medium like hydroponics. Use seeds with strong genetics to avoid damping off. 

Hot and Humid Conditions

A small grow room or improperly monitored growing area, humidity can easily increase, and this can be fatal to your plants. This usually happens right after watering or when there are just too many plants inside the growing area. The plant leaves will exhibit the following symptoms:

• Leaves closing 
• Leaves are drooping

Correct humidity problems right away. For a room with high humidity, improve conditions by using a fan or opening a window or door. Let clean, fresh air enter to remove moisture. Use a small fan and place on the floor to reduce moisture and prevent mold growth.  Take note that molds and mildew develop in very humid environments.

Underwatering/overwatering

If your leaves are drooping or your plants look unwell, you may suspect overwatering or underwatering. Correct these conditions right away. If the soil is too dry, water your plants ASAP. 

If the soil is moist or wet and yet the plant is drooping, use a fan to dry it out. Remember to water only when the soil is dry and never when it’s moist and wet.

There may be more cannabis leaf symptoms; nevertheless, always monitor your plants and be on the lookout for any problems. Correct problems right away to prevent anything that can interrupt your plants’ growth and bud development.