After you’ve spent time and money on your grow room, it’s not a good feeling to walk into your greenhouse and spot wilting plants. Or leaves twisting and curling. Or brittle stems. Even worse, there may be no visible symptoms at all until the end of the growth cycle. When you should be getting bud, you get duds instead.

Worried you might have plant pathogens in your grow room? Here are five common cannabis pathogen symptoms and what they might mean.

1. Plants are stunted

2. Leaves are turning yellow

3. Leaves are deformed

4. Plants are wilting

5. Flowers are too small and aren’t potent

Having a cannabis emergency?

1. My cannabis plants are stunted

Stunting can sneak up on you. At first, plants might just seem stuck in an early growth stage. They just don’t progress. Another sign of stunting is if the stem points where leaves or branches begin are too closely spaced.

Disease, pests or poor growth conditions may be to blame for cannabis plant stunting. Set up the conditions for success by understanding cannabis cultivation practices.

Pathogens in the grow room could cause stunting. Stunting is an early-stage sign of several pathogens, so it can be hard to narrow it down based on this symptom alone.  Common pathogenic culprits of stunting include:

• Beet curly top virus (BCTV). If your cannabis plant has BCTV, it may show stunting. Other tell-tale symptoms of BCTV include leaf deformation, especially with a curly top appearance.

• Hop latent viroid (HLVd). HLVd (sometimes called HpLVd) can cause stunting during the vegetative stage of cannabis growth. During this stage, the cannabis plant should be growing rapidly and developing stems, branches, and leaves, but HLVd can cause stunting. HLVd may be difficult to spot at this stage, however. The most notorious symptom of HLVd is dudding (more on that coming up).

• Lettuce chlorosis virus (LCV). LCV can also cause stunting, though this is not the most obvious symptom of LCV. Other symptoms of LCV in cannabis include interveinal chlorosis and leaf distortion.

If you’re plants aren’t growing like they should, it might be growth conditions, but if it’s a pathogen, you don’t want to let it spread unchecked in your grow room. If you don’t have a plant pathogen testing regimen, try isolating or destroying the impacted specimen. Then make sure your grow conditions are dialed in to promote plant health.

2. My cannabis plant leaves are turning yellow

Yellowing, or chlorosis, can impact the entire leaf or only specific areas. Yellowing may start at the leaf margins and progress inward. Or the leaves may turn yellow while the veins stay green; this is called interveinal chlorosis.

Leaf yellowing may be caused by disease, pests or chemical damage (like herbicide drift or too much fertilizer). Make sure the conditions in your grow room are dialed in. A pH imbalance may cause yellowing. You might also have light or temperature stress or root rot from poor drainage.

Plant pathogens can also cause leaf yellowing. Some of the usual suspects include:

• Beet curly top virus (BCTV). Leaf yellowing in cannabis plants is a hallmark symptom of BCTV, as well as leaf curling and deformation. BCTV can also cause stunting and purple veins, as well as premature flowering and overall plant decline and death. BCTV can be transmitted by the beet leafhopper, so controlling for pests can help reduce spread.
• Fusarium oxysporum (F. oxysporum). F. oxysporum is a soil-borne fungus that may cause severe yellowing. With F. oxysporum, yellowing often starts on the lower leaves and moves upwards. If you cut into the stem, you may see dark discoloration in the veins, a clear indicator of F. oxysporum. The plant's veins (the vascular system) may also wilt to wilt.
• Hop latent viroid (HLVd). HLVd can cause chlorosis, along with other symptoms like brittle stems and leaf deformation. However, HLVd is most notorious for causing dudding – reduced flower mass and potency – right at the end of the growth cycle.
• Lettuce chlorosis virus (LCV). If your plant leaves are turning yellow between the veins, but the veins are still green, you may have LCV. The leaves may become curled and twisted or appear smaller than normal. Later, you may get brown or black patches in the leaves, or entire leaves may wilt, dry up, and fall off. Later in the growth cycle, LCV can result in smaller, malformed, less potent flowers.

3. My cannabis plant leaves are deformed

It just doesn’t look right.

The cannabis plant leaves are deformed. Maybe they’re curled up or twisted. Maybe the leaves are rough or brittle.

It could be pests. It could be that your grow room isn’t optimal. But if you’ve got your lighting, nutrients, humidity, temperature, watering, and pH dialed in, you might start to wonder if a plant pathogen is spreading in your grow room.

Some of the common pathogenic perpetrators of leaf deformation include:

• Beet curly top virus (BCTV). If your cannabis leaves are curled and deformed, BCTV might be spreading in your grow room. One of the first signs of BCTV is stunted growth, but one of the most tell-tale signs is leaf curling or twisting, especially at the edges. Leaves may turn yellow between the veins, and sometimes the veins may turn purplish or reddish.
• Lettuce chlorosis virus (LCV). Leaf distortion may be caused by LCV. Cannabis plants with LCV may also have necrosis (localized death of plant tissue), interveinal chlorosis (yellowing between the veins), or purple discoloration (especially on the undersides of leaves or along the veins).

4. My cannabis plants are wilting

Are your cannabis plant leaves or stems drooping and sad? Maybe the drooping started on one side of the plant and spread, and now the whole plant just looks deflated.

Of course, check your growth conditions. Your watering regimen might be off. Your plants may be root-bound.  Ensure your temperature, humidity, nutrients, and pH are all good. Wilting may also result from transplant shock or physical damage.

Some plant pathogens might also cause wilting. Some wilting suspects might include:

• Fusarium oxysporum (F. oxysporum).  If your plant is wilting, you may have F. oxysporum. Vascular wilt is the most common symptom. If you cut into the plant, you may see browning in the veins. Your plant may also wilt on one side or suffer overall collapse. F. oxysporum infected plants may show chlorosis, stunting, and root rot.
• Phytophthora or Pythium. Wilting is often the first sign that a plant has Phytophthora or Pythium. The plant may then start to yellow, especially the lower leaves. As the disease progresses, roots may be dark and rotting. You may see cankers or lesions at the base of the stem near the soil line. The cankers may turn brown as the infection spreads. Phytophthora and Pythium work fast, so act fast to isolate or destroy this specimen.

5. My cannabis plant is a dud. My cannabis flowers are too small and aren't potent.

It’s the end of the growth cycle, right when you expect your investment and all your hard work to pay off, and then BAM!

You notice that your stems snap easily under pressure. Your plants show a general lack of vigor. They don’t have a robust plant structure. Worse, they produce low yields and potency. The flower mass is smaller and has lower trichome production.

You might have pests, or there might be a sudden change in growth conditions that is impacting your cannabis plants.

Or you could have the pathogen that is #1 on cannabis growers’ least wanted list:

• Hop latent viroid (HLVd). HLVd is a sneaky little pathogen. It’s notorious for reducing the mass and quality of flowers. Late in the growth cycle, plants suddenly produce lower yields with reduced trichome density. HLVd duds your bud with few warning signs until late in the growth cycle. Some other symptoms to watch out for include stunted growth, with shorter and fewer branches. Stems may be brittle and easy to break. You may get leaf deformation and yellowing.

According to an article published in the Canadian Journal of Plant Pathology in November of 2023, over their 3-year study, the average HLVd infection rate was 25.6% in Canada (Punja, Wang, Lung, & Buirs, 2023). Infection rates in the U.S. are thought to be even higher, so testing is essential to weed out HLVd-positive plants and protect yield and potency. A study conducted by Dark Heart Nursery estimated that 90% of cannabis grown in California was infected with HLVd.

Other plant pathogens, like Fusarium oxysporum, Phytophthora, Pythium and Lettuce chlorosis virus can also cause dudding. However, these pathogens typically show other symptoms earlier in the growth cycle.

HLVd is the scary one that spreads like a ghost and then duds your bud right at the end.

How do I protect my cannabis plants?

Sanitize tools and equipment regularly, enforce hygiene protocols and eliminate infected plants. Control pests that can transmit pathogens, especially HLVd. Learn more about plant pathogens.

If you think your cannabis plant has a pathogen, consider isolating or destroying it so that the pathogen can’t spread. It may be time to begin a testing regimen with a trusted plant pathogen provider.

Looking at a plant and knowing what is going on inside is almost impossible. Test frequently, especially before new clones enter your grow room, and eliminate infected plants before HLVd spreads in your grow room.

Don’t wait and wonder. Test and know.

Contact Agdia

Contact Agdia and learn about our Amplifire® Pro testing machine and AmplifyRP® test kits.