Lecture 18: Emerging diseases of cannabis in controlled environment

1. What are the major emerging cannabis diseases discussed in controlled environments?

• Root rot

• Crown rot

• Fusarium wilt spread by flies

• Powdery mildew (management evaluated specifically in marijuana plants)

• Pythium root rot (a major threat in hydroponics)

• Hemp leaf spot caused by Bipolaris (first reported in Kentucky)

• Hop latent viroid (HLVd) found in Cannabis sativa in California

What symptoms do root and crown rot typically cause in cannabis?

• Severe wilting

• Plant collapse in field-grown cannabis

• Brown, necrotic (dead) roots

• Dark, sunken lesions at the plant crown

• Visible mycelium on infected tissue

• Internal brown/black discoloration in xylem and pith when stems are cut open

In what growing environments does root and crown rot severely impact cannabis?

• Controlled environments

• Field-grown cannabis

• Hydroponic systems (especially for Pythium)

What environmental factors influence the spread and severity of root and crown rot?

• High moisture

• High plant density

• Environmental stress

• Wounding of plants

What does root rot typically look like?

• Roots turn brown

• Roots become soft and necrotic

• Tissue sloughs off easily

• Rot is visible before above-ground symptoms appear

How does crown rot appear on cannabis plants?

• Dark brown or black lesions at the base of the stem

• Sunken, decayed crown tissue

• Mycelium sometimes present at the crown

What internal symptoms are visible when cutting open stems infected by root/crown pathogens?

• Brown or black vascular discoloration

• Darkening of xylem

• Darkening of pith

• Evidence of internal fungal spread

Why do symptoms sometimes only appear when a cannabis plant experiences stress?

• Stress reduces plant defenses

• Latent infections accelerate under heat or water stress

• Pathogens block xylem more effectively during stress

How can these infections impact cannabis crops overall?

• They can devastate entire crops

• They reduce yield

• They kill plants suddenly

• They require intensive management

Why is Fusarium management particularly important in cannabis cultivation?

• Fusarium can spread rapidly

• It causes vascular wilt, root rot, and crown rot

• It survives long-term in soil/media

• It infects multiple stages of growth

What fungal species are especially problematic for cannabis?

• Fusarium oxysporum

• F. proliferatum

• F. solani

What diseases can Fusarium cause in cannabis?

• Crown rot

• Root rot

• Damping off

• Vascular wilt

How do Sciarid and shore flies contribute to Fusarium spread?

• They act as aerial vectors

• They carry fungal propagules on their bodies

• They transfer the pathogen between plants in greenhouses

How do root-feeding insects increase fungal and oomycete infection severity?

• They wound plant tissue

• They make infection entry easier

• They create synergistic interactions that worsen disease severity

At what temperature range does Fusarium grow best?

28°C to 34°C

What pH range is optimal for Fusarium growth?

Slightly acidic to neutral pH 5–7

What substrates can Fusarium survive in?

• Soil

• Rockwool

• Peat-based growing media

How does Fusarium typically infect cannabis roots?

• Contaminated soil or media contacts the roots

• Fungus secretes cell-wall–degrading enzymes

• Root tissues rot

• Mycelium invades xylem

• Water and nutrient transport becomes blocked

What are the above-ground symptoms of Fusarium infection?

• Wilting

• Yellowing

• Stunting

• Collapse under stress

• Crown rot or root rot depending on infection site

At which growth stages can Fusarium infect cannabis?

• Propagation

• Vegetative stage

• Flowering stage

• Cutting stage (causing damping-off)

What does Fusarium mycelium look like on cannabis tissue?

• White

• Orange

• Pink

What do Fusarium spores look like under a microscope?

• Long

• Curved

• Canoe-shaped

What virus stunts and distorts hemp and is spread by leafhoppers?

• Beet curly top virus

What virus in hemp is spread by whiteflies and not by seed?

Lettuce chlorosis virus

What viroid causes stunted growth and flower loss in hemp?

Hop latent viroid (HLVd)

What pathogen causes cannabis bud rot?

Botrytis cinerea

Under which environmental condition does bud rot develop most rapidly?

High humidity

How does Botrytis spread in cannabis?

• Rapid colonization of tissues

• Infection of leaves, buds, stems, and flowers

What organisms cause post-harvest decay in cannabis?

• Botrytis cinerea

• Penicillium species

• Fusarium species

• Aspergillus species

• Cladosporium westerdijkieae

What general pathogen management practices are recommended for cannabis?

• Improve air circulation

• Control humidity

• Remove diseased plant parts

• Destroy infected debris

• Use resistant varieties

• Monitor for emerging pathogens (ex: Sclerotinia)

• Maintain strict sanitation post-harvest

What three elements are collectively essential for reducing pathogen impact in cannabis production?

• Integrated disease management

• Vector control (flies, insects)

• Continuous monitoring

Which pathogens cause damping off in cannabis?

• Fusarium oxysporum

• Fusarium proliferatum

• Fusarium solani

What humidity-related practice helps manage damping off?

Have appropriate humidity levels

How can airflow help control damping off?

Improve air circulation

What biological approach is recommended for damping off during propagation?

Apply biocontrols during propagation

What sanitation step helps prevent the spread of damping off?

Remove diseased cuttings

What plant replacement measure helps reduce damping off issues?

Replace stock plants regularly

Which pathogens cause Fusarium root and crown rot?

• Fusarium oxysporum

• Fusarium proliferatum

• Fusarium solani

What testing practice helps prevent Fusarium root and crown rot?

Test mother plants to ensure they are free of disease

What plant replacement practice helps reduce Fusarium root and crown rot?

Replace stock plants regularly

What biological strategy is recommended during propagation and vegetative stages for Fusarium root and crown rot?

Apply biocontrols during propagation and vegetative stages of growth

What cultural practice prevents Fusarium root and crown rot?

Avoid injury to roots

Which pathogens cause Pythium root and crown rot?

• Pythium myriotylum

• Pythium dissotocum

• Pythium aphanidermatum

Which watering practice helps prevent Pythium root and crown rot?

Avoid overwatering

Which cultural practice helps reduce Pythium infections?

Avoid injury to roots

What biological control strategy is recommended for Pythium root and crown rot?

Apply biocontrols during propagation and vegetative stages of growth

What water treatment method helps control Pythium?

Treat irrigation water with UV or chlorine

Which pathogen causes powdery mildew in cannabis?

Golovinomyces spp.

How can airflow help manage powdery mildew?

Improve air circulation

How do temperature and humidity management help control powdery mildew?

Manage temperature and humidity

What plant removal practice reduces powdery mildew spread?

Prune and de-leaf plants

What products can be applied to control powdery mildew?

Products such as Regalia Maxx, MilStop, ZeroTol, or others

How can light be used to manage powdery mildew?

Treat plants with UV-C light

. What sanitation practice helps reduce powdery mildew?

Remove and destroy diseased leaves

What genetic strategy can reduce powdery mildew susceptibility?

Plant resistant varieties

Which pathogens cause bud rot in cannabis?

• Botrytis cinerea

• Fusarium spp.

vHow does airflow help manage bud rot?

Improve air circulation

How can growers use temperature and humidity control to prevent bud rot?

Manage temperature and humidity

What sanitation step helps remove bud rot sources?

Remove diseased bud

What plant selection practice helps reduce bud rot?

Avoid varieties with dense inflorescences

Which pathogens cause post-harvest decay in cannabis?

• Botrytis cinerea

• Penicillium spp.

What environmental controls help prevent post-harvest decay during drying?

Maintain appropriate humidity, temperature, and air movement in drying rooms

What handling practice helps prevent post-harvest decay?

Avoid damage to buds during harvesting and trimming

What removal practice helps control post-harvest decay?

Remove infected post-harvest buds