Like every other plant, Cannabis is attacked by a wind range of diseases including fungi, bacteria, viruses, and nematodes. At least 88 species of fungi have been isolated from Cannabis and more are being discovered every year.
Fungal Diseases of Cannabis
Grey Mold or Bud Rot Disease of Cannabis
The most common and significant disease of Cannabis is grey mold or bud rot, caused by the fungus Botrytis cinerea. Botrytis cinerea attacks many crop plants and weed species worldwide. It thrives in high humidity and cool to moderate temperatures. Under these conditions gray mold can reach epidemic proportions and completely wipe out a Cannabis crop within a week. Botrytis can attack the Cannabis crop at any stage of growth.
Hemp canker is believed to be the second most important disease of Cannabis. The disease is caused by the fungus Sclerotinia sclerotiorum. Symptoms of hemp canker may appear as water-soaked lesions on stems and branches of plants nearing maturity. The lesions collapse into cankers and become darkly discolored. Affected areas take on a shredded appearance and the pith becomes filled with a white cottony mycelium. Plants remain in this condition or wilt and fall over.
Damping off Disease of Cannabis
Damping of disease of Cannabis is caused primarily by Pythium species (technically they are not fungi), Pythium aphanidermatum and Pythium ultimum. Several fungi have also been associated with damping off disease. These include Rhizoctonia solani, Botrytis cinerea, Macrophomina phaseolina, and several Fusarium species, Fusarium solani, Fusarium oxysporum, Fusarium sulphurem, Fusarium avenaceum, and Fusarium graminearum.
Damping off fungi kill seeds in soil or seedlings shortly after they emerge from the soil. Fungi invade stems of seedlings at the soil line, causing a brown watery soft rot, then the plants topple over. damping off is a widespread problem, attacking all cultivars of Cannabis.
Leaf Spot Diseases of Cannabis
The most common leaf spot diseases of Cannabis are yellow and brown leaf spot diseases. The yellow leaf spot disease is caused by two Septoria species. Brown leaf spot is caused by about eight Phoma and Ascochyta species. These leaf spot diseases rarely kill the plants but sharply reduce the crop yields.
Other leaf diseases include downy mildew (caused by Pseudoperonospora species), olive leaf spot (caused by Pseudocercospora species and Cercospora species), pink rot (caused by Trichothecium roseum), brown blight (caused by Alternaria and Stemphylium species), anthracnose (caused by two Colletotrichum species) and white leaf spot (caused by Phomopsis ganjae). Powdery mildews, black mildews, and rusts are also common problems.
Stem Diseases of Cannabis
Some fungi causing leaf diseases can also attack the stems. These fungi include Trichothecium roseum, Phoma sp, Stemphylium sp, Colletotrichum sp, and Phomopsis species. Fusarium species are the most serious causes of stem cankers.
Root Rot and Wilt Diseases of Cannabis
Some root rots cause serious losses. Fungi associated with root rot diseases of Cannabis include Fusarium solani, Rhizoctonia solani and Sclerotium rolfsii. The most common wilt diseases of Cannabis include fusarium wilt (caused by Fusarium oxysporum), verticillium wilt (caused Verticillium species), and premature wilt or charcoal rot (caused by Macrophomina phaseolina).
Bacterial Diseases of Cannabis
The most common bacterial disease of Cannabis is bacterial blight caused by the bacterium Pseudomonas syringae pv. cannabina. Symptoms of bacterial blight resemble those caused by brown leaf spot. Bacterial blight has only been described on fiber cultivars in Europe. Rare bacterial diseases include crown gall by Agrobacterium tumefaciens, bacterial wilt by Erwinia tracheiphila, and xanthomonas leaf spot by Xanthomonas campestris pv. cannabis.