Fungi
According to the literature at least 88 fungi species attack cannabis. Here you can find a list of the most common ones.- Gray mold
Cause or pathogen: Botrytis cinerea
Description and Symptoms:
Gray mold is one of the most serious threats to cannabis plants and it can destroy the crop within a week. It is common for regions with high humidity and cool to moderate temperatures.
When infecting the seedlings, gray mold cause damping off.
In fiber cannabis plants it causes stem disease – it causes gray-brown mat of mycelium, which is covered with fungal spores. Stems become chlorotic and often snap at canker sites.
In “drug type” cannabis plants, gray mold infests flowering tops. Large moister-retaining female buds are more vulnerable, especially during flowering, near harvest time. Fan leaflets turn yellow and wilt and pistils begin to brown. It is described in the literature that Afghan cannabis strains have no resistance to gray mold and this often affects hybrids with Afghan strain as well. [1]
Recommendations: Reduce humidity by improving ventilation and do not overcrowd plants. [4]
- Hemp canker (White mold)
Cause or pathogen: Sclerotinia sclerotiorum
Description and Symptoms:
This is the second most common fungal disease observed in cannabis. It has attacked fiber cannabis plants in Europe, North America, Australia and Tasmania. It was also observed in “drug type” cannabis in India.
It causes watersoaked lesions on the stem and branches of plants near maturity. The lesions collapse into cankers and become discolored. Cannabis plants remain in this condition or wilt and fall over. [1]
Recommendations: There are several fungicides available for protection from white mold (vinclozolin). [5]
- Damping off
Cause or pathogen: Pythium aphanidermatum and Pythium ultimum (oomycetes), Rhizoctonia solani, Botrytis cinerea, Macrophomina phaseolina, Fusarium (F. solani, F. oxysporum, F. sulphurem, F. avenaceum, F. graminearum)
Description and Symptoms:
Fungi invade stems of seedlings at the soil line, causing a brown watery soft rot, then the plants topple over.
Pythium aphanidermatum is considered one of the water molds because it survives and grows best in wet soils. Warm temperatures favor the pathogen, making it an issue in most greenhouses. [1,2]
Recommendations: Poor drainage is a contributing factor to the dispersal of Pythium, as with other water mold fungi. A key to controlling disease development is to keep soil well drained and to avoid over irrigating. There are several fungicides available (Previcur). [2, 6]
- Yellow spot leaf
Cause or pathogen: Septoria species
Description and Symptoms:
Typical early symptoms are brown sunken leaf spots on leaves and cotyledons. Occasionally, the lesions have yellow haloes. These fungi are transmitted by seeds, which could cause rapid spread. [3]
Recommendations: Water the plants but keep the foliage dry. If watering the plants overhead, do it at a time of day that allows the foliage to dry quickly. Avoid watering the plants in the evening as that might allow the leaves to stay wet through the nighttime and that will favor disease. There are some fungicides available (Serenade). [7]
- Brown spot leaf
Cause or pathogen: Phoma and Ascochyta species
Description and Symptoms:
Lesions are found on lower leaves early in the season and upper leaves later on. They are usually elliptical and although chlorotic at first, soon become buff to brown in colour, often splitting longitudinally. Initially lesions have a dark brown margin with papery white centre. The fungus often invades damaged leaf tissue such as that caused by liquid urea or nitrogen. [8]
Recommendations: Ensure good air circulation around clematis stems. Remove infected leaves as they are detected. Irrigate plants in a manner that keeps water off the foliage. Apply a fungicide to protect plants. [9]
- Downy mildew and olive leaf spot
Downy mildew is caused by Pseudocercospora species. Downy mildew infections are first noticeable as light green spots on the upper side of the leaf that turn brown with time. Downy clumps of delicate white filaments and spores form on the undersides of leaves during humid weather. If the leaves are infected when young, they may become distorted. [10]
Olive leaf spot is caused by Cercospora species. A diagnostic feature is the presence of tiny black dots (pseudostromata) that form in leaf substomatal cavities within the grayish-tan lesions. Infection and lesion formation initially occur on older leaves before progressing to newer ones. Lesions at maturity are 1/8 inch in diameter and appear light gray-colored to dark tan with a brown to purple border. Severely affected leaves wither and die from coalescing lesions.
Recommendations: Don’t overcrowd plants and allow water to drain. Some fungicides may be helpful. [10]
- Pink rot
Cause or pathogen: Trichothecium roseum
Description and Symptoms:
It usually affects stems, petioles and dead leaves. Trichothecium roseum causes pink lesions, which initially appear white. It is reported that it reduces the germination potential of seeds. [12]
Recommendations: Improve greenhouse ventilation to maintain temperature and humidity. [13]
- Brown blight
Cause or pathogen: Alternaria and Stemphylium species
Description and Symptoms:
These infections are typical for hot weather and high humidity. Alternaria and Stemphylium species can cause cankers and leaf spots. Usually they infect lower leaves and form brown or black spots, then they affect the stem. [14]
Recommendations: Some fungicides could help.
- Anthracnose
Cause or pathogen: Colletotrichum species
Description and Symptoms:
Anthracnose affects developing and maturing plants. It is characterized by necrosis of leaves, fruits and stems.
Recommendations: Some fungicides could help. [15]
- White leaf spot
Cause or pathogen: Phomopsis ganja
Description and Symptoms:
Phomopsis species are developed in wet weather conditions during the spring. Cool weather and prolonged leaf wetness increases the appearance and severity of white leaf spots.
Recommendations: Some fungicides could help. [16]
- Stem cankers
Cause or pathogen: Trichothecium roseum, Phoma, Stemphylium, Colletotrichum, Fusarium and Phomopsis species
Description and Symptoms:
Reddish-brown stem lesions develop during the early reproductive stages of plants. Lesions will usually be in the vicinity of a stem node. A diagnostic symptom of stem canker is that green stem tissue will usually be present both above and below individual stem cankers. As the disease progresses, cankers will enlarge, longitudinally, turn dark brown to black in color, become slightly sunken and eventually completely girdle stems. At this point, the free flow of nutrients and water is disrupted in the plant.
Recommendations: Some fungicides could help. [17]
- Root rot
Cause or pathogen: Fusarium solani, Fusarium oxysporum, Macrophomina phaseolina, Rhizoctonia solani, Sclerotium rolfsii, Verticillium species
Description and Symptoms:
Initial symptoms include wilting – either the entire plant may wilt or only parts of the plant may wilt. Partially wilting plants are only partially infected. Partialling wilted plants may recover at night till the fungus spreads through more of the plant. On wilting tissue, the leaves soon begin to yellow then turn brown and die. Internal discoloration or streaking of the sapwood occurs in most plants.
Recommendations: Try to avoid stressing plants, especially the roots. Maintain/provide proper moisture and soil drainage. Avoid high nitrogen fertilizers – use balanced fertilizers or fertilizers with slightly higher phosphorus levels. Avoid deep cultivation around plants while they are growing. [18]
Sources
1. A review of Cannabis Diseases
2. Pythium aphanidermatum
3. Septoria leaf spot of parsley
4. Grey mould
5. Efficacy of Fungicides on Sclerotinia sclerotiorum and Their Potential for Control of Sclerotinia Stem Rot on Soybean
6. Efficacy of fungicides against damping-off in papaya seedlings caused by Pythium aphanidermatum
7. Septoria leaf spot on tomatoes: More ways to prevent spots before your eyes
8. Didymella exitialis (Ascochyta spp.)
9. Clematis Diseases
10. Management of leaf spot diseases of trees and shrubs
11. Cercospora Leaf Spot
12. Encyclopedia of Food Mircobiology
13. Post-Harvest Diseases and Disorders of Fruits and Vegetables: Volume 2
14. Early Blight Alternaria – Treatment For Tomato Plant Leaf Spots And Yellow Leaves
15. Characterization of Colletotrichum Species Responsible for Anthracnose Disease of Various Fruits
16. Phomopsis cane and leaf spot management
17. Stem Canker
18. Verticillium Wilt
