Fungi: At least 88 species of fungi attack Cannabis and more are being discovered every year (McPartland & Hughes 1994, McPartland & Cubeta 1996). By far the most significant is gray mold, caused by Botrytis cinerea (teleomorph Botryotinia fuckeliana). B. cinerea thrives in temperate regions with high humidity and cool to moderate temperatures. Under these conditions gray mold can reach epidemic proportions and completely destroy a Cannabis crop within a week (Barloy & Pelhate 1962). B. cinerea attacks many crop plants and weed species worldwide.
Gray mold presents in three scenarios, depending on plant maturity and cultivar. Seedlings succumb to damping off, discussed below. In fiber cultivars gray mold presents as a stem disease. It arises as a gray-brown mat of mycelium which becomes covered by masses of conidia (fungal spores). Stems become chlorotic at margins of the mat. Enzymes released by B. cinerea reduce stems to soft shredded cankers. Stems often snap at canker sites. Gray mold may encircle and girdle stems, wilting everything above the canker. Fiber varieties become more susceptible after canopy closure. In field experiments in the Netherlands the disease was found from the beginning of July (Van der Werf and Van Geel, 1994). Van der Werf et al. (1995) note Hungarian Kompolti Hibrid TC is more susceptible to gray mold than other fiber varieties. Dempsey (1975) says the Russian cultivars JUS-1 and JUS-7 are resistant, but these may no longer be available (de Meijer 1995).
In drug cultivars, gray mold infests flowering tops. Large moisture-retaining female buds are most susceptible. Fan leaflets first turn yellow and wilt, then pistils begin to brown. Whole inflorescences soon become enveloped in a fuzzy gray mycelium then degrade into a gray-brown slime. Drug varieties are most susceptible during flowering near harvest time. Dense tightly-packed buds of Cannabis afghanica Clarke tend to hold moisture and easily rot (Clarke 1987). Afghan cultivars evolved in very arid conditions and have no resistance to gray mold. This unfavorable trait is often expressed in hybrids that have only a small percentage of C. afghanica Clarke heritage.
For the second most important disease, Termorshuizen (1991) lists hemp canker. This diease is caused by Sclerotinia sclerotiorum. The fungus primarily attacks fiber cultivars in Europe, but it has caused up to 40% losses in North America (Hockey 1927) and damaged hemp in Australia (Synnott 1941) and Tasmania (Lisson & Mendham 1995). Hemp canker has also appeared on drug cultivars in India (Bilgrami et al. 1981). Symptoms begin as watersoaked 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. By September large black sclerotia develop on the stem surface or within pith of dead stalks.
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. Most damping off is caused by two Protoctistan Pythium species (technically they are omycetes, not fungi), P. aphanidermatum and P. ultimum. Several fungi also cause damping offRhizoctonia solani, Botrytis cinerea, Macrophomina phaseolina, and several Fusarium species, F. solani, F. oxysporum, F. sulphurem, F. avenaceum, F. graminearum. Together they make damping off a ubiquitous problem, attacking all cultivars of Cannabis (Bush Doctor 1985).
The two most common leaf spot diseases are yellow leaf spot caused by two Septoria species (McPartland 1995d), and brown leaf spot caused by about eight Phoma and Ascochyta species (McPartland 1995c). These diseases rarely kill plants but sharply reduce crop yields. Two common diseases of fiber varieties are downy mildew, caused by two Pseudoperonospora species, and olive leaf spot caused by a Pseudocercospora species and a Cercospora species. Pink rot, caused by Trichothecium roseum, has recently killed greenhouse-grown drug cultivars and seems to be on the rise. Less frequently seen but equally virulent diseases include brown blight (caused by two Alternaria and two Stemphylium species), anthracnose (caused by two Colletotrichum species) and white leaf spot (caused by Phomopsis ganjae). Powdery mildews, black mildews, and rusts are caused by high-visibility fungi, but rarely cause serious problems (McPartland 1983).
Some leaf disease fungi also infest stems, especially Trichothecium roseum, Phoma, Stemphylium, Colletotrichum, and Phomopsis species. The most serious causes of stem cankers are Fusarium speciesF. graminearum and F. avenaceum occur in cooler climates, F. sulphureum and F. sambucinum in warmer climates.
Some root rots cause serious losses. Barloy & Pelhate (1962) considered root rot caused by Fusarium solani the worst disease of hemp in France. Pandotra & Sastry (1967) report a virulent strain of Rhizoctonia solani destroying 80% of drug plants in northern India. Root rot by Sclerotium rolfsii predominates in southern temperate zones and the tropics, on both fiber and drug cultivars (Ferri 1961).
Above-ground symptoms of root rots are hard to distinguish from wilt diseases. Three wilt diseases are importantfusarium wilt caused by two forms of Fusarium oxysporum, verticillium wilt caused by two Verticillium species, and premature wilt (also called charcoal rot) caused by Macrophomina phaseolina. Fusarium wilt received attention as a potential biocontrol to eliminate illegal marijuana plantations (Hildebrand & McCain 1978, Noviello et al. 1990). Wilt diseases are more severe in Cannabis fields harboring root-wounding nematodes or broomrape.
