Hemp diseases & pests

Cannabis is afflicted by over 100 diseases caused by fungi, bacteria, viruses, nematodes, plants, and abiotic challenges (genetics, nutrients, stress and pollutants) but only about a dozen cause serious problems.

With the exception of striping virus, the most common diseases of hemp (i.e., Hypochnus solani and Pythium sp.) can be controlled by treatment of the seeds with Clorox (25% sodium hypochlorite) diluted with an equal volume of water and adjusted to pH 9. Soak the seeds for 10 minutes, then rinse thoroughly with fresh water. (24, 25)

Cannabis' resistance to diseases and pests can be strengthened by the judicious use of methyl salicylate, which is a major component of wintergreen oil. Aspirin (acetyl salicylic acid) also is effective. Dissolve two tablets in a quart of water and apply the solution as a foliar spray.

Fungi & Bacteria --- More than 90 species of fungi attack Cannabis. Seedlings can be infected by damping-off fungi; the flowers and leaves are vulnerable to grey mold, yellow, brown and olive leaf spots, mildew and brown blight, pink rot, and viruses. The stalk and stems are susceptible to attack by grey mold, hemp canker, Fusarium oxysporum canker and wilt, stem nema, charcoal rot, anthracnose, and striatura ulcerosa. The roots can be diseased by Fusarium, rhizoc, sclerotium root rot, and nematodes. Alternaria alternata Keissler, a common fungal pathogen of many plants, can destroy up to 45% of hempseed in a crop.

K. Roder investigated hemp diseases, and isolated 7 strains of Sphaerella cann., 13 strains of Phoma or Phyllosticta, and 13 forms of Phomopsis, Coniothyrium, Vermicularia, Fusarium, and Cylindrocarpon. None of these species can infect the roots of hemp directly, unless the roots are weak or injured.

Gray mold (Botrytis cinerea) and hemp canker (Sclerotina sclerotiorum) can severely damage a hemp crop in wet years. In temperate regions with high humidity and low temperatures, botrytis can completely destroy a hemp crop within a week. Botrytis and hemp canker can be controlled by spraying alternately with the fungicides vinchlozolin (0.5 kg/ha) and iprodion (0.5 kg/ha) at two-week intervals from June through August. Tetramethylthiuran disulfide also is effective. Van der Werf, et al., however, found from three years of field experiments that one or two applications did not increase actual stem yield. They concluded that, "although fungal diseases may severely reduce hemp yield in the Netherlands, the use of fungicides is not worthwhile". They recommended breeding cultivars less susceptible to B. cinerea in particular. Botrytis can be controlled by applications of sulfur followed by "AQ-10", a microbial product. Bentonite clay also can control botrytis; it is better to use California calcium-bentonite rather than the more common sodium variety. Potassium bicarbonate ("Kali-Green") also kills botrytis. (26)

Botrytis appears as "damping-off" (moldy germination) on seedlings; it can be prevented by aeration of the soil, by hydrogen peroxide, or by the Burgundy Formula. Dissolve 1 lb of calcium sulfate and 1 lb of washing-soda crystals in 10 gallons of water. Soak the seeds in this solution, then rinse with fresh water.

The Chestnut Formula also prevents damping-off. Mix 2 parts copper sulfate and 11 parts ammonium carbonate in a glass jar. Dissolve 1 oz in 2 gallons of water. Soak the seeds in this solution, then rinse them with fresh water.

Botrytis is a stem disease in fiber cultivars, arising as a gray-brown mycelial mat, later covered with conidia. The stem becomes chlorotic along the edge of the mat, then reduces to soft, shredded cankers; the plant often snaps at that point, or wilts above it. The mature floral bracts of female drug cultivars are most susceptible to infestation. Leaflets turn yellow, then wilt, and pistils become brown. Mycelia envelope the infloresences, which dissolve into gray-brown slime.

Infection by Botrytis is directly affected by the level of calicum in the plant; the higher the amount of Ca, the lower the incidence of Botrytis. Sprays of Ca-silicate (2,000 ppm) and Ca-formate (2,000 ppm) provide effective control of fungi and are safe alternatives to highly toxic fungicides.

Damping off is caused by other several fungi, mostly by the Protoctistan oomycetes Pythium aphanidermatum and P. ultimum. Other causative fungi include Fusarium solani, F. oxysporum, F. avenaceum, F. graminearum, and F. sulpurem, Rhizoctonia solani, and Macrophomina phaseolina.

The severity of infection by facultative parasites is affected by the levels of nitrogen and potassium; low N and high K provide resistance; high N and low K invite parasites.

Wilt is caused by Fusarium oxysporum f. sp. cannabis Nov. & Snyd. It first appears on 3-month old plants as a yellowish-green color and small dark spots on the lower leaves, which die but remain hanging. The plant may bend to the side of the stem that is affected.

In 1999, Ag/Bio Con, a Montana company, offered the world a mutant strain of Fusarium oxysporum as a mycoherbicide against cannabis --- an utterly insane promotion that has instigated lawsuits to cease and desist. Fusarium mutates very easily and spreads to other crops. It is an environmental disaster.

Verticillium wilt is caused by Verticillium(two species); Macrophomina phaseolina causes premature wilt (also known as charcoal rot).

Hemp twig blight is caused by Botryosphaeria maronii (Cav.) Charles & Jenkins. The leaves quickly wilt and droop and turn brown, but remain attached. The symptoms first appear on the tips of branches. The lower parts become bleached.

Two species of Colletotrichum cause anthracnose in cannabis.

Hemp canker, caused by Sclerotinia sclerotiorumi, is one of the most important diseases of cannabis. It appears as wet lesions on the branches of plants as they mature; the lesions become dark cankers. The pith fills with white mycelia. Black sclerotia then develop in the stem and in the pith. The plants wilt and collapse. The optimal temperature range for its growth and maturation of is 24-33o C. Sclerotinia can be controlled somewhat by proper drainage of the fields.

Yellow leaf spot is caused by two species of Septoria; brown leaf spot is caused by species of Phoma and Ascochyta; Phomopsis ganjae causes white leaf spot.

Severe leaf infections also are caused by strains of Sphaerella (stem browning), Phyllosticta, Cercospora, Microdiplodia, Macrosporium cann., and Pseudoperonospora (olive leaf spot), and Didymella arcuata. (32)

Didymella arcuata is found in association with the fungus Ascochyta cann. Lasch. The optimal temperature for the germination and growth of D. arcuata is 19-26o C. Its sporulation is promoted by light; its vegetative development is stimulated by darkness. (33)

Leaf spots caused by Cercospora cannabis Hara & Fukui are yellow-tan or brown. They are circular at first but become irregular and distinct.

Trichothecium roseum causes pink rot on cannabis drug cultivars in greenhouses.

Hemp rust (Melampsora cannabina) appears as orange patches on the leaves. It can be controlled by spraying with thiocarbamate.

Bacteriosis (stripe disease) is caused by Pseudomonas cannabina Sutic & Dows. var. italica Dows. Scattered reddish-violet spots (under 2 mm), surrounded by a pale yellow halo, appear on leaf blades, followed by rapid shedding. Small necrotic elongate cavities, filled with bacteria, appear on the stems.

Some less common but nonetheless virulent diseases of cannabis are anthracnose (Colletotrichum atramentarium) and brown blight, which is caused by species of Alternaria and Stemphylium.

Many of the bacterial and fungal diseases of cannabis and other crops can be controlled by applications of Bacillus subtilis, which is available in several commercial products (i.e, Serenade, manufactured by Agraquest).

Several bacteria that are pathogenic to humans have been found on Cannabis, including: Salmonella muenchen, Klebsiella pneumoniae, Euterobacter cloacae, E. agglomerans, Streptococcus (Group D), Thermoactinomyces candidus, T. vulgaris, Micropolyspora faeni, Aspergillus fumigatus, A. niger, A. flavus, A. tamarri, A. sulphureus, A. repens, Penicillium chrysogenum, P. italicum, Rhizopus stolonifer, Alternaria alternata, Curvularia lunata, and Histoplasmus capsulatum. Aspergillus can be killed by baking cannabis at 150o C for 15 minutes, but only about 15% is destroyed by smoking through a waterpipe. Microbial pathogens and toxins, however, are not destroyed by heating or other methods of sterilization.

Insects --- Several species of mites are injurious to stored hempseed: Tyroglyphidae (3 sp., especially T. farinae), Glycyphagidae (6 species, especially G. destructor), and predaceous Cheyletus eruditus. The maximum number of mites are found in July-August. T. farinae occurs constantly. Once introduced into storage, the mites can persist for several years. Tyrophagus can be controlled by treatment of the seed with fungitoxic preparations (i.e., Panogen and Aldogen). The mechanical damage of hempseed stimulates the breeding of mites. The miticide "Cinnamite" (cinnamaldehyde), produced by Mycogen, is very effective. (23)

Spider mites also plague hemp. Their effect is seen as minute white spots on the top of leaves. The mites appear as tiny black specks on the the underside of leaves. They can be controlled by introduction of another mite, Phytoseiulis persimilis.

The seedlings can be attacked by hemp flea beetles, cutworms and white grubs. The stalks and stems can be infested by European corn borers and hemp borers, weevils, and by modellid and longhorn grubs. The larva of the Death's Head moth (Acherontia atropos) occasionally bore into hemp stalks. Flea beetles, white root grubs, wireworms, fungus gnats, root maggots, termites and ants will attack the roots of cannabis.

The click beetle (Agriotes mancus), also known as wireworm in its juvenile form, infests the top six inches of soil and eats the roots of plants, including cannabis. Wireworm can be controlled by introduction of beneficial nematodes (Heterohabidis and Steinernema spp.).

Marijuana thrips (Oxythrips cannabensis Knechtel) is a sucking insect that is host-specific to hemp. Common thrips (Frankliniella occidentalis) eat the calyx of cannabis, causing deformation and affecting viability. Thrips can be controlled by the predators Amblysieus cucumeris, A. degenerens, and Orius tristicolor. The aphid Phorodon cannabis Pass. also is found on hemp.

The tarnished plant bug (Lygus spp.) has been found on hemp crops throughout Canada It feeds on the apical meristem, which appears malformed and stunted. Brown lesions are visible on the stem where the pest has been feeding. It can be controlled by the predatory Spined Soldier Bug (Podisus spp.).

The hemp flea beetle (Psylliode attenuata) hibernate in the soil until the temperature of the soil and air rises to 10o C (50o F). Then they emerge and feed on leaves and stems of seedlings. The pest can be controlled by planting ‘bait’ hemp with sodium fluosilicate (15 kg/ha) one month before the mass planting of the crop. The bait plants are treated again after 10 days. This procedure reduces the number of hemp fleas by 90%. In rare, extreme cases, methyl parathion can be used (50% solution, 8 fluid oz/acre).

The larvae of hemp borer moths (Grapholita delineana) damage the stalks to such extent that the quality does not meet the industrial standard. Up to three generations can develop each year. The caterpillars live in the stems and flowers of female plants, where they damage developing seeds (50% or more). After feeding, the caterpillars burrow into the soil and overwinter there. In Hungary, where they were first observed, the period between August 20 and September 7 (when daylight is reduced from 15 to 14 hours) is critical for diapause of the larvae. An earlier harvest will prevent the overwintering of most of the larvae; thus the population of the next year can be considerably reduced. (27)

The borers can be fought with 50% methyl parathion (20 fl oz/acre). After harvesting, the stubble and waste stalks should be burned, and the field plowed. Organophosphate insecticides are much more effective than organochlorine compounds. Treatment methods vary according to the intended use of the crop (fiber or seed). Two or three applications of Fenthion (500 gr/ha) are up to 98% effective. (28)

The wasp Trichogramma evanescens Westw. parasitizes the eggs of the hemp moth; 80,000-120,000 wasps/ha are released at one time. Biological control is more efficacious than of chemical methods. Several Hymenoptera species of parasites and predators will prey on G. delineana. Commercially available Persimilis (Phytoseiulus persimilis) will destroy spider mites that infest cannabis (See Appendix 1, #34). (29)

The European corn borer also damages hemp crops. The hemp greenfly (Phorodon cannabis) can damage fiber hemp, but it has never been a major problem.

The most common rootknot nematode, Meloidogyne incognita, occurs on fiber cultivars of cannabis. The northern rootknot nematode (Meloidogyne hapla) has appeared in European hemp fields in recent years. Nematodes can be controlled somewhat by planting resistant varieties of cannabis, and by applications of aqueous extracts of several common plants (i.e., pigweed, marigold, hyssop and mustard). Aqueous urea is very effective against nematodes, but it must be buffered to prevent damage to the plants. Lesion nematodes (Pratylenchus spp.), reniform nematodes (Rotylenchus spp.), and spiral nematodes (Helicotylenchus and Scutellonema spp.) also have been found in hemp crops.

The hemp sawfly (Trichiocampus cannabis Xiao et Huang) is one of the main pests of hemp in China. The larva feeds on hemp leaves, making many holes. It produces two generations/year, and the mature larvae overwinter in the soil. The sawfly is controlled by Fenitrothion.

The leaf roller moth (Tortricidae, Grapholitha zinana) can damage up to 100% of hemp sowings. The larvae damage the infloresences and seed. N. Kozinets, who discovered the pest in 1964, advised spraying hemp sowing with DDT (15-20 kg/ha) during the flight of the adult moth. (30)

Irrigation too early and continuing humid conditions will promote mass attack of hemp stems by Pyraustis nubialis Hb. The fiber yield from attacked plants deteriorates by one grade. The pest can be controlled by introduction of Trichogramma. The number of caterpillars can be reduced by over 60%, and plant damage decreased by 80%. (31)

Cannabis foliage and infloresence possesses phytocidal activity. The antibacterial properties are effective against Bacillus mesentericus, B. subtilis, B. mycoides, B. cereus, Micrococcus albus, M. aureus, and Clostridium welchii. Hemp is inactive against Salmonella, Pseudomonas, Azotobacter, and Candida. The maximum inhibitory effect from leaf extracts occurs in August; the flowers are most potent in September, after which time there is a marked decline in activity. Phosphate fertilizer promotes the antibacterial activity, whereas K- and N-fertilized plants are less active. (34)

eace:

HIE great post. My solution for these diseases and pests are organic in nature, of course. With every bad bacteria there are good bacteria, both competing for the same resources. By introducing more helpful bacteria and providing an environment that is more favorable to them, rather than the harmful ones, controlling parasites, pests, and diseases is much easier. Nature has its own way of checks and balances that can be tilted to our favor.