Hemp (Cannabis sativa) is one of the earliest plants to have been cultivated. This plant has been used since the beginning as a raw material for making clothes. The stem of the improved varieties contains 20-30% fiber. These varieties have several properties that differentiate them from cotton, linen, jute, etc. Hemp fibers are resistant to torsion, friction, rot, are elastic, and have a longer length than the others. Hemp seeds contain significant amounts of oils and are used in the manufacture of vegetable fats. Oil is also used in the manufacture of varnishes, paints, and soaps. Press cakes and seeds are used in animal feed. The wood resulting from the extraction of fibers is used for the manufacture of paper, OSB boards, soundproof boards, or in the furniture industry.

The main diseases of hemp

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Cannabis mosaic virus

The disease occurs during the flowering phase of the plant. The leaves at the top of the plant have short, isolated, yellow-green stripes. The attacked leaves turn yellow and dry out. The attacked plants have a small size, the fibers have lower quality, and the seed production is low. The virus is transmitted from one year to another through infected seeds, and during the vegetative growth period, the virus is transmitted by aphids.

Prevention and control methods:

• using healthy seeds;
• rational fertilization;
• carrying out the maintenance works properly;
• applying insecticides to control the aphid population.

Leaf and stem rot of hemp (Pseudomonas cannabina)

The first symptoms appear on young plants. Numerous small protrusions appear on the stems, arranged in parallel rows that gradually disappear. Mature plants have gray, elongated waxy spots on the stems. Next to them, there are small blisters filled with yellow mucilage. Eventually, the blisters burst. Ulcers also appear on buds. On the leaves, the disease produces asymmetrical, purple spots, bordered by a light-colored ring.

Prevention and control methods:

• carrying out treatments with specific fungicides.

Downy mildew (Pseudoperonospora cannabina)

Yellow spots appear on the top side of the leaves, and on the underside, near the spots, a gray-whitish fuzz forms. The attacked leaves no longer grow, curl up, turn brown, and eventually dry out. The occurrence of the disease is favored by the rainy weather.

Prevention and control methods:

• gathering and destroying plant remains after harvest;
• a correct crop rotation;
• reharvesting hemp to the same field after 3-4 years;
• deep plowing;
• carrying out treatments with specific fungicides.

White mold (Sclerotinia sclerotiorum)

The disease can occur in all stages of vegetative growth and can cause significant damage in rainy years. The stem blackens close to the soil surface and it is covered with a white mold. After a while, the leaves turn yellow, wither, and fall off. In the affected area, the fibers deteriorate, become fragile, and the stem breaks. In the fuzz formed on the surface of the stem, fungal sclerotia develop in the form of small, hard, black masses.

Prevention and control methods:

• avoiding damp soils;
• using healthy seeds;
• avoiding preceding plants that are prone to white mold attack;
• a correct crop rotation;
• reharvesting hemp to the same field after 3-4 years;
• no fungicides are applied in the advanced vegetative growth phases.

Septoria leaf spot (Septoria cannabis)

The disease attacks all the aerial organs of the plant. On the leaves appear small, round spots, at first gray color. The disease progresses and the spots turn yellow, being bordered by a reddish ring. In the center of the spots develop many small black dots, which represent the fructification of the fungus. The attacked leaves dry up and fall off. The disease is favored by hot and humid weather. This disease can degrade the quality of the fibers.

Prevention and control methods:

• carrying out treatments with specific fungicides.

Brown leaf spot (Didymella cannabis)

On the leaves, there are small spots that become bigger and reach up to 1.5 cm in diameter. The original color of the spots is brown, then they become whitish and are surrounded by a brown ring. The attacked tissues peel off and fall, giving the leaf a sifted appearance.

Prevention and control methods:

• carrying out treatments with specific fungicides.

Stem canker (Mycosphaerella cannabis)

The fungus appears on the leaves and stems forming circular, brown spots. As the disease progresses, the spots turn white and surround the stem. The fructification of the fungus develops near these spots. This fungus does not cause significant damage, but in combination with other diseases, it can cause a decrease in fiber and seed production.

Grey mould (Botrytis cinerea)

The attack manifests on the stem, at a distance from the ground of about 40-50 cm. The disease mainly affects male plants. On the stem and on the leaves appear elongated spots on the surface of which a gray mold develops. Over time, the affected tissues turn white, crack and exfoliate. The attacked plants can be recognized by the withered tip.

Prevention and control methods:

• avoiding damp soils;
• using healthy seeds;
• avoiding preceding plants that are prone to grey mould attack;
• a correct crop rotation;
• reharvesting hemp to the same field after 3-4 years;
• no fungicides are applied in the advanced vegetative growth phases.

The main pests of hemp

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Wooly apple aphid (Eriosoma lanigerum)

It produces several generations per year and it colonizes the aerial parts of the plants. The adults and larvae feed on the intracellular fluid of the plants. Following the attack, the tissues atrophy and eventually dry out. This species can transmit a number of diseases.

Control methods:

• carrying out treatments with specific insecticides.

Green peach aphid (Myzus persicae)

It may cause significant damage. This insect is polyphagous and attacks several plant species. It forms a colony on the aerial parts of the plant. The adults and larvae feed on the intracellular fluid of the plant. Following the attack, the hemp no longer grows normally, and eventually, the plants dry out.

Control methods:

• carrying out treatments with specific insecticides.

European fruit lecanium (Parthenolecanium corni)

The adults and larvae colonize the aerial parts of the plant. They feed on the intracellular fluid of the plant, and after the attack, the plants no longer grow normally and dry out. It is a difficult species to control because its body is covered with a thin shield of wax.

Control methods:

• carrying out treatments with specific insecticides.

Greenhouse whitefly (Trialeurodes vaporariorum)

The adults have their bodies covered with a white waxy substance that gives them a dusty look. The adults and larvae feed on the intracellular fluid of the plant, causing disorders of the physiological processes of the plant after the attack. In the case of a severe attack, the plants can dry out completely.

Control methods:

• carrying out treatments with specific insecticides.

Red spider mite (Tetranychus urticae)

It overwinters as a fertilized female, below the plant remains found on the soil surface. Females create a specific web surrounding the stem, after which they begin to lay eggs. The larvae and adults sting and suck the intracellular fluid of the attacked plants. Following the attack, the development of the plant is slowed down. This pest can also transmit a number of diseases.

Control methods:

• carrying out treatments with specific insecticides.

Thrips (Thrips sp.)

They are small insects, difficult to see with the naked eye, which attack a large number of cultivated plants. They colonize the tops of the stems, the flowers, the leaves, causing the flowers to fall and the stagnation of the plants.

Control methods:

• carrying out treatments with specific insecticides.

Hop flea beetle (Psylliodes attenuata)

It produces one generation per year and overwinters as an adult in the surface layer of the soil or underplant remains. The adults appear in spring when temperatures exceed 8° C. This species also causes damage to hop cultures. In the event of a severe attack, the culture is compromised. The larvae develop in the roots, digging small superficial galleries, thus creating gateways for phytopathogenic fungi.

Prevention and control methods:

• carrying out treatments with specific insecticides.

Eurasian hemp moth (Grapholita delineana)

It appeared in Europe after the exchange of seeds with Korea, China, and Japan. It produces three generations a year and overwinters in the larval stage in a silky cocoon beneath the plant remains or in the soil. It can destroy up to 70% of the crop. It is a monophagous species and feeds on all aerial parts of the plant.

Prevention and control methods:

• deep plowing before setting up the culture;
• destroying the plant remains after harvest;
• the common red mite feeds on the eggs of this species;
• carrying out treatments using specific insecticides.