Get to know the pests and pathogens below. Keeping plants healthy is your greatest defense and prey often prefer to pick on damaged or weak specimens. For treatment, consult a professional. While there are many pesticides and fungicides on the market, there are few that are approved for use on hemp. Do your research before treating with virtually anything, as failed pesticide/herbicide/fungicide tests can render your flower useless.
Insects and soil dwellers
Hemp Russet Mites - A prolific pest invisible to the naked eye, these mites can destroy cannabis crops quickly. With some varieties you will see 'canoeing' in the leaves as well as crispy tips on foliage that almost looks like a nutrient burn. They are most destructive on flowering plants, where they consume the bud from the inside out, causing it to rot.
Broad Mites - Tiny and destructive, another mite that can only be seen using a scope. Damage can be seen in twisted and drooping new growth along with leaves that look blistered or wet.
Caterpillars – There are many different species but the most destructive are the Eurasian Hemp Borer and the Corn Earworm. The Borer chews its way into the stalk, killing the plant almost immediately. The Corn Earworm is most prevalent during flower when it burrows into ripening buds. Wherever it sets foot, or feces, the flower rots. Damage is visible from dead leaves on otherwise healthy looking flowers, but generally, by the time you spot the damage it's too late.
Symphylans - These small but visible soil dwellers voraciously chew away on root tips. In large colonies they can render areas of your fields useless. There damage is often visible in rings of slowed growth or death in your cop. Once an area is infested they are very difficult to get rid of. If you are concerned they are present, put soil from around the base of an infected plant into a 5 gallon bucket and fill it with water. Symphylans will float to the surface.
Spider Mites - Small but visible pest that is generally more of a threat in greenhouse and indoor grows. Damage causes leaf “flecking,” which appears as tiny white speckling on the leaf surface. Large populations may cover plants with webbing as they suck the life out of its vegetation.
Aphids - Small cream-, clear- or all the way to light green-colored insects that are visible to the eye. Damage can slow growth and cause wilting and leaf yellowing. Another more common indoor threat, but large populations can also hit outside - generally accompanied by ants who feed off their nectar.
Whiteflies - Small but visible flies that have been known to invade plants toward the end of flower. Damage causes white spots on tops of leaves; with large populations, leaves may yellow and die.
Thrips – Visible, with elongated and wingless bodies during their feeding stage. Heavy damage causes extensive leaf scarring. They prefer to feed on expanding immature leaves.
Ants – Excessive populations in fields can consume plant stems and cause instant death. They have a taste for the cambium layer and can chew through the outside of a stalk quickly.
Common Fungal Pathogens
Bud Rot - Bud rots can be caused by various fungi, but one of the most common is Botrytis. Botrytis species cause a bud rot also referred to as “Botrytis gray mold” due to the fuzzy gray growth that often accompanies symptoms in high-moisture conditions, especially within bud clusters. Botrytis infection of flowers causes the bud to become dry and brown. Botrytis spores require free moisture on the plant surface to germinate and cause infection. Therefore, the disease is most likely to occur in areas or times of year that experience humid or rainy conditions, or in fields that utilize overhead irrigation. In addition to being an important pathogen in the field, Botrytis is an important post-harvest pathogen of flower.
Powdery Mildew – Powdery mildew fungi grow along the plant surface, appearing powdery (hence the name) white. In addition to being unsightly, powdery mildew infections can render flowers unfit for the fresh cut flower market. Powdery mildew does not usually kill the plant; however, infection of the foliage decreases the photosynthetic capabilities of the plant and thereby limits plant growth. Unlike many fungi which require free moisture on the leaf surface to germinate, powdery mildew fungi germinate on dry foliage. Nonetheless, decreasing humidity, increasing airflow, and increasing light penetration through the plant canopy can all help limit powdery mildew infection.
Root rot – Root rots caused by Pythium and Phytophthora lead to symptoms of chlorosis and wilting. Roots will typically be underdeveloped with few fine root hairs and can appear brown or red in color (rather than healthy white). Both of these pathogens are known as “water molds,” and are favored by high soil moisture. Therefore, root rots caused by Pythium and Phytophthora are often issues in fields that are overwatered or in areas of the field with poorly drained soil. Pythium and Phytophthora are also important seedling pathogens and can cause death of seedlings, a disease referred to as “damping off.”
