Most Prevalent Diseases

How to use this page: This page includes descriptions and images of diseases that are commonly observed on industrial hemp plants in the north-central region of the United States. The diseases listed below are known to cause damage to industrial hemp plants; however, economic thresholds are not known.

Common Diseases

  • Gray mold
  • Leaf spots
  • Fusarium head blight

Root Diseases

Fusarium wilt and Fusarium root rot

Fusarium wilt is most commonly caused by Fusarium oxysporum in the soil. Fusarium root rots are caused by many species within the genus Fusarium and are widespread in the soil. Yellow leaves are often observed first as the vascular tissue is damaged, followed by more severe wilting. Dark and sunken cankers may be visible on the stem at the soil line. Infected roots may appear pink to brown and have internal staining when cut crosswise. Warm soils with poor drainage are conducive to disease, with damaged roots and girdled roots being more prone to infection.

 

Hemp plants are being grown outside with a stake for support. The center plant is wilting and has downturned branches and leaves. Some of the leaves have turned yellow while other leaves are still green. Fusarium wilt can cause mature plants to wilt and die
A hand holds a hemp stem that has been cut horizontally. The cross section of the stem shows a white margin with streaks of brown rot radiating from the center of the stem.  A cross section of a hemp stem shows Fusarium infection
A portion of a hemp root is magnified under a dissecting microscope. The root is yellow with a splotch of bright pink in the center. Magnified hemp root, arrow points at pink staining caused by Fuasrium

Pythium

Multiple Pythium species can infect hemp. Infected plants will be stunted, yellow, and eventually wilt. Brown lesions on the stem may be visible. White, cottony mycelium growing from the plants may also be observed. Excessive rainfall and poorly drained fields are conducive to disease.

 

A hemp seedling is uprooted and horizontal on top of a clump of soil that has been dug up. A white cotton like growth of mycelium grows at the point where the stem meets the soil line. Hemp seedling infected with Pythium. The arrow points to mycelium growing from the plant.
A tangle of hemp roots that are light tan to brown rather than white. Infected hemp roots are brown rather than white.
Several hemp plants have turned yellow and brown and the tops are bent over from wilting. There are healthy, green hemp plants in the background. The plants in the white square have wilted from a Pythium infection.

Foliar Diseases

Cercospora leaf spot

Cercospora leaf spot is caused by Cercospora cannabis and possibly other species. Lesions begin as small yellow spots that become tan to brown with a dark margin. Lesions may merge and cause larger spots. It is sometimes confused with other leaf spots. Humid and warm weather are conducive to disease.

 

A single green hemp leaf has numerous spots distributed unevely on the leaflets. The spots are white with brown and yellow halos around the white spot. Cercospora cannabis, Nancy Gregory, University of Delaware, Bugwood.org
Two hemp leaves have spots with white centers and brown margins on all the leaflets. Some of the spots have merged together and several tips of the leaflets have turned brown. The black arrows point to lesions caused by Cercospora cannabis.
A hemp leaf under the canopy has several spots with white centers and brown margins. The leaves in the background also have spots. All the leaves with spots are yellow-green in color. Cercospora leaf spot on older leaves lower in the canopy.

Downy mildew

Downy mildew in hemp is caused by the pathogen Pseudoperonospora cannabina. Lesions first appear as yellow-green spots on the upper leaf surface. As lesions progress, they become angular and brown. Leaves with many lesions can become distorted and plants may defoliate. Lesions may look like chemical damage or other types of leaf spots. However, the underside of the leaves can provide a crucial sign of downy mildew. During wet periods, gray to black down-like fuzz can be observed. High humidity and free water on the leaves are conducive to disease.

 

The upper portion of a cannabinoid hemp plant has leaves that are twisted and contorted. There are gray spots on the underside of the leaves. Infected hemp leaves deform and fall off with time.
The underside of a single hemp leaf has numerous areas with dark gray to black, fuzzy downy mildew growth. The margins of the growth are contained by the veins of the leaf. Pseudoperonospora cannabina sporangiophores (black areas) grow on the underside of leaves.
The topside of a single hemp leaf has yellow spots confined between the veins of the leaf. Some spots have dark brown centers. There is water on the ends of the leaflets. The background has hemp grown on black Yellow spots caused by downy mildew.

Hemp leaf spot

Hemp leaf spot sometimes called Bipolaris leaf spot is caused by Drechslera gigantea. Lesions begin as light green spots on the leaves that progress to round, white-to-tan spots. It may be confused with other leaf spots.  Non-hemp plants in or near the field may be infected as well. Warm and wet conditions are conducive to disease.

 

A cannabinoid hemp plant has all its leaves covered in small white spots. The plant is green and beginning to flower. Some of the leaves are beginning to curl on the ends. Bipolaris causes numerous spots on leaves.
The topside of a single, green, hemp leaf has sparse white spots on the leaflets. A few of the spots are brown. All spots have thin brown margins. Lesions caused by Bipolaris
A single beige spot with a brown margin is magnified with a dissecting microscope. There are black sting like conidiophores in the center of the spot. There are several holes near the spot with jagged edges. Magnified lesion, the arrow points to conidiophores.

Powdery mildew

Powdery mildew in hemp is caused by multiple species from different genera. A powdery white appearance can be patchy or widespread on leaves and flowers. Warm to high daytime temperatures and humid nights, shade or low light intensity are conducive to disease.

 

A hemp leaf is almost entirely covered with a white powdery appearance. There are only a few parts of the leaf where there is exposed green. Golovinomyces causes white powder appearance on leaves.
A green hemp leaf has sporadic white patched on the leaflets. The patches are irrefular in size and distribution Powdery mildew white patches
The top of a hemp plant has small leaves that have a white appearance that looks like powder sugar. The larger leaves in the background do not have white appearance but there are several that are twisted and brown Powdery mildew on the new growth of a plant

Rust

Rust on hemp is caused by the pathogen Uredo kriegeriana. Lesions appear as light to dark brown with a yellow halo on the topside of leaves. Orange pustules with visible spores are found on the underside of the leaves. Moderate temperatures and high humidity appear conducive to disease; however, the exact conditions are unknown for this species.

 

The underside of a singele hemp leaf has sparse brown spots on the leaflets. Some spots have a bright orange center. Pustules with orange spores on the underside of a leaf
The topside of a hemp leaf has sporadic spots. The spots are tan to dark brown with dark brown margins and yellow halos. Lesions appear as brown spots on the tops of leaves
A close up of the underside of a hemp leaflet. There is a orange fungal growth coming out of a spot. The growth is powdery orange. Rust pustules contain bright orange spores

Septoria leaf spot

Septoria leaf spot on hemp is caused by Septoria cannabis. Spots begin as yellow flecks and progress to darker brown lesions with yellow halos. The disease is often observed in older leaves first. Warm, humid, and rainy conditions are conducive to disease.

 

 A light greenish-yellow hemp leaf has brown somewhat angular brown spots and yellow halos.The spots are contained by the veins of leaf. Septoria cannabina
A whole cannabinoid hemp plant has the majority of leaves brown and curled. The plant is being grown on white plastic and there are broadleaf weeds next to the plastic. A person wearing boots is in the background. Severe Septoria infection

Crown and Bud Diseases

Gray mold

Gray mold is caused by Botrytis cinerea. Infection causes browning and dieback of grain or flower tissue and can also affect leaves, especially on injured plants. Gray mycelia may be visible and overwintering sclerotia look like rat droppings. Cloudy conditions, high humidity, and moderate temperatures are conducive to disease.

 

A cannabinoid hemp plant is grown on black plastic. The inner parts of the branches is brown with curled leaves, the outer branches and leaves have a few brown spots. Damage within the canopy caused by Botrytis cinerea
A small hemp plant is grown in a green pot. The top of the plant  is covered in fuzzy gray mold and leaves are twisting and wilting. Botrytis cinerea causes gray fuzzy mold growth.

Fusarium head blight

Fusarium head blight is caused by multiple Fusarium species. Brown, dead tissue can be observed in both grain and flower structures. White mycelia growth is often observed when heads are broken open. Warm and humid conditions are conducive to disease.

 

The grain head of a hemp plant has brown leaves. The head is broken open and a white to gray mold has infected leaves and seeds. Fusarium mold growth within the grain head
Three grain hemp plants are standing in the field. The center plant is completely. The plant on the left is green and healthy. The plant on the right has some leaves that are brown. Advanced Fusarium infection.

White mold

White mold is caused by Sclerotinia sclerotiorum. Portions of branches or whole plants can brown and wilt. White, fluffy mycelial growth is often observed. Black sclerotia may also be observed and are overwintering structures. Cool to moderate temperatures, high humidity, and moist soil are conducive to disease.

 

A hemp stem of a cannabinoid hemp plant is brown with white moldy patched and a black sclerotium that looks like a dropping is in the center of the stem. The surrounding branches and leaves are brown and dying. White mold sclerotium is circled on the stem.
A cannabinoid hemp plant has a stem that is beige with white mold growing out of the stem and a branch.The top part of the stem is green as are many of the leaves. The area behind the plant has sorghum cover crop growing. Infected cannabinoid hemp stem
A cannabinoid hemp branch attached to a green stem is covered in white mold. The leaves on the branch are brown and curled. Hemp branch infected with white mold.