NDA lab detects Phytoplasma Witch’s Broom: A devastating bacterial disease that can affect hemp crops in Nevada

By Shouhua Wang, Ph.D., Nevada Department of Agriculture Plant Pathologist

A healthy hemp plant that has green and fully developed leaves.

Now in its third year, the Nevada Department of Agriculture (NDA) has registered more than 100 hemp operations throughout the state, many of them researching the viability and durability of different varietals in Nevada’s climates.

In the 2018 growing season, the NDA Plant Pathology Lab detected a phytoplasma bacterium causing witches’ broom disease on hemp crops in several production areas. The disease seems to be primarily affecting “Cherry Wine” and “Berry Blossom” varieties, and it has affected these strains severely enough that crop harvest value has been negatively impacted.

 A diseased hemp plant that is much smaller than the healthy plant and has severe yellowing and clustered Accredited plant pathology lab is first in the U.S. to identify the pathogen

This is the first confirmed identification of the phytoplasma pathogen on cannabis crops in the United States. Hemp growers should pay attention to this type of disease by checking plants periodically for symptoms and taking steps to prevent its spread to other plants.

Bacteria are microscopic single-celled organisms. Most bacteria cause soft rot, fire blight, wilt or root rot on plants. Phytoplasmas are different from most bacterium and typically cause witches’ broom, systemic yellowing, stunting, chronic decline and abnormal growth. Unfortunately, phytoplasmas cause untreatable damage, so it’s important for hemp growers to take preventative steps.

Hemp is prone to phytoplasma infection, and once infected, plants exhibit severe witches’ broom symptoms, as is sometimes seen on alfalfa. Witches’ broom symptoms typically resemble crowded growth of leaves on single branches, creating a cluster where the leaves are small, yellowish and not fully developed. Early infected plants may be severely stunted or killed by the bacteria. While plants infected later may not die during the season, symptomatic plants are useless for harvest. In the middle or late season, some plants may show clusters of witches’ broom symptoms on certain branches while the rest of plant looks normal, which suggests only a portion of the plant is infected.

Preventing phytoplasma diseases

Because phytoplasma bacterium are generally not treatable, the best way to manage this disease is through prevention. Check for healthy plant stock with no signs of disease, especially on “Cherry Wine” and “Berry Blossom” varieties. Seeds should come from sources free of phytoplasma witches’ broom during seed production.

If the disease does occur in a field, the best strategy is to remove all symptomatic plants as early as possible, and monitor for potential insect vectors such as leafhoppers, planthoppers and psyllids. These insects can acquire phytoplasma from diseased plants and transmit it from plant to plant. Control of insect vectors can minimize the spread of the disease but will not cure or reverse existing phytoplasma infection.

The NDA Plant Pathology program provides diagnostic services to growers for any plant disease problems. If phytoplasma witches’ broom is suspected on a hemp plant, submit samples to the NDA Plant Pathology Lab. For sample submission guidelines and more information about plant pathology, visit http://agri.nv.gov/plant_pathology.


Photo captions

  • First Picture – A healthy hemp plant that has green and fully developed leaves.
  • Second Picture – A diseased hemp plant that is much smaller than the healthy plant and has severe yellowing and clustered leaves.
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