Occurrence and Distribution of Common Diseases and Pests of U.S. Cannabis: A Survey

Hemp and marijuana, both common names for Cannabis sativa, are recent additions to U.S. agricultural and horticultural industries (McPartland 2018; NCSL 2022). Hemp was reintroduced in 2014 under a pilot research program and legalized in 2018 (Agricultural Act of 2014; Agricultural Improvement Act 2018). Hemp (Δ⁹-THC content < 0.3%) can now be grown in all 50 U.S. states. Marijuana (Δ⁹-THC content > 0.3%), although classified as a Schedule I narcotic by the U.S. Drug Enforcement Administration, is legal in 37 states either for medical or for recreational use, and voters approved marijuana usage in two additional states in 2022. Legalization of medical marijuana in Colorado (1996) and in Oregon and Washington (1998), followed by recreational-use legislation in Colorado and Washington (2012), began a series of statewide legalization laws (NCSL 2022). As a result of these laws, cannabis acreage has increased during the past decade, with hemp reaching over 200,000 ha in 2019 (MJBiz 2019; USDA-NASS-ESMIS 2019). Marijuana acreage is not well documented, but nearly 3 million kg was produced in 2021 (Downs and Williams 2022).

Although C. sativa is often described as a pest-free crop, it remains susceptible to a considerable number of diseases and pests (McPartland et al. 2000). Several diseases and arthropod pests have been confirmed in recent years (Ajayi and Samuel-Foo 2021; Cranshaw et al. 2019; Gauthier et al. 2019; Gauthier and Thiessen 2022; Kesheimer et al. 2021; Punja et al. 2019; Thiessen et al. 2020). However, statewide restrictions and fears of introducing illegal cannabis into federally supported facilities have slowed research and limited understanding of the occurrence and distribution of diseases and pests (Marcu 2019; US-DEA 2019). Currently no reliable database exists to track confirmed diseases and arthropod pests of cannabis, as with other commodities.

In 2021, the Plant Protection Working Group, a part of the Multistate Research Project S1084: Industrial Hemp Production, Processing, and Marketing in the U.S., implemented a census documenting biotic problems on hemp and marijuana across the United States. This survey was intended to determine the distribution and occurrence of common diseases and arthropod pests on cannabis by targeting diagnosticians, researchers, and industry specialists at universities, government agencies, and private industries. The resulting data will serve as a foundation for regional and national prioritization of research and regulatory activities.

Materials and Methods

The survey was designed and implemented using Qualtrics survey software (Qualtrics International, Seattle, WA) and was intended for agronomy/horticulture, entomology, and pathology specialists who were qualified to confirm the identity of arthropod pests and/or pathogens. The survey was distributed to state and university diagnostic laboratories, research and industry specialists, and extension programs across the 48 continental states between July 2021 and June 2022. Solicitations included government agencies, industry partners, and universities. Outreach included direct emails, listservs, and phone calls. The survey was designed to capture responses only from those who were trained to confirm causal pathogens and arthropod pests. Respondents were initially asked about their role in the cannabis industry and whether that role included diagnostics or confirmation of pathogens and/or pests (Supplementary Fig. S1). The survey terminated if respondents indicated that they were not qualified to confirm causal agents. Questions focused on the frequency of specific diseases and pests. No distinction was made between indoor versus outdoor production or whether the crop was considered hemp or marijuana. A total of 76 variables were presented, and respondents were given the option of disregarding questions that were unrelated to their position or those about which they were uncertain.

States were sorted into five geographic regions, similar to those of the National Plant Diagnostic Network (NPDN; https://www.npdn.org/). Figure 1 and Supplementary Figure S2 define the states included in each region. Occurrence was documented for each disease and arthropod for each region, but state-specific results were not evaluated due to potential variability as a result of limited responses in some states.

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To compare disease and arthropod occurrence on a national scale (across all regions), the percentage of incidence of each disease or arthropod pest from each region was assessed with analysis of variance using GLIMMIX of SAS 9.4 (SAS Institute, Cary, NC), with each region representing a replication. Replication was then treated as a random effect of the model. Tukey's test was used for post-hoc means pairwise comparisons at an alpha of 0.05.

Results

A total of 163 valid responses were collected, and 148 were included in the final dataset. The most common reason responses were removed from the analysis was failure to include state or location. Survey responses were collected from 39 states: No responses were received from Idaho, Massachusetts, Minnesota, New Hampshire, New Mexico, Rhode Island, South Dakota, Washington, or West Virginia. The largest percentage of respondents were from the South (41%), followed by the Northeast (21%), Great Plains (15.2%), North Central (12.4%), and West (10.4%).

Disease distribution was variable across regions (Figs. 1A, 2, 3 and 4; Supplementary Fig. S3). All fungal diseases in the survey were reported in the Northeast and South, the regions with the highest rainfall. Nationwide comparison among fungal diseases showed that fungal diseases such as Botrytis bud rot, powdery mildew, Cercospora leaf spot, Fusarium damping off, and Fusarium canker/basal rot were reported significantly more often than fungal diseases such as sooty mold, Exserohilum leaf spot, Stemphylium leaf spot, and Corynespora leaf spot (P < 0.0001) (Fig. 2A). Fungal diseases, notably Cercospora leaf spot, Fusarium canker, powdery mildew, Botrytis bud rot, and Fusarium head blight, had the highest occurrences in both the Northeast and South (Fig. 3). Warm-season fungal diseases such as Southern blight were more prominent in the South, whereas cool-season diseases such as white mold were more prominent in the Northeast (Fig. 3). Fewer diseases were reported in the arid Great Plains and West. In the West, only 15 out of the 30 fungal diseases were reported. The Great Plains had fewer diseases than other regions, and none reported with more than 50% occurrence. Bacterial (Ralstonia) blight, specific to tropical and subtropical climates, was reported in the Great Plains and South (Fig. 4). More viral diseases were reported in the Great Plains and West than in other regions (Fig. 4). Hop latent viroid (HLVd) was reported in all regions, although it was more prominent in the West and Great Plains, where more hops are grown (Fig. 4). Beet curly top virus (BCTV) was reported in the West and Great Plains, where its vector, the beet leafhopper, feeds primarily on native plants of arid regions (Fig. 4). More prominent viral diseases in both the Great Plains and West are relevant, with published reports of the diseases in both regions, such as a recent report of Hop latent viroid (HLVd) in Washington State (Jarugula et al. 2023) and Beet curly top virus (BCTV) in Arizona (Hu et al. 2021), California (Melgarejo et al. 2022), Colorado (Chiginsky et al. 2021), Oregon (Rivedal et al. 2022), and Washington State (Jarugula et al. 2023). Oomycetes were consistently reported across regions (Fig. 4). There were limited reports of nematodes overall (Fig. 4).

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Most arthropods in the survey were reported in all regions (Figs. 5, 6, 7, 8, 9, 10, 11, and 12; Supplementary Fig. S3), except for beet leafhoppers, fire ants, oblique banded leafrollers, the darker spotted straw moths, and rice root aphid. Nationwide comparison among lepidopterans showed that corn earworm was reported significantly more often than yellowstriped armyworm, beet armyworm, oblique banded leafroller, and darker spotted straw moth (P < 0.0001) (Fig. 5A). Corn earworm was the most reported Lepidopteran in all regions (Fig. 6). Three borer species were reported in all regions (Fig. 7), although occurrence was higher in the Northeast, where forest cover is denser than in other regions. Tarnished plant bug was the most reported bug in all regions (Fig. 8). Cannabis aphid, specific to cannabis, was reported in all regions (Fig. 9). Rice root aphid was not reported in the North Central region despite its wide host range; fewer overall responses were received from the North Central region (Fig. 9). All beetles were reported in all regions (Fig. 10). Mites were reported in all regions, including the host-specific hemp russet mite (Fig. 11). However, there were more reports of mites in the Northeast and South (Fig. 11). Fire ants were reported in the West, Great Plains, and South, which include the warmer latitudes across the United States. Beet leafhopper was limited to the arid western portion of the United States and was not reported in the central or eastern portion of the United States (Fig. 12).

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Conclusions

Until recently, national and statewide regulations have slowed research and limited our understanding of the occurrence and distribution of diseases and arthropod pests of cannabis. This survey included reports of diseases and pests of hemp and marijuana from 39 states and serves as the first report of its kind in which a nationwide summary is published.

Diseases and pests included in this survey ranged from generalists to host-specific organisms, as well as from ubiquitous to regionally specific agents. For example, Pythium root rot and two-spotted spider mites are universal and have broad host ranges; they were reported in all regions, as expected. Regionally specific diseases and pests with broad host ranges such as beet leafhopper and southern blight were reported in limited geographic areas. Still, some cannabis-specific agents, such as Septoria leaf spot and hemp russet mites, were reported across the United States, possibly due to years of widespread clandestine production, feral cannabis populations, and interstate shipment of infected hemp clones.

Cannabis production has experienced volatility during the past several years. Hemp acreage initially focused on hemp for floral material (cannabinoid production), but markets significantly decreased beginning in 2019 (Shepherd and Mark 2021). Growing interest in grain and fiber hemp has led to increases in acreage (Sandy 2020; USDA-FSA 2022). Marijuana legalization has led to increased indoor and greenhouse production, as well as outdoor field production (Hansen et al. 2022). As acreage increases, reports of diseases and pests will also increase, likely combinations of cross infection from previously planted crops and introductions via movement of plant material.

This survey included reports from 148 diagnosticians, researchers, and industry leaders from across the United States. Although the survey targeted professionals in diagnostics-related disciplines, it was limited to the self-reported data based on various means for diagnoses. Due to the recent expansion of cannabis production into the legal domain, unfamiliarity with cannabis-specific agents may impact accuracy of identification.

Results from this survey present an overview of the occurrence and distribution of common diseases and pests of C. sativa in the United States, as well as provide a foundation for future regional and national prioritization of research and regulatory activities. Focused research on pest and disease management is critical to building a sustainable cannabis industry.