In October of 2020, a grower in Boyle County, KY, reported mold and blight symptoms on flowers of field-grown hemp. Plants were approaching harvest, and the mold was affecting 100% of the cultivar ‘White CBG’ being grown for cannabinoid (CBD) extraction. Mycelium colonized the flower heads and any seeds within bracts. Affected flower bracts were necrotic, and mycelium and necrosis in the most severe cases also encompassed adjacent (sugar) leaves. Necrotic symptomatic tissue was collected, disinfested in 10% bleach for one minute, and cultured on acidified potato dextrose agar (APDA). Each isolate was single-spored, transferred to PDA, stored in 15% glycerol at -80°C and maintained at room temperature under blacklight blue and fluorescent bulbs on a 12-hour light-dark cycle. Colonies produced white-pink mycelia with a dark red pigment on the undersides. Conidia collected after 7-9 days were falcate and septate (5 to 6). No microconidia were produced. Macroconidia measured 35.4-49.7 µm x 3.4-5.8 µm (n=50). The strains produced blue-black fertile perithecia on carrot agar when induced according to the method of (Bowden and Leslie, 1999). To confirm pathogenicity, flowers of hemp cultivars ‘Lifter’, ‘Trump Towers’, ‘Wife’ and ‘White CBG’ were inoculated in the greenhouse with a representative fungal strain (20Hemp010). Plants were inoculated at two different stages: when the styles were still green or after they had become senescent. Macroconidia were collected from 7- to 9-day-old cultures grown under a 12-hour light-dark cycle. Plants were spray-inoculated with a 5 x 105 per ml conidial suspension in 0.05% Tween 20 until runoff. Flower heads were individually covered with clear plastic bags and incubated for 72 h at 95-100% humidity under greenhouse benches to avoid direct light. Bags were removed after 72 h and returned to the bench. Greenhouse conditions were 23-25°C with a 14-hour photoperiod and 50% RH. Symptoms developed 7 dai in 1% of the flowers inoculated when styles were green, and 36% of the flowers that had senescent styles. Symptoms were similar to those initially noticed in Boyle County, including necrotic flower bracts and sugar leaves, and visible fungal growth. Symptoms were more severe on plants inoculated when styles were necrotic. Recovered fungi were morphologically similar to 20Hemp010. Genomic DNA was extracted from the mycelium with the Zymo Research Quick-DNA Fungal/Bacterial Miniprep Kit. A fragment of the translation elongation factor 1-alpha 1 gene was amplified with primers EF1 and EF2 as described by (O’Donnell et al. 1998). Amplicons were sequenced and the consensus (MZ407909) was compared with the NCBI GenBank Refseq database by BLASTn. The top hit was Fusarium graminearum with 100% identity (JF270185.1). Pairwise alignments via MycoBank Fusarium MLST and Fusarium-ID also revealed a top hit of F. graminearum with 100% identity (AY452957.1). Conidial and colony morphology were also consistent with F. graminearum (Leslie and Summerell, 2006), thus we conclude that this species was the causal agent of the flower blight and mold. The same disease was subsequently confirmed on hemp in Breathitt and Franklin Counties in KY in 2020. This is the first report of this disease in KY, although F. graminearum has been reported previously causing a similar flower blight on hemp in NY and NC (Bergstrom et al., 2019, Thiessen et al. 2020). Fusarium graminearum is common in KY as a cause of Fusarium head blight on wheat and Gibberella ear rot on corn. In cereals, fungal infection is facilitated by the production of the mycotoxin deoxynivalenol (DON), which is harmful to humans and livestock (Desjardins and Hohn, 1997). As hemp production in Kentucky continues to rise for production of CBD products and edible grains, accumulation and concentration of DON in these products could become a concern.

Hemp and marijuana, both Cannabis sativa L., are revitalized crops to U.S. agricultural and horticultural industries. Hemp (Δ⁹-Tetrahydrocannabinol content <0.3%) was reintroduced in 2014 under a pilot research program and legalized in 2018. Hemp can now be grown in all 50 states. Marijuana (Δ⁹-THC content > 0.3%), though classified as a Schedule I narcotic by the U.S. Drug Enforcement Administration (DEA), is legal in 40 states and the District of Columbia for medical and/or recreational use. Although C. sativa is often promoted as a pest-free crop, multiple diseases and arthropod pests have been identified and confirmed in recent years. There are limited options for control of diseases and pests affecting hemp. A survey of diagnosticians, researchers, and industry leaders conducted in 2021-22 sought to determine the distribution and occurrence of 76 common diseases and pests on C. sativa across the U.S. A total of 148 responses were collected and grouped by U.S. region: Western, Great Plains, North Central, Northeastern, and Southern. Survey results suggest that while some pathogens and pests are widely distributed across the U.S., others occur more frequently in specific regions. This finding may indicate variations in economic importance by region. Results from this survey provide a foundation for regional and national prioritization of research and regulatory activities.

Hemp (Cannabis sativa L.) is a versatile crop that can be used to produce a wide range of products including food, fiber, and medicine. In 2021, U.S. hemp was valued at over $824 million. Emerging threats to hemp production in Kentucky and the southeastern U.S. are head blight in floral and grain hemp. During the 2020 growing season, severe losses, some as high as 100%, resulted from head blights. Necrotic floral tissues and seed heads from research trials were analyzed to confirm species identity and pathogenicity. Previously unreported causal species in Kentucky included F. avenaceum, F. incarnatum-equiseti species complex, and F. sporotrichioides; F. graminearum was also confirmed. These species are known to cause head blight of cereal grains. The mycotoxin production potential of each of these species warrants further studies into the parameters that influence infection and the interaction between these Fusarium spp. and hemp.

As hemp becomes established as a commodity in the U.S., continued cultivation demands a greater understanding of the pathogens that affect the consumable portions such as flowers and grain. Four Fusarium spp. have been confirmed to cause Fusarium head blight on hemp in Kentucky. Several Fusarium species, including F. graminearum, that are known to produce mycotoxins have been confirmed pathogenic on hemp. Fusarium mycotoxins are regulated in grains used for human and animal consumption. Determining which Fusarium species infect hemp is the first step to producing safe material. While field disease is under investigation, there have been no studies regarding stored hemp. Harvested and stored floral material for production of cannabidiol (CBD) were collected from seven Kentucky producers from 2019 and 2020 harvests. Material was screened using a Fusarium-selective medium and DNA sequencing. At least 12 different species were isolated, most from the Incarnatum-equiseti species complex (75.6%). Species from the Sambucinum (16.3%), Oxysporum (0.8%), Fujikuroi (5.7%), and Solani (1.6%) species complexes were also identified. Additional research is essential to determine whether these Fusarium species are pathogenic or saprophytic, and if they can produce toxins dangerous for humans and animals. Such information is crucial to determine how to store hemp, manage infected material, and promote successful production of hemp products.

Upon reintroduction of hemp in 2014 and legalization in 2018, labeled pesticides have remained limited. Further, consumer demand has aimed the market toward organic or chemical-free production systems. In efforts to manage diseases and pests in fields and greenhouses, producers turn toward biological and biorational formulations. Efficacy of these fungicides against common aerial diseases of hemp is largely unpublished. Challenges of efficacy testing, however, further delay or discourage research. In this study, we evaluated screening methods against some common biological products. The aim was to test a screening model in order to examine products against fungal pathogens and to identify demonstrable differences under controlled conditions. Thus, in this study, we prescreen 11 biological and biorational fungicides against four common fungal leaf and flower pathogens using three bioassays. Confirmation that the major modes of action for these products have measurable activity against major pathogens of hemp serves as a first step toward more complex field studies.