Reemergence of Septoria Leaf Spot caused by Septoria Cannabis on Hemp in Kentucky Confirmed by Sequence Data

Authors: RAHNAMA, MOSTAFA - University Of Kentucky; SZARKA, DESIREE - University Of Kentucky; LI, HUA - University Of Kentucky; DIXON, ED - University Of Kentucky; Castlebury, Lisa; GAUTHIER, NICHOLE - University Of Kentucky

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2021
Publication Date: 10/17/2021
Citation: Rahnama, M., Szarka, D., Li, H., Dixon, E., Castlebury, L.A., Gauthier, N. 2021. Reemergence of Septoria leaf spot caused by Septoria cannabis on hemp in Kentucky, confirmed by sequence data. Plant Disease. 105:2286-2289. https://doi.org/10.1094/PDIS-12-20-2620-SC.
DOI: https://doi.org/10.1094/PDIS-12-20-2620-SC

Interpretive Summary: Industrial hemp is becoming and important crop in many parts of the United States. However, it has not been grown commercially in the the U.S. in the last 50 years. As a result, research on diseases of industrial hemp using modern methods, including DNA sequencing, is lacking. In 2018, a leaf spot of in industrial hemp emerged in Kentucky causing losses in yield. In this study, the fungus causing the leaf spot was isolated and characterized using morphological characters and DNA sequences. The identity of the fungus was determined to be Septoria cannabis by comparing it with historical descriptions and specimens at the U.S. National Fungus Collections. This research will be used by extension agents, plant breeders, plant pathologists, and plant quarantine officials to accurately identify and manage this disease. In this review the major groups of fungi causing plant diseases are discussed with examples provided from each of the groups. Representative species are discussed and images are provided. This review article will be used by extension agents, plant breeders, plant pathologists, and plant quarantine officials as well as students to enhance their awareness of plant pathogenic fungi and to reinforce the need for continued vigilance in the protection of plants from destructive fungi to keep agriculture safe in America and around the world.

Technical Abstract: Industrial hemp was reintroduced to the US in the 2014 Farm Bill after more than 50 years of prohibition. Limited research on diseases was conducted during this time, but modern technology now allows for more accurate identification of plant pathogens associated with industrial hemp. Septoria leaf spot is one of several common leaf spot diseases in Kentucky hemp fields. This disease primarily affects lower leaves and those in the inner canopy beginning in early- to mid-July, approximately four to six weeks after transplanting. Samples of Septoria leaf spot were collected in 2018 from several locations in Kentucky. Morphological characteristics of eight isolates were documented. To validate pathogenicity, each of the eight isolates was tested on 6-week-old greenhouse grown hemp plants (cv ‘Victoria’ and a proprietary variety). Morphological characteristics of isolates used in this study matched those of S. cannabis from published reports; characteristics such as conidial septations and pycnidial size and color were distinct from S. neocannabina, also reported from industrial hemp. A phylogenomic analysis was performed by sequencing diagnostic loci (EF, TUB, RPB2, LSU, ITS, ACT and CAL for three isolates to infer the evolutionary history using the maximum likelihood method. Sequences from all loci from the three isolates were identical and all three sequenced isolates clustered separately from other Septoria spp. The resurgence of Septoria leaf spot has likely coincided with the reintroduction of hemp. This is the first report of Septoria leaf spot on Cannabis sativa that includes sequence data for the causal agent. Although distribution across Kentucky appears widespread, host range is unknown. As hemp acreage increases across the U.S., proper identification of causal pathogens and an update of modern diagnostic protocols is essential.