Researchers Find Cause of Hemp Blight in North Carolina

During the hemp-growing seasons of 2017 and 2018, several cultivars of industrial hemp plants across numerous counties in North Carolina started to show signs of foliar, stem, and floral blight. In order to determine the exact cause of this blight, samples of the affected plants were collected and sent for scientific analysis at the Plant Disease and Insect Clinic at North Carolina State University.

Now, Assistant Professor Lindsey Thiessen, Ph.D, and Research Assistant Tyler Schappe, M.S., have published their initial findings, naming the fungal disease Exserohilum rostratum as the cause of the hemp blight.

Industrial hemp farming

Industrial hemp is an important crop in North Carolina. Before the passage of the Farm Bill, North Carolina already had more than 500 registered hemp cultivators operating under the Industrial Hemp Pilot Program run by the state Department of Agriculture. Now that the Farm Bill has solidified hemp’s position as an important agricultural commodity, this number is only expected to rise.

This effect may also be compounded by the state’s reliance on tobacco farming. Tobacco is the state’s leading field crop, and is among the top five agricultural products in the state in terms of the revenue that is generated by its farming — but falling cigarette sales and rising sale prices means many tobacco farmers are turning to hemp farming as an alternative.

The move makes sense; much of the equipment and knowledge needed to grow both crops are similar enough that farmers can easily make the switch without having to do too much external study. And with the legalization of hemp-derived CBD products as per the Farm Bill, farmers should have little trouble in lining up buyers for their crop.

Analyzing the blighted crop

In order to better protect the crops of North Carolina’s hemp farmers, it is important to identify any new disease risk that could limit the yield and quantity of industrial hemp flower production. With this in mind, the researchers at North Carolina State University set out to identify the cause of the 2017 and 2018 blights.

The blighted leaves were observed to have brown-black lesions with dark margins on their surface. Inside the legions, significant amount of conidia (spores produced asexually by various species of fungi) were found.

The conidia were multiseptate and varied slightly in shape, from rostrate, ellipsoidal to narrowly obclavate, straight, to slightly curved, and they were olive-brown in color. Conidiophores were cylindrical and olive-brown with swollen conidiogenous cells containing circular conidial scars.

Isolates for testing were produced by transferring a single spore into water agar, then again onto potato dextrose agar (PDA). After 2-3 days on PDA, the mycelia observed turned from their initial white to a dark brown-black color.

Identifying the fungal disease

The identity of the fungal disease was confirmed in two ways: by molecular identification using DNA and the polymerase chain reaction (PCR) technique, and by satisfying Koch’s Postulates for the identification of a pathogen.

For the PCR method, DNA was extracted from a representative pure culture of the isolated pathogen using the DNeasy Powersoil kit. By amplifying relevant regions of the DNA (ITS1, 5.8S, and ITS2 regions) researchers were able to identify the pathogen belonged to the Setosphaeria species (syn. Exserohilum) using the Genbank database. Additionally, NCBI-BLAST searches using the ITS and RPB2 sequences identified the pathogen culture as Setosphaeria longirostrata (99.8% pairwise identity, syn. Exserohilum rostratum) and S. rostrata (99.5% pairwise identity, syn. Exserohilum rostratum).

To satisfy Koch’s Postulates, a representative isolate of S. rostrata (syn. Exserohilum rostratum) was used to inoculate a healthy hemp plant; here of the variety ‘Carmagnola’. Two week old ‘Carmagnola’ saplings were inoculated with a conidial suspension containing S. rostrata and monitored and compared to a control plant that did not receive the same inoculation. After 8 days, dark brown lesions had formed on the leaves of the inoculated plants. After 3 weeks of growing time, the lesions were surface sterilized and then placed on PDA to culture. The only microorganism re-isolated from the legions was the S. rostrata and the spores developed looked identical to those from the blight-affected samples.

From this molecular identification and morphological similarities, the researchers at North Carolina State University concluded that Exserohilum rostratum (syn. Setosphaeria rostrata) was the fungal infection responsible for the hemp blight episodes in North Carolina.

Does Exserohilum rostratum pose any immediate danger?

As hemp cultivation expands across the US it is important to identify any infections that might affect the cannabis crop, both from a viewpoint of protecting the health of the overall crop, and to ensure that consumers are not at risk.

Since the effects of the infection appear to be highly visible — in the form of dark brown-black lesions — it is unlikely that infected hemp material could move through the supply chain far enough to reach the point of being processed for human consumption. Additionally, while it does not appear to be mandated by the state, there are hemp testing laboratories in the state that offer quality screening which could detect a fungal infection if a cultivator was concerned about their crop.

For the farmers who may encounter Exserohilum rostratum during cultivation, there should perhaps be slightly more concern. While E. rostratum is a rare cause of infection in humans, there are scattered cases of the fungus leading to allergic fungal sinusitis and even more rarely, a form of fungal corneal inflammation known as keratomycosis in farmers.

It will be easier to get a handle on any concerns that may or may not be warranted after the group from North Carolina State University publish their full findings.

*Image: Conidia found on the hemp leaves. (https://doi.org/10.1094/PDIS-08-18-1434-PDN)