Exserohilum Leaf Spot Causing Problems in NC Hemp

— Written By Lindsey Thiessen

Hemp Leaf Spot Problems

North Carolina industrial hemp producers are combatting a leaf spot issue that is caused by Exserohilum rostratum. This disease was first identified during the industrial hemp pilot program in 2017, and has not been widespread in the last two years. In the last week, however, we are seeing this disease in several hemp strains of hemp across the state. Other leaf spots may look similar to this disease, and accurate diagnosis is important.

Symptoms of Exserohilum Leaf Spot

This disease can affect any portion of the plant, including stems, flowers, and leaves. The leaf spot is most distinct (Fig. 1) as lesions are small and round with dark brown margins and a gray-brown center. As lesions age, spores may be produced inside the gray-brown center, and look like fine ground pepper flakes. Lesions may coalesce to form larger, irregular-shaped lesions.

Exserohilum leaf spot

Figure 1. Leaf spots on industrial hemp leaves caused by Exserohilum rostratum.

Other similar diseases include bacterial leaf spot, Fusarium leaf spot, and Septoria leaf spot. North Carolina has not had much incidence of Septoria leaf spot, but there is information from University of Kentucky available for Septoria Leaf Spot.

Bacterial leaf blight

Figure 2. Bacterial leaf spot and blight on industrial hemp leaves characterized by angular lesions.

Fusarium leaf spot

Figure 3. Fusarium leaf spots on hemp leaves. Wilting of plant caused by a stem canker from the same Fusarium spp.

Management Strategies

There are currently no chemical management strategies that are effective and legal to apply in industrial hemp. Cultural practices may help reduce incidence by removing the pathogen from the production system or making conditions less suitable for growth of the fungus.

  • Maintain weed-free fields to limit fungal survival on grasses or other weedy hosts
  • Destroy crop debris at the end of the growing season to limit overwintering inoculum
  • Improve air flow through the plant canopy to limit humid conditions that favor fungal growth

Have more questions?

For more help on managing industrial hemp foliar diseases, contact your cooperative extension agent.