Reemergence of Septoria Leaf Spot Caused by <i>Septoria cannabis</i> on Hemp in Kentucky

Abstract

Hemp reemerged in 2014 after being illegal for over 50 years and restricted for 90 years. Today, hemp is primarily grown for production of cannabidiol (CBD), with limited acreage dedicated to fiber. One of the most frequent and destructive leaf spot diseases of hemp cultivars grown for CBD is Septoria leaf spot. Symptoms are mostly observed in lower leaves and inner canopy. Leaf spots begin as small, irregular, brown to gray spots that rapidly expand to about 5.0 to 7.5 mm in diameter. Pycnidia are scattered, round, dark brown to black in color, and measure 38.8 to 145.0 μm in diameter. Conidia are hyaline, curved but occasionally straight, pointed at the apex, and contain 3 to 4 septa. Morphological characteristics were similar to those reported for <i>Septoria cannabis</i>. Gene sequences from seven diagnostic loci (<i>elongation factor</i>, β-<i>tubulin</i>, <i>RNA polymerase II</i>, <i>large subunit</i>, internal transcribed spacer, <i>actin</i>, and <i>calmodulin</i>) did not match any published accessions. There are no published sequences for <i>S. cannabis</i> available for comparison. Phylogenetic analysis of concatenated sequences showed that isolates from hemp grouped separately from other <i>Septoria</i> spp. Similarity of morphological characteristics and lack of sequence data matching other <i>Septoria</i> spp. led to the conclusion that isolates collected from hemp in Kentucky are <i>S. cannabis</i>. This new information will serve as an update for Septoria leaf spot diagnostics, especially as hemp acreage continues to increase across the United States.