First report of anthracnose leaf spot caused by Colletotrichum fioriniae on hemp (Cannabis sativa).
In August 2018, a field hemp sample with leaf spots was submitted to the University of Kentucky Plant Disease Diagnostic Lab from Jackson County, Kentucky, USA. Leaf spots affected all plants in the field, with spots forming on the lower half of plants in early August as plants entered the reproductive stage. Disease severity on mature leaves was ~20%, and incidence was 100%. All three cultivars in the field were equally affected: 'Cherry × Cherry', 'Trump 1' and 'Sweetened'. Symptoms included numerous, scattered, round leaf spots that expanded to necrotic blotches. Spots began as light green specks and developed into round or angular spots with tan centers. Spots were circular but occasionally became irregular as they expanded, reaching 3 to 4 mm in diameter with a darker brown margin and yellow halo. Centers of spots cracked or dropped out, causing shot-hole or frog-eye symptoms. Spots sometimes coalesced to form necrotic regions up to 20 mm across. Necrotic areas on or near leaf margins resulted in symptoms typical of anthracnose leaf diseases on other hosts, such as scorching, puckering, and twisting. Conidia were isolated from leaf lesions to identify the pathogen. Based on morphological characteristics, pathogenicity, and multi-gene sequencing, the causal agent was identified as Colletotrichum fioriniae. This fungus has been reported as a fruit rot pathogen on many fruits grown in Kentucky, including apple, blueberry, grape, peach, and strawberry. It is the most commonly reported Colletotrichum species causing bitter rot of apple in the state, causing an average annual yield loss of 30%. It is also the causal agent of ripe rot of blueberry in Kentucky. Additionally, latent or endophytic infections by C. fioriniae have been observed and reported on a wide range of native and cultivated hosts including poison ivy, wild leek, and brambles; ornamental plants including rhododendron and winterberry; and insects including hemlock scale. Following this confirmation, reports of anthracnose leaf spot caused by C. fioriniae have been verified in additional hemp fields in Kentucky. This is thought to be the first report of anthracnose leaf spot caused by C. fioriniae in the world.