Evaluating Leaf Spot Diseases on Hemp in Kentucky: A 2-Year Study

Modern hemp was reintroduced to the US in 2014 and legalized in 2018 (Agricultural Act of 2014; Agricultural Improvement Act 2018).   Acreage has fluctuated in the past years, reaching about 100,000 acres in the southeastern US and over 500,000 acres in the US in 2019.  The market has experienced some volatility in the past two years, but economists suggest that the outlook for hemp as a stable commodity is positive.  Hemp has potential for a wide range of products from food to fiber to medicine.  Like other agricultural crops, hemp is susceptible to a wide range of diseases.  Leaf spot diseases have been known to causes losses by extreme blighting and plant death, or by harvests being rejected by processors.   The most severe diseases of Kentucky hemp include Bipolaris leaf spot (Bipolaris gigantea), Cercospora leaf spot (Cercospora flagellaris), and Septoria leaf spot (Septoria cannabis).   Thus, a two-field, two-year study was designed to answer important questions regarding cultivar susceptibility and disease development. Data indicated that the Trump group (T1 and Wife) were the most susceptible to Bipolaris gigantea, Cercospora flagellaris, and Septoria cannabis.  The Otto II group (Otto II and Endurance) was least susceptible to leaf spots diseases.   Cherry group and BaOx groups were moderately susceptible. Time of infection was determined by disease emergence in fields.  Septoria leaf spot was the first disease to appear in fields, with disease developing in mid-July and peaking in late-August.  Septoria leaf spot was predominantly in the lower to inner canopy of plants, and it did not expand to upper plant parts or flowers.  Leaf drop resulted, and new foliage emerged and remained uninfected.   Bipolaris leaf spot appeared in fields by mid-August, peaked in mid-September, and remained active through harvest. This disease developed on lower and upper plant canopies, and in this study, it infected sugar leaves and flower bracts.  Cercospora leaf spot developed symptoms in late-September.   The disease was severe in upper plant canopies as late as harvest.  As a result of this study, new grower recommendations have been developed and distributed to farmers at field days and educational events.  With this new crop, research was required in order to develop appropriate management recommendations.  This study represents the connection between applied research and extension.