Curly Top Disease of Hemp () in California Is Caused by Mild-Type Strains of Often in Mixed Infection.

Interest in industrial hemp () as a potential crop led to the establishment of commercial fields in a number of counties in California in 2019 and 2020. Plants in these fields developed different types of virus-like symptoms. The most prevalent type was stunted and bushy plants with distorted, upcurled, and yellowed leaves, which were similar to those associated with curly top disease (CTD) caused by the beet curly top virus (BCTV). This beet leafhopper-vectored virus is endemic in California and can cause economic losses to processing tomato production. Using a multiplex PCR test, BCTV infection was detected in 89% of hemp samples with CTD-like symptoms from Fresno, San Bernardino, and Ventura counties. Other symptom types had low incidence of BCTV infection and were associated with other factors. Hemp plants in California were infected only with the mild-type strains, BCTV-CO and BCTV-Wor, and often in mixed infection (43% of samples). Finally, using an infectious clone of a BCTV-CO isolate from hemp, we demonstrated that agroinoculated hemp plants developed these CTD-like symptoms, thereby fulfilling Koch's postulates for the disease.