Prevalence, Identity, Pathogenicity, and Infection Dynamics of Botryosphaeriaceae causing Avocado Branch Canker in California

Botryosphaeria branch canker and dieback of avocado has expanded in avocado growing areas in recent years. Twenty-one avocado groves in major California producing regions were surveyed in 2018 and 2019. Monthly inoculations of wounded, green and lignified branches of ‘Hass’ and ‘Lamb Hass’ were conducted. Botryosphaeriaceae were the predominant fungi recovered from cankered tissues collected across traditional and high-density orchards, causing symptoms on all sampled cultivars. These fungi were also recovered in asymptomatic twigs and other organs, thus exist as potential reservoir for future infections. Molecular analyses of 173 isolates showed that Neofusicoccum luteum had the greatest incidences across sites and cultivars, with 83 and 29% recovered from ‘Hass’ and ‘Lamb Hass’, respectively. Pathogenicity tests on excised (‘Hass’, ‘GEM’, and ‘Hass’ mutants), attached shoots from potted (‘Hass’) and mature avocado trees (‘Hass’ and ‘Lamb Hass’) showed that all species were pathogenic on wounded, green, and mature branches. Monthly inoculations of wounded, green and lignified branches of ‘Hass’ and ‘Lamb Hass’ showed that both stem types were susceptible throughout the inoculation periods, regardless of the avocado phenological stage. In temperature-dependent growth and infection studies, growth of three Botryosphaeriaceae was higher between 20 and 30 °C, but only Lasiodiplodia theobromae significantly grew and caused external lesions at 35 °C. Lasiodiplodia theobromae also grew more on perseitol-amended media, all indicating its adaptation to warmer temperatures and capacity in metabolizing the avocado-produced sugar. Overall, this study extended our knowledge of the biology of Botryosphaeriaceae on avocados, which will be useful to tailor management strategies.