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POSTERS: New and emerging diseases

Extent and significance of Botryosphaeria and Anthracnose diseases in avocado orchards of California
Herve Avenot - University of California Davis, Kearney Agricultural Research & Extension Center. Marcelo Bustamante- University of California Davis, Kearney Agricultural Research & Extension Center, Mirella Zaccheo- University of California Davis, Kearney Agricultural Research & Extension Center, Themis Michailides- University of California Davis, Ke

Severe Botryosphaeria (Bot) canker and dieback and anthracnose blight have been observed in California avocados in the past few years. These diseases, caused by a complex of species in the Botryosphaeriaceae and Glomerellaceae family members, can easily infect woody trees directly or through injuries. To determine the extent of Bot canker and anthracnose problems in avocado-producing counties in California, 18 experimental and commercial groves were surveyed and sampled in the fall of 2018. Isolations from symptomless and infected plant organs (branch, twig, leaf…) were performed on acidified potato dextrose agar. Although, Botryosphaeria and Colletotrichum spp. were recovered from all sampled tissues of the Hass and Lamb Hass cvs., they were most frequently isolated from symptomatic branches and leaves, respectively. The predominance of Bot species in infected branch and twig tissues confirmed their role in avocado branch canker formation, either acting solely or synergistically with Colletotrichum spp. However, the later pathogens were less common within the xylem elements, but along with Alternaria, they dominated the fungal population within the extra-cambial tissues. These pathogens were also detected in symptomless tissues revealing the presence of latent infections, while their respective fruiting structures (pycnidia and acervuli) were observed on infected twig and leaves. The highest incidence of Colletotrichum spp. was found in a high-density Hass orchard in San Luis Obispo, suggesting that the local conditions are likely conducive for anthracnose development.