In November 2019, a severe outbreak of fruit rot was observed in olive orchards in Crete, southern Greece. Symptoms appeared primarily on fruits and stalks, resembling those caused by anthracnose. Typical symptoms were fruit rot, shrinkage and mummification, associated commonly with stalk discoloration and fruit drop. Disease incidence was estimated up to 100% in some cases and an unprecedented increase in olive oil acidity reaching up to 8% (percentage of oleic acid) in severely affected olive groves was recorded. Thirty-two olive groves were then surveyed, and samples of fruit, stalk, leaf and shoot were collected. Visual, stereoscopic and microscopic observations revealed several fungi belonging to the genera Alternaria, Botryosphaeria, Capnodium, Colletotrichum, Fusarium and Pseudocercospora. Fungal infection in fruits was commonly associated with concomitant infestation by the olive fruit fly Bactrocera oleae along with increased air temperature and relative humidity conditions that prevailed in October-November 2019. Twenty representative fungal strains isolated from symptomatic fruits and stalks were characterized by morphological, physiological and molecular analyses. By rDNA-ITS region and EF1-α gene sequencing analysis, these isolates were identified as Alternaria spp., A. infectoria, Botryosphaeria dothidea, Colletotrichum boninense sensu lato, Fusarium lateritium, F. solani species complex and Stemphylium amaranthi. Pathogenicity tests on punctured fruits revealed that all isolates were pathogenic; however F. solani isolates along with B. dothidea were the most virulent and wounds were necessary for efficient fungal infection. Moreover, as little as 10 spores of F. solani were sufficient of causing significant infection in punctured fruits. Fusarium solani was also capable of infecting olive fruits in the presence of B. oleae, with no additional wounding, in artificial inoculation experiments. Moreover, it was capable of colonizing and affecting olive blossoms. Further analyses in olive oil extracted from fruits artificially inoculated with F. solani indicated a significant increase in oil acidity, K232, K270 and peroxide value, whereas total phenols content was significantly decreased. To the best of our knowledge, this is the first report of Fusarium solani associated with olive fruit rot and olive oil degradation worldwide.
Survey of Florida Olive Groves during Olive Fruit Development: Monitoring for Stink Bugs and Olive Fruit Flies
