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Olive "escudete", caused by Botryosphaeria dothidea, as result of the interaction fly-mosquito-fungus
Abstract
The Dalmatian disease or escudete, caused by Botryosphaeria dothidea, is the most important disease of table olive fruit. The typical escudete (small shield) symptom is a sunken, necrotic, and circular (≈8mm) lesion that appears in green fruits. The olive fly (Bactrocera oleae) and mosquito (Prolasioptera berlesiana) whose larvae prey on fly eggs, are disputed as vectors of the pathogen. Here, we study the interaction among these three agents: fly-mosquito-fungus. In the field, we periodically sampled for the presence of the mosquito in fruits, which were naturally punctured by the olive fly or artificially by a needle for three seasons. Under lab conditions, the presence of the fungus in the mosquito and the fly was studied by microscopy, isolation on media, and DNA sequencing. According to our results, the mosquito is attracted by ovipositor punctures on the olive surface made by the olive fly. When the mosquito deposits its own egg adjacent to the fly egg, it also inoculates the fungus in the puncture. This allows the mosquito larvae to feed on B. dothidea mycelium when the fly egg is absent; a need that arises due to frequent false punctures (no oviposition) made by the fly to avoid predation by the mosquito larvae. Microscopic studies show that the mosquito female has a special structure (mycangia) to carry B. dothidea spores in the last abdominal segments close to the ovipositor, which suggests a mutualistic association between B. dothidea and P. berlesiana.
The presence and infestation level of Bactrocera oleae was monitored in an organic olive orchard divided into differently treated parcels with kaolin (K), spinosad (S) and with kaolin and spinosad (K + S) in alternate rows. The treatments did not seem to affect olive fruit fly population dynamics, while statistically significant protective effects were recorded against total and harmful infestation, but not against the active one. Eventually, neither kaolin nor spinosad were shown to have a particular detrimental effect against naturally occurring B. oleae parasitoids.
Over two consecutive seasons, 16 olive orchards with trees exhibiting dieback symptoms of branches were surveyed in Southern Spain. The six most frequent fungal species recovered were morphologically, molecularly (by sequencing of the ITS, TUB, and LSU regions), and pathogenically characterized. The resistance of the 11 most important table cultivars to Neofusicoccum mediterraneum and Botryosphaeria dothidea, the causal agent of “escudete” (small shield) of fruit, was also studied by inoculation of branches and immature fruits, respectively. The species Cytospora pruinosa, N. mediterraneum, Nothophoma quercina, Phoma incompta, and Phomopsis sp. were identified. Only N. mediterraneum and P. incompta were able to induce the typical dieback symptoms and cankers that affected plants development. N. mediterraneum was the most virulent of the evaluated species although difference of virulence among their isolates were observed. The remaining fungal species were weakly pathogenic on potted plants. According to resistance tests, cv. Gordal Sevillana followed by the cvs. Manzanilla Cacereña and Manzanilla de Sevilla were the most susceptible to branch dieback caused by N. mediterraneum. Furthermore, the fruits of cv. Aloreña de Atarfe followed by those of cvs. Ocal and Manzanilla de Sevilla were the most susceptible to B. dothidea. Knowledge of the etiology and cultivar resistance of these diseases will help to establish better control measures.
Over two consecutive seasons, 16 olive orchards with trees exhibiting dieback symptoms on branches were surveyed in southern Spain. The six dominant fungal species recovered were characterized by means of phenotypic observations, DNA analysis (by sequencing of the internal transcribed spacer, b-tubulin, and large subunit nuclear ribosomal DNA regions), and pathogenicity tests. Additionally, three isolates collected from Tunisian olive trees showing similar dieback symptoms, one isolate of Colletotrichum godetiae, and a reference isolates of Neofusicoccum mediterraneum were included. The resistance of the 11 most important table cultivars to N. mediterraneum and Botryosphaeria dothidea, the causal agent of “escudete” (small shield) of fruit, was studied by the inoculation of branches and immature fruit, respectively. The species Cytospora pruinosa, N. mediterraneum, Nothophoma quercina, Comoclathris incompta, and Diaporthe sp. were identified. Only N. mediterraneum and C. incompta were able to induce the typical dieback symptoms and cankers that affected the development of the plants. The species N. mediterraneum was the most virulent among the evaluated species, although differences in virulence among its isolates were observed. The remaining fungal species were weakly pathogenic to nonpathogenic on plants. According to resistance tests, ‘Gordal Sevillana’ and ‘Manzanilla Cacereña’ were the most susceptible to branch dieback caused by N. mediterraneum. Furthermore, the fruit of ‘Aloreña de Atarfe’ and ‘Manzanilla de Sevilla’ were the most susceptible to B. dothidea. Knowledge of the etiology and cultivar resistance of these diseases will help to establish better control measures.
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