Abstract
In this study, the presence and variability of Colletotrichum spp. was evaluated by comparing fungal isolates obtained from olive trees under long-time phytosanitary treatments with trees without any phytosanitary treatments (treated and untreated, respectively). Olive fruits of trees of the highly susceptible ‘Galega vulgar’ cultivar growing in the Alentejo region were used as samples. From the 210 olive trees sampled (half from treated and half from untreated orchards), 125 (59.5%) presented Colletotrichum spp., with a significant lower number of infected trees in treated (39) when compared to untreated orchards (86). The alignment and analysis of beta-tubulin (tub2), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), chitin synthase (CHS-1) and histone H3 (HIS-3) gene sequences allowed the identification of all 125 isolates as belonging to the C. acutatum complex. The vast majority of the isolates (124) were identified as C. nymphaeae and one isolate, from an untreated tree, was identified as C. godetiae. Isolates were divided into five different groups: Group A: 39 isolates from treated trees matched in 100% with C. nymphaeae sequences from the database; Group B: 76 isolates from untreated trees matched in 100% with C. nymphaeae sequences from the database; Group C: one isolate from untreated trees presenting a single nucleotidic difference in the HIS-3 sequence; Group D: eight isolates from untreated trees presenting differences in two nucleotides in the tub2 sequences that changed the protein structure, together with differences in two specific nucleotides of the GAPDH sequences; Group E: one isolate, from untreated olive trees, matched 100% with C. godetiae sequences from the database in all genes. Considering the similarities of the sampled areas, our results show that the long-time application of fungicides may have caused a reduction in the number of olive trees infected with Colletotrichum spp. but an increase in the number of fruits positive to Colletotrichum spp. within each tree, which may suggest different degrees of virulence of Colletotrichum isolates from trees growing different management regimes. It is imperative that the fungicides described as causing resistance are applied at appropriate times and intervals, since their efficiency decreases when applied incorrectly and new and more virulent species may arise.
Highlights
The olive (Olea europaea L.) is one of the most cultivated fruit crops in countries characterised by Mediterranean climates and its area is continuously expanding to several countries around the world [1], mainly due to the olive oil health benefits [2]
A total of 125 (59.5%) from the 210 sampled trees were positive to Colletotrichum spp
Despite the overwhelming majority of isolates being identified as C. nymphaeae, the present study demonstrated that the tub2, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ACT, Histone 3 (HIS-3) and CHS-1 sequences of the isolates showed a low heterogenicity
Summary
The olive (Olea europaea L.) is one of the most cultivated fruit crops in countries characterised by Mediterranean climates and its area is continuously expanding to several countries around the world [1], mainly due to the olive oil health benefits [2]. The construction of the Alqueva dam in the beginning of this century, the biggest water reservoir in Western Europe, has rapidly increased the area of olive orchards in Alentejo region, which is currently responsible for over 70% of the Portuguese olive oil production. Despite the recent increase of super-intensive regime, using mainly ‘Picual’ and ‘Arbequina’ two Spanish cultivars, the Portuguese ‘Galega vulgar’ is still the most cultivated variety, mostly due to the unique characteristics of its olive oil [3]. In Alentejo, the disease occurs epidemically and only two species within the C. acutatum complex -C. nymphaeae and C. godetiae- were detected and C. nymphaeae highly prevails (>95%) [6]. In olive, anthracnose typically affects the fruits near maturation [11], the pathogen can be present on flowers, leaves, shoots and branches [4,12,13,14]
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