olive anthracnose - SIPaV

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Journal of Plant Pathology (2012), 94 (1), 29-44

Edizioni ETS Pisa, 2012

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INVITED REVIEW OLIVE ANTHRACNOSE S.O. Cacciola1, R. Faedda1, F. Sinatra2, G.E. Agosteo3, L. Schena3, S. Frisullo4 and G. Magnano di San Lio3 1 Dipartimento

di Gestione dei Sistemi Agroalimentari e Ambientali, Università degli Studi, Via S. Sofia 100, 95123 Catania, Italy 2 Dipartimento “G.F. Ingrassia”, Università degli Studi, Via S. Sofia 87, 95123, Catania, Italy 3 Dipartimento di Gestione dei Sistemi Agrari e Forestali, Università Mediterranea, Località Feo di Vito, 89122 Reggio Calabria, Italy 4 Dipartimento di Scienze Agro-ambientali, Chimica e Protezione delle Piante, Univeristà degli Studi, Via Napoli 25, 71100 Foggia, Italy

SUMMARY

INTRODUCTION

Olive anthracnose is the most important fungal disease of olive fruits worldwide. It occurs in humid olivegrowing areas of many countries and causes heavy yield losses and lowering of oil quality. Several species or genotypes of Colletotrichum have been indicated as responsible for olive anthracnose in different countries, including C. gloeosporioides, C. acutatum (both in a broad and narrow sense) and the molecular groups A4 and A6 of C. acutatum, C. fioriniae, and C. simmondsii. Recently, the molecular group A4 of C. acutatum has been described as a new species named C. clavatum. Conidia of the different species and genotypes of Colletotrichum causing olive anthracnose are the infective propagules; they are produced in acervuli on infected drupes and are dispersed by rain splash and via windblown rain droplets, causing secondary infection cycles. Mummified drupes on the tree have been regarded as the inoculum reservoir. The incidence and severity of anthracnose vary considerably depending on the environmental conditions, the susceptibility of the olive cultivar and the virulence of the pathogen’s population. Susceptibility of the drupes to anthracnose increases with maturity, although green fruit of susceptible cultivars may be severely affected in favorable environmental conditions. Control is based on integrated disease management that includes chemical treatments, selection of resistant cultivars and early harvesting.

Anthracnose may be regarded as the most damaging disease of olive fruits worldwide. It was first reported from Portugal by J.V. d’Almeida (1899) and named gaffa. In Italy, it is known as lebbra (leprosy) and in Spain as aceituna jabonosa (soapy fruit) or momificado (mummification) (Graniti et al., 1993; Martìn et al., 2002; Moral et al., 2008). In Spain, the term repilos is commonly used to indicate collectively anthracnose and two other olive diseases, peacock spot and cercosporiosis, which, like anthracnose, infect fruit and cause defoliation and canopy blight (Trapero Casas and Roca, 2004). The most typical symptoms of olive anthracnose (OA) is fruit rot and mummification (Fig. 1a, b). In moist conditions, infected fruits show a soft to dark brown rot with an abundant production of an orange gelatinous matrix (Fig. 1c) embedding conidia emerging from acervuli, while, in dry conditions, the fruits mummify and lose weight due to dehydration. Affected fruits fall prematurely to the ground and only a few mummies remain attached to the tree. Fruit drop may also be the consequence of infections in the peduncle (Oliveira et al., 2005). Mature drupes are mostly affected, but in favorable environmental conditions green fruits of susceptible cultivars may also be severely affected (Fig. 1c). In cultivars with oblong drupes, rot often starts from the apical end (Fig. 1c) but, in cultivars with large, subspherical drupes, the first symptoms appear as circular sunken lesions and acervuli form in concentric rings starting from the centre of the lesion (Fig. 1b). In ripening drupes, an internal brown rot of the pulp around the pit may be observed before the appearance of external symptoms on the outer surface (Agosteo et al., 2005). Trees affected by anthracnose show also chlorosis and necrosis of the leaves, defoliation, and dieback of twigs and branches (Graniti et al., 1993; Prota, 1995). In spring, infected leaves exhibit extensive yellowing of the leaf blade and, at this stage, the pathogen can be isolated from infected tissues with a high frequency (Cacciola et al., 1996). In late spring and early summer, infect-

Key words: Olea europaea, Colletotrichum clavatum, epidemiology, chemical control, IPM

Corresponding author: G. Magnano di San Lio Fax: +39.0965.312827 E-mail: [email protected]


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Fig. 1. a. Rot and mummification of green olive drupes affected by anthracnose. Note the apical rot on oblong drupes of cv. Ottobratica. b. Drupes of the olive cv. Itrana with circular sunken lesions. c. Symptoms of anthracnose caused by Colletotrichum clavatum on green olive drupes. Note the pinksalmon gelatinous matrix embedding conidia. d. Anthracnose infection on an olive leaf.

ed leaves fall to the ground and trees appear defoliated. Brown-necrotic spots with poorly defined margins, mostly starting from the margin of the leaf blade (Fig. 1d) and subsequently enlarging and coalescing, can be observed on the leaves fallen to the ground and, although less frequently, on those still attached to the tree. Acervuli of the pathogen often develop on necrotic leaf areas (Martelli, 1961; Bompeix et al., 1988; Cacciola et al., 1996; Sergeeva et al., 2008b). Symptoms on the canopy were tentatively ascribed to the toxic action of metabolites produced by the pathogen in infected drupes (Ballio et al., 1969). As a matter of fact, blight occurs only on twigs bearing a high proportion of infected drupes (Moral et al., 2009b). Defoliation of trees may be heavy and have detrimental effects on fruit production in the following year (Bompeix et al., 1988). This facies of the disease was described initially in Italy, Greece and Portugal (Saponaro, 1953; Martelli, 1959; Avezedo, 1976; Zachos and Makris, 1963), and later in Spain where it was noticed during the serious epidemic outbreak of 1997 (Moral et al., 2008; Oliveira et al., 2005; Trapero et al.,


1998; Trapero Casas, 2003). Anthracnose infections to olive flowers were first reported by Gorter (1956) in South Africa and recently by Sergeeva et al. (2008a) in Australia. Moral et al. (2009b) by artificial inoculations of young olive trees demonstrated that C. acutatum may cause blossom blight and observed that differences of susceptibility of olive cultivars to flower infections are not correlated with susceptibility to fruit infections. No cultivar was immune and, in very favorable environmental conditions, the disease caused blight of all inflorescences in very susceptible cultivars. The conclusion was that, in years with low flowering and a high incidence of anthracnose, the infection of flowers can affect fruit set. More interestingly, these experiments revealed that most of the drupes borne by plants inoculated at the flowering stage were infected, but infections remained latent until ripening (Moral et al., 2009b). On the basis of this observation, fungicide treatments before flowering have been suggested to protect the flowers from early infections (Sergeeva, 2011). OA is also a post-harvest disease with decay developing from quiescent infections that took place in the field. Its infections affect both fruit yield and commercial quality of the oil. In fact, oil produced from infected olives has off-flavor, a reddish color and shows chemical alterations such as high acidity, a considerable reduction of β-sitosterol, polyphenols and α-tocopherol (Mincione et al., 2004; Trapero and Blanco-López, 2008a, 2008b). Processing infected fruit soon after harvest helps reducing off-flavor changes in the oil. It has recently been observed that fruit infections can reduce significantly seed germination (Moral et al., 2009a). GEOGRAPHIC DISTRIBUTION AND ECONOMIC RELEVANCE

Anthracnose occurs in many olive-growing countries of the world, including Portugal, Spain, Greece, Italy, Montenegro, Japan, Uruguay, Argentina, Brazil, South Africa, California, China, India, Australia and New Zealand (Margarita et al., 1986; Bompeix et al., 1988; Mugnai, et al., 1993; Latinovic and Vucinic, 2002; Duarte et al., 2010; Sergeeva and Spooner-Hart, 2010). However, some records, such as the recent one from Tunisia (Rhouma et al., 2010), require confirmation as based on published evidences they may concern occasional rots of mature drupes caused by opportunistic Colletotrichum species. After the first report by de Almeida (1899) fom Portugal, severe epidemic outbreaks of OA were registered from the early 1920s in the Greek island of Corfu (Petri, 1930). Petri (1930) gave an account of the geographic distribution of the disease in the Mediterranean basin,


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maintaining that its incidence was high in Corfu and low in Spain, whereas it did not occur in France or Italy. In the same period, Biraghi (1934) reported that OA was present only in Greece, Portugal and, sporadically, in Spain. In Italy, the disease was recorded for the first time by Ciccarone (1950) who found it in some humid olivegrowing areas of Apulia, in the province of Lecce (southern Italy). In the 1950s, anthracnose spread epidemically in Apulia; 5,000 ha were estimated to be affected in 1953 and over 40,000 ha in 1960 (Saponaro, 1953; Martelli, 1960). In the same period, the disease was recorded from other Italian regions, namely Calabria and Sardinia (Martelli, 1959; Marras, 1962). An early report from Sicily (Graniti, 1954) has only recently been confirmed (Scarito et al., 2003). On its first appearance in Italy, in some olive-growing areas OA caused 80-100% yield loss. However, after an epidemic outbreak, it remained restricted to some humid areas of Calabria and Apulia, such as the Gioia Tauro plain (Calabria), an olive-growing area of over 32,000 ha, where the very susceptible cv. Ottobratica prevails (Graniti et al., 1993), and in the Salento peninsula. An occasional OA outbreak was registered in Umbria (central Italy), where the inoculum was likely introduced with infected young trees imported from Calabria (Agosteo et al., 2003). Severe epidemics of OA occur in wet regions of Portugal, where the susceptible cv. Galega is grown (Talhinhas et al., 2005). However, in favorable environmental conditions and high inoculum pressure, the disease can also severely affect the less susceptible cvs Arbequina and Picual (Talhinhas et al., 2005). During a 4-year survey (2004-2007) it was found that in Algarve (southern Portugal), the proportion of olive orchards affected by anthracnose ranged between 65 and 70%, with a peak of 100% in autumn 2006, following a period of prolonged rain and mild temperatures. The percentage of infected fruits was 22% in 2004 and 2005, 85% in 2006 and 36% in 2007 (Talhinhas et al., 2009). In Spain, it has been estimated that the overall losses in net income for the olive industry caused by anthracnose is over $ 93.4 million per year (Moral et al., 2009b). In this country, the disease is particularly severe in southern Córdoba, northern Málaga, Seville and western Granada provinces, where the susceptible cvs Hojiblanca and Picudo are grown (Moral and Trapero, 2009). During the period 1970-95 epidemics of little consequence occurred due to low rainfall, while in the warm and rainy autumn of 1997 a very severe epidemic outbreak was recorded in the Córdoba and Málaga provinces with up to 100% yield losses (Trapero Casas, 2003). In the Austral hemisphere, severe OA outbreaks have recently been observed in South Africa (G. Magnano di San Lio, unpublished information) and Australia in olive cvs Barnea, Manzanillo, Kalamata, UC 13A6,

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Azara and Sevillano (Spooner-Hart and Sergeeva, 2010; http://www.olivediseases.com). Yield losses of up to 70% have also been registered in olive cv. Arauco in Argentina, province of Mendoza (Docampo et al., 2010). THE CAUSAL AGENT

The fungus causing OA was originally described as a distinct species, Gloeosporium olivarum, by de Almeida (1899) in Portugal. Subsequently, G. olivarum was found to be indistinguishable from G. fructigenum Berk., the agent of bitter rot of apples. As a consequence of the revision of the genus Gloeosporium (von Arx, 1957), both species were transferred to Colletotrichum gloeosporioides (Penz.) Penz. et Sacc., the anamorph of Glomerella cingulata (Stonem.) Spaulding et v. Schrenk. C. gloeosporioides sensu von Arx or C. gloeosporioides sensu lato (s.l.) is a genetically heterogeneous complex of species, with a very wide host range, to which almost 600 species already described as Colletotrichum, Gloeosporium and other fungal genera, have been referred (Graniti et al., 1993; Sutton, 1992). More recently, two anamorphic species of Colletotrichum, C. gloeosporioides (Penz.) Penz. et Sacc. and C. acutatum J. H. Simmonds ex J. H. Simmonds, were reported to be associated with OA in Spain by Martín and GarcíaFigueres (1999) and Moral et al. (2008), according to whom C. acutatum prevails in areas where the disease occurs epidemically. C. acutatum was first reported as a distinct species on pawpaw (Carica papaya) in Queensland (Australia) by Simmonds (1965) who later validated the species with a broader concept (Simmonds, 1968). C. acutatum has been referred to as causal agent of anthracnose diseases on a large number of crops and non-cultivated plant species (Shi et al., 1996; Zulfiqar et al., 1996; Johnston and Jones, 1997; Freeman et al., 1998). Its teleomorph, Glomerella acutata J.C. Guerber et J.C. Correll, was first obtained in vitro by crossing different self-sterile monoconidial strains of C. acutatum (Guerber and Correll, 2001) and it was recently observed on naturally infected fruits of highbush blueberry in Norway (Talgø et al., 2007). G. acutata, however, has never been observed on olive, while G. cingulata, the teleomorph of C. gloeosporioides, has been found only on artificially inoculated olive trees (Cacciola et al., 1996). C. acutatum sensu lato (s.l.) was first introduced by Johnston and Jones (1997) to accommodate isolates that clustered with C. acutatum sensu Simmonds and others that showed a wide range of morphological and genetic diversity. Like C. gloeosporioides, C. acutatum s. l. is a species complex showing high phenotypic and genotypic diversity. It has been recovered from a wide range of hosts, often in close association with other Col-


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letotrichum species (Peres et al., 2005), and includes variants with distinct morphological and molecular characters (Sreenivasaprasad and Talhinhas, 2005; Hyde et al., 2009). Lardner et al. (1999) used RAPD analyses and morphological and cultural features to separate C. acutatum s.l. into seven distinct taxa, including five morphologically and genetically uniform groups, designated as A, B, C, D, E, and two species, Glomerella miyabeana (Fukushi) Arx and C. acutatum f. sp. pineum Dingley et J.W. Gilmour. Subsequently, the analysis of the internal transcribed spacer (ITS) regions of ribosomal DNA (rDNA) and a fragment of the β-tubulin-2 gene enabled the rearrangement of a global C. acutatum population into eight different molecular groups (A1 to A8), that


showed some degree of correlation with phenotypic characteristics, host association patterns and geographical distribution (Sreenivasaprasad and Talhinhas, 2005). According to Sreenivasaprasad and Talhinhas (2005), groups A2, A3, A4, A5 and A6 comprised isolates from olive. The same above mentioned genes have more recently been utilized to reassess three different genetic groups within C. acutatum s.l. and to describe two new species, C. fioriniae (Marcelino et S. Gouli) R.G. Shivas et Y.P. Tan comb. et stat. nov. and C. simmondsii R.G. Shivas et Y.P. Tan sp. nov. (Shivas and Tan, 2009). The new species corresponded to group C (also known as A3 molecular group) and group D (also known as A2 molecular group), respectively, whereas a third group was

Fig. 2. Dendrogram obtained by cluster analysis of RAPD profiles of Colletotrichum isolates of worldwide origin and from various hosts. Figures on branches are the bootstrap values as percentage of bootstrap replication from 1,000 replicate analysis. The dendrogram clearly differentiates all isolates of C. clavatum (formerly C. acutatum molecular group A4) from isolates of C. acutatum, C. simmondsii and C. fioriniae (formerly C. acutatum molecular groups A5, A2 and A3, respectively), as well as from isolates of C. gloeosporioides, C. musae and C. circinans used as outgroups (after Faedda et al., 2011).


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Fig. 3. UPGMA phylogenetic tree of Colletotrichum isolates of worldwide origin and from various hosts based on ITS15.8S-ITS2 of ribosomal DNA, showing the position of C. clavatum sp. nov. in relation to other known Colletotrichum species and genetic groups of C. acutatum. Bootstrap interiorbranch values (1,000 replicates) ≥50% are displayed between the nodes. The GenBank accession numbers of the reference sequences are in brackets preceded by the corresponding isolate codes. The scale bar indicates a 0.001 U of genetic distance from Kimura’s two-parameter model (after Faedda et al., 2011).

Fig. 4. UPGMA phylogram of Colletotrichum isolates of worldwide origin and from various hosts inferred from partial βtubulin-2 gene DNA sequences, showing the position of C. clavatum sp. nov. in relation to other known Colletotrichum species and genetic groups of C. acutatum. Bootstrap interiorbranch values (1,000 replicates) ≥50% are displayed between the nodes. The GenBank accession numbers of the reference sequences are in brackets preceded by the corresponding isolate codes. The scale bar indicates a 0.001 U of genetic distance from Kimura’s two-parameter model (after Faedda et al., 2011).

defined as C. acutatum sensu Simmonds or C. acutatum sensu stricto (s.s.) and corresponded to group A (also known as A5 molecular group). Very recently, C. acutatum group B or genetic group A4, as identified by Lardner et al. (1999) and Sreenivasaprasad and Talhinas (2005), respectively, was formally designated as a novel species and named C. clavatum G.E. Agosteo, R. Faedda et S.O. Cacciola (Faedda et al., 2011). Based on

RAPD genomic fingerprinting, ITS and β−tubulin DNA sequences, this species appeared to be clearly distinct from C. acutatum s.s., C. fioriniae and C. simmondsi, as well as from the genetic groups A1, A6, A7 and A8, all previously referred to collectively as C. acutatum s.l. (Figs 2, 3, 4). C. clavatum is the prevalent Colletotrichum species associated with epidemic outbreaks of OA in Greece,


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Fig. 5. Supposed route of the olive anthracnose epidemics caused by Colletotrichum clavatum in Italy. The years of first records of olive anthracnose epidemics in different regions are indicated. Epidemic outbreaks of olive anthracnose reported in Portugal in 1890s and in Spain in 1930s were probably caused by C. simmondsii while based on circumstantial evidences it can be supposed that C. clavatum is the prevalent causal agent of severe epidemics reported in Andalusia (southern Spain) since the late 1990s.

Italy, Montenegro, some restricted olive-growing areas of Portugal and perhaps Andalucia (southern Spain). By contrast, C. gloeosporioides was rarely isolated in areas where OA anthracnose is endemic and epidemic explosions of this disease are recurrent (Cacciola et al., 2001, 2007; Agosteo et al., 2002; Talhinhas et al., 2009; Sergeeva et al., 2010; Faedda et al., 2011). Talhinhas et al. (2009, 2010) identified both C. acutatum s.l. and C. gloeosporioides s.l. in Colletotrichum populations associated with OA in Portugal, but observed that C. acutatum was prevalent and more aggressive than C. gloeosporioides (only 3-4% incidence), whose infections strictly depended on favorable environmental conditions, high inoculum pressure and high host susceptibility. In agreement with previous observations, pathogenicity tests conducted in Spain by Martìn et al. (2002) using detached drupes, confirmed that C. gloeosporioides isolates are less virulent than those of C. acutatum. On the other hand, C. gloeosporioides was the sole species found in olive orchards in Sicily (southern Italy), a region where rot of olive drupes has been observed only occasionally and on mature fruits in late autumn (Scarito et al., 2003). These observations would suggest that C. gloeosporioides behaves as a weak oppor-

tunistic pathogen and very probably is not responsible for epidemic OA outbreaks. In a study of Colletotrichum populations associated with OA in diverse olive-growing regions of Portugal, five molecular groups of C. acutatum s.l. were identified (Talhinhas et al., 2005, 2010; Sreenivasaprasad and Talhinhas, 2005), but group A2 (C. simmondsii) included about 80% of the isolates, followed by group A4 (C. clavatum) with 12% of the isolates and group A5 (C. acutatum s.s.) with 3-4% of the isolates, while groups A3 (C. fioriniae) and C. acutatum group A6 occurred only sporadically. However, the distribution pattern and the proportions of molecular groups in different regions varied greatly (Talhinhas et al., 2010). In southern Italy, the population of Colletotrichum associated to OA is constituted prevalently by C. clavatum (Faedda et al., 2011). Two vegetative compatibility groups (VCGs) that correlated to both the geographic origin and RAPD patterns were identified within the Italian population of this Colletotrichum species using nit-mutants (Agosteo et al., 1997, 2002). One VCG comprised isolates from Apulia, while the other VCG comprised isolates from Calabria, Sardinia and Umbria as well as the reference isolate CBS 193.32 from Greece (Petri, 1930), deposited by Petri in the collection of the


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Centraalbureau voor Schimmelcultures, Baarn, The Netherlands. The presence of two distinct VCGs in Apulia and Calabria and the genetic similarity between the isolates from Greece and the Italian isolates suggest the hypothesis of separate introductions of C. clavatum into regions of southern Italy from the Balkans (“founder effect”), very probably during the 1930-1940s (Agosteo, 2010). Following the initial accidental introductions, the pathogen spread from Calabria to other Italian regions, namely Sardinia and more recently Umbria (Fig. 5). An alternative hypothesis is that the presence of distinct VCGs in C. clavatum populations from different regions indicates that a diversification process took place as a consequence of geographic isolation. C. simmondsii (formerly, group A2 of C. acutatum s.l.), which is the main causal agent of olive anthracnose in Portugal (Sreenivasaprasad and Talhinhas, 2005; Talhinhas et al., 2005, 2010), has not been found in olive in Italy although it is a common causal agent of strawberry anthrachnose in various Italian regions, including Apulia and Calabria. Likewise, C. acutatum s.s. (formerly, group A5), which has not been isolated from olive in Italy so far, is the prevalent species in olive-growing areas of the Austral hemisphere (Sergeeva et al., 2010; Faedda et al., 2011). In a recent study of olive isolates from New South Wales, South Australia, Queensland and Western Australia, most isolates were identified as C. acutatum s.s. and a few proved to be C. simmondsii. As to Spain, both direct and circumstantial evidence indicate that C. simmondsii and C. clavatum are the causal agents of OA in this country. From the data of Martín and García-Figueres (1999) and Sreenivasaprasad and Talhinhas (2005) it can be inferred that C. simmondsii is the prevalent species on olives affected by anthracnose in Catalonia, while it seems that C. clavatum is the dominant pathogen in Andalusia (Oliveira et al., 2005; Moral et al., 2008, 2009b; Moral and Trapero, 2009; Faedda et al., 2011). Inoculation of leaves of oleander and detached fruit of almond, apple, olive, orange, and strawberry showed that olive isolates from Andalusia were highly specialized (Moral et al., 2009b). Accordingly, with previous pathogenicity tests, it had been observed that C. clavatum isolates from olive did not infect oleander leaves (Cacciola et al., 1996). In a detailed study on the genetic structure of the Colletotrichum population associated with OA in Algarve (Portugal), Talhinhas et al. (2009) found a relatively high genetic diversity and identified four different molecular groups corresponding to as many species, i.e. C. simmondsii, C. fioriniae, C. clavatum and C. acutatum s s., but no single sub-population was dominant. This finding is in contrast with other reports suggesting little or no intra-regional heterogeneity of populations of the OA causal agent, such as in the case of C. simmondsii in other regions of Portugal, C. clavatum in southern Italy

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and C. acutatum s.s. in South-Africa and Australia. In conclusion, several species of Colletotrichum are associated with OA and, with few exceptions, a different species dominates in a particular olive-growing area, thus suggesting an adaptive potential to different hostgenetic background and environmental factors. Further investigations on the phylo-geography and host association patterns of the diverse Colletotrichum species on a global scale and cross-inoculation studies with isolates from olive and other host plants are needed to elucidate the biology and ecology of these species in natural conditions and to explain their geographic distribution in different olive-growing regions. LIFE CYCLE AND EPIDEMIOLOGY

The incidence and severity of OA vary considerably depending on the environmental conditions, the susceptibility of olive cultivars and the virulence of the pathogen population. It is commonly thought, in fact, that the pathogen populations causing epidemics in various olive-growing countries are particularly adapted both to the host and the environment. There is also a great deal of evidence indicating that serious losses occur only where virulent populations of the pathogen are present. The variability of the Mediterranean climate considerably affects disease incidence and severity. Infections of drupes are favored by mild temperature and high rainfall in autumn, while severe infections of leaves occur during wet springs. In favorable conditions such as rains, mild temperature, high inoculum pressure and severe attacks of the Mediterranean olive fly (Bactrocera oleae Gmelin) during fruit ripening, the disease may cause up to 100% loss of fruit production on susceptible cultivars (Graniti et al., 1993; Moral et al., 2008). In the Mediterranean region, anthracnose disease is responsible for considerable losses in certain humid olive-growing areas of Portugal, Greece, Andalusia and Catalonia in Spain and Calabria, Apulia and Sardinia in southern Italy, where it can be regarded as the most damaging disease of olive. Accurate identification of the causal agent and the knowledge of its life-style are needed in order to develop effective disease control measures. Although the disease cycle of OA has been studied since the first reports of its epidemic outbreaks in the Mediterranean, some aspects are still controversial. Under laboratory conditions, germination of conidia of Colletotrichum species formerly referred to collectively as C. acutatum s.l. takes place between 5 and 30°C, while the optimum growth temperature is comprised between 20 and 27°C. Infections of drupes occur between 10 and 30°C and require over 93% relative humidity. In the same conditions, acervuli are formed on the surface of the drupes after six,


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seven or nine days at incubation temperatures of 20, 25 and 15°C, respectively (Graniti et al., 1993). These laboratory results are indicative of what happens in olive groves, where infections occur when the season is mild and humid, have a longer incubation period during the cold months and are inhibited by dry and hot weather. Conidia (Fig. 6a, 6b, 7a) are the infective propagules while, conceivably, ascospores do not play a significant role in disease epidemiology, for the sexual stage of C. acutatum has not been observed on olive so far. Conidia are produced in acervuli in a hydro-soluble mucilaginous matrix (Fig. 6b) and are dispersed by rain splash and via windblown rain droplets. The pathogen sporulates more profusely on drupes of susceptible olive cultivars. On infeceted drupes optimum temperature for conidial production is around 20°C (Moral et al., 2010). Free water is needed for germination of conidia on the host surface. Conidia form germ tubes and melanized appressoria (Fig. 7b) from which infection pegs develop


to penetrate the cuticle. Appressoria form only on solid surfaces such as microscope slides or host cuticle. The pathogen, which is able to produce extracellular cutinase, can enter the drupes directly through their epicarp, but the severity of symptom expression and the rate of colonization increase if drupes are wounded (Moral et al., 2008). It has been observed that the susceptibility of olive cultivars in the field is enhanced if fruits are wounded by Mediterranean olive fly. A correlation between the incidence and severity of anthracnose infections and attacks of this insect was observed in several European olive-growing areas (Graniti et al., 1993; Trapero Casas, 2003; Kramer-Haimovich et al., 2006). A similar correlation was also reported between OA and attacks of Queensland fruit fly [B. tryoni (Froggatt)] in Australia (Sergeeva and Spooner-Hart, 2010; http://www.olivediseases.com). Oviposition lesions and exit holes of olive fly may represent a point of entry of infections, and the feeding activity of maggots in the

Fig. 6. a. Scanning electron micrographs of conidia of Colletotrichum gloeosporioides. Conidia of this Colletotrichum species are cylindrical with both ends rounded. b. Conidia of C. gloeosporioides embedded in a gelatinous matrix with a fibrillar structure.

Fig. 7. a. Fusiform conidia of Colletotrichum simmondsii with cuspidate ends. b. Appressoria of C. clavatum produced on slide cultures. Note mononucleate conidia and both mono- and binucleate hyphal elements (Giemsa stain). Scale bars = 10 µm.


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flesh favors the colonization of the drupe by the fungus, also inducing an early ripening of the drupes. This insect may also contribute as a vector to the dissemination of conidia, although in Calabria (southern Italy) the percentage of flies contaminated by the form of C. acutatum recently identified as C. clavatum was shown to be quite low (