18 Oct Botrytis

Botrytis

Background

In the asexual state, Botrytis produces different tissues including mycelia, macroconidia, microconidia, and sclerotia. Macroconidia are ellipsoidal to obovoid shape and rise from conidiophore branches into botryose clusters. They are pale brown and range in size from 9–23×8–15 μm. Microconidia are more spherical and much smaller than macroconidia (about 1 μm), and function as male spermatia (Groves and Loveland 1953; Faretra et al. 1988; Beever and Parkes 1993; Fukumori et al. 2004). Sclerotia are irregularly hemispherical, convex and normally have a concave surface. They are usually black, with sizes ranging between 1 and 10 mm (Whetzel 1945), and function as survival structures during winter and serve as a maternal parent in the production of apothecia.

The sexual state forms fruiting bodies called apothecia: a cup- or open saucer-shaped ascoma at the top of a stalk, that acts as a platform to discharge ascospores from the ascus. Botrytis apothecia vary in size depending on the species, between 1 and 25 mm high and 1–6 mm diam. (Hennebert and Groves 1963; Bergquist and Lorbeer 1972). Apothecia are brown and become darker when mature (Hennebert and Groves 1963; Bergquist and Lorbeer 1972; Faretra and Antonacci 1987). Generally, multiple apothecia can develop on a single sclerotium. Mature apothecia normally can be observed 2 months after fertilization (Faretra et al. 1988; Hennebert and Groves 1963; Van Der Vlugt-Bergmans et al. 1993). In the genus Botrytis, both homothallic and heterothallic reproductive lifestyles have been reported. Homothallic (self- fertile) species can undergo sexual reproduction and form apothecia and generate progeny in the absence of a mating partner, e.g. B. porri and B. globosa (Buchwald 1953; Elliott 1964). By contrast, heterothallic (self-sterile, self-incompatible) species require isolates with compatible mating types in order to complete the sexual cycle. B. cinerea is considered a typical heterothallic fungus (Elliott 1964; Faretra et al. 1988). Mating is controlled by the mating-type locus with two alleles, MAT1-1, and MAT1-2 (Faretra et al. 1988), each carrying two distinct, non-homologous genes (Amselem et al. 2011).

Species identification and numbers

Approximately half of the Botrytis species are named after the host that they are derived from (listed in Table). One hybrid species, B. allii which originated from hybridization between B. byssoidea and B. aclada (Nielsen and Yohalem 2001; Yohalem et al. 2003) could not be placed in the phylogeny (Staats et al. 2005) and was omitted from Table 3. The genus Botrytis predominantly comprises narrow host range pathogens that infect a single, or a few (often related) host species. There are two exceptions to this rule: B. cinerea can infect more than 200 host species (Jarvis 1977 ), and B. pseudocinerea has been isolated from several unrelated host species (Fournier et al. 2005; Leroch et al. 2013).

The taxonomic classification and nomenclature in Botrytis have rarely been comprehensively reviewed. Morphological descriptions of most species have been published in the 19th and the first half of the 20th century in separate papers, many of which are not easily accessible. The most recent taxonomic compilation of the genus is in a monograph by Jarvis (1977), which also lists ~25 excluded or doubtful species, and briefly describes the historical debates between mycologists and the confusion in the classification of Botrytis species. Morphological features were often inadequate to distinguish species and the variability among isolates of the same species further complicated the situation (Jarvis 1977). Recent studies have identified B. cinerea and B. pseudocinerea as species that are very similar in morphology, yet recognized as distinct taxa that diverged several million years ago (Walker et al. 2011). Even more puzzling, the morphology and narrow host range of B. fabae separate this species clearly from B. cinerea and B. pseudocinerea, but phylogenetic studies revealed it to be a sister species of B. cinerea (see below). These examples illustrate the limitations of morphological characters for Botrytis species identification.

Table  Botrytis. Details of the isolates used in the phylogenetic tree

Ex-type (ex-epitype) strains are bolded and marked with an * and voucher strains are bolded

^a sequences obtained from a different isolate than the one listed in the table.

Molecular phylogeny

Holst-Jensen et al. (1998) were the first to use nuclear ribosomal ITS sequences to infer a phylogeny of the family Sclerotiniaceae, including several members of the genus Botrytis. The relationships among many Botrytis species could not be resolved because of the limited number of informative characters, however, the study permitted the conclusion that Botryotinia asexual morphs along with Botrytis sexual morphs constitute a monophyletic lineage (Holst-Jensen et al. 1998). The phylogeny of the Sclerotiniaceae was further refined by Andrew et al. (2012) using three protein-coding genes: calmodulin, glyceraldehyde 3-phosphate dehydrogenase G3PDH, and heat shock protein HSP60.

Staats et al. (2005) performed a comprehensive phylogenetic analysis of the genus Botrytis, at that time comprising 22 recognized species and one hybrid. Using three protein-coding genes (G3PDH, HSP60, and the DNA-dependent RNA polymerase subunit II gene RPB2), they corroborated the morphological and host plant-based classification of Botrytis spp. and divided the genus into two (rather widely separated) clades. Clade I contained species that only infect eudicot plants, while Clade II contained species that can infect either eudicotyledonous or monocotyledonous plants. The use of the same three genes facilitated the discovery of Botrytis sinoallii, a new species infecting Allium spp., and its distinction from other Botrytis spp. infecting the same hosts (Zhang et al. 2010b); B. fabiopsis, a new species infecting broad bean, very distant from B. fabae (Zhang et al. 2010a); and B. caroliniana, a new species infecting blackberry (Li et al. 2012).