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Stud Mycol. 2006; 55: 163–173.
PMCID: PMC2104728

Re-evaluating the taxonomic status of Phaeoisariopsis griseola, the causal agent of angular leaf spot of bean

Abstract

Angular leaf spot of Phaseolus vulgaris is a serious disease caused by Phaeoisariopsis griseola, in which two major gene pools occur, namely Andean and Middle-American. Sequence analysis of the SSU region of nrDNA revealed the genus Phaeoisariopsis to be indistinguishable from other hyphomycete anamorph genera associated with Mycosphaerella, namely Pseudocercospora and Stigmina. A new combination is therefore proposed in the genus Pseudocercospora, a name to be conserved over Phaeoisariopsis and Stigmina. Further comparisons by means of morphology, cultural characteristics, and DNA sequence analysis of the ITS, calmodulin, and actin gene regions delineated two groups within P. griseola, which are recognised as two formae, namely f. griseola and f. mesoamericana.

Keywords: Ascomycetes, DNA sequence comparisons, Mycosphaerella, Phaeoisariopsis, Phaseolus vulgaris, Pseudocercospora, systematics

INTRODUCTION

Angular leaf spot (ALS) of beans (Phaseolus vulgaris) is caused by Phaeoisariopsis griseola (Sacc.) Ferraris. The disease is of major importance in tropical and subtropical areas, causing yield losses of up to 80 % (Schwartz et al. 1981, Saettler 1991, Liebenberg & Pretorius 1997). The disease affects pods and foliage, and is particularly destructive in warm, humid areas (Saettler 1991). Pod symptoms consist of circular to elliptical red-brown lesions, while leaf lesions start as small, brown or grey spots that become angular and necrotic, being confined by leaf veins. Leaf spots eventually coalesce, causing premature defoliation (Correa-Victoria et al. 1989, Saettler 1991). Furthermore, the disease also affects the quality and marketability of seed across bean-producing areas of the world (Pastor-Corrales et al. 1998).

In the Great Lakes Region of Africa, losses attributed to ALS have been estimated to be around 374 800 t (Wortmann et al. 1998). Disease control is best achieved via the selection of resistant varieties. Breeding for resistance against ALS is complicated, as the pathogen is highly variable with regard to pathogenicity, which means that durable resistance is difficult to achieve (Pastor-Corrales et al. 1998). High levels of pathogenic and genetic variation have been reported in P. griseola by various authors (Guzmán et al. 1995, Boshoff et al. 1996, Busogoro et al. 1999, Mahuku et al. 2002, Wagara et al. 2004).

There are indications of at least two main, morphologically distinguishable domestication events for the common bean, which in turn gave rise to two main gene pools, namely large-seeded beans of Andean origin, and small to medium-sized beans of Middle-American origin (Brown et al. 1982, Gepts & Bliss 1985, 1986, Gepts et al. 1986, Koenig & Gepts 1989, Sprecher & Isleib 1989, Koenig et al. 1990, Singh et al. 1991a, b, Miklas & Kelly 1992, Skroch et al. 1992, Chacón et al. 2005).

Several fungal pathogens of P. vulgaris, in particular Phaeoisariopsis griseola, causal organism of ALS, Colletotrichum lindemuthianum (Sacc. & Magnus) Briosi & Cavara, the causal organism of anthracnose, and Uromyces appendiculatus (Pers.: Pers.) Unger var. appendiculatus, the causal organism of bean rust, have undergone parallel micro-evolution with the host. Although there is considerable variation within gene pools, differences are particularly evident when the reactions of isolates to differential lines of known Andean and Middle-American origin are compared. Isolates originating from the Andes are virulent only on large-seeded lines, whereas those originating from countries such as Central America, Mexico, Bolivia and Brazil are generally virulent on lines from both groups (Steadman 1995, Liebenberg 1996, Pastor-Corrales 1996, Chacón et al. 1997, Araya & Steadman 1998, Sandlin et al. 1999, Araya et al. 2004). Using isozyme analysis, Correa-Victoria (1987) could distinguish two groups in 55 P. griseola isolates from Africa, the U.S.A. and Latin America. All 26 isolates from Africa clustered in one group, whereas Latin American isolates clustered in both groups. However, recently the presence of both groups was reported from Africa (Liebenberg 1996, Wagara et al. 2004), which was also supported by data derived from isozyme analysis (Boshoff et al. 1996). Guzmán et al. (1995) used RAPD analysis to divide 62 P. griseola isolates from Brazil, Wisconsin (U.S.A.) and Malawi into two broad groups. Isolates in the Andean group, collected predominantly from Andean bean host genotypes, were more pathogenic on Andean genotypes, whereas those from the second group, originating predominantly from Middle-American bean genotypes, were more pathogenic on Middle-American bean genotypes. The 11 Brazilian isolates fell in the second group, whereas 39 of the 42 Malawian isolates belonged to the Andean group. This grouping reflects the preference for small-seeded beans in Brazil, and large-seeded beans in Malawi. A third, more virulent group reported in Africa (CIAT 1996, Liebenberg 1996) appears to be a variation of the Andean group (Mahuku et al. 2002).

Buruchara (1983) observed differences in conidial size and amount of septation between isolates. However, he concluded that, due to the extent of variation within groups, these characteristics could not be used for grouping isolates. Several authors have attempted to associate lesion size with pathogenicity differences. Verma & Sharma (1984) observed two types of lesions in the field that differed in size, but found no significant differences in the number and size of lesions caused by the two groups of isolates, or in their radial growth in culture. Lesion size can vary considerably, but Correa-Victoria (1987) found no significant correlation between disease severity and lesion size, and no correlation between spore production and lesion size, but reported it to be highly dependent on the host cultivar (Correa-Victoria 1987). Lesion size may be affected by the interaction between host gene pool and pathogen origin (Liebenberg et al. 1996). These phenomena gave rise to questions as to the extent of differences between the Andean and Middle-American groups.

Ferraris (1909) erected the genus Phaeoisariopsis Ferraris for four Isariopsis-like species, including Isariopsis griseola Sacc. (Saccardo 1878), the type species, characterised by having synnematous conidiophore fascicles and pigmented conidiophores and conidia. In subsequent years several diverse elements were included in the genus (Ellis 1971, 1976, von Arx 1983). Chupp (1954) described a bean pathogen in his monograph under Cercospora columnaris Ellis & Everh., but cited the older name Phaeoisariopsis griseola as synonym. In his notes he stressed to favour the retention of Phaeoisariopsis. Deighton (1990) reassessed the genus, and considered the synnematous arrangement of conidiophores to be unsuitable as sole character for generic differentiation. Subsequently he confined Phaeoisariopsis to a few species similar to P. griseola, having non-geniculate conidiogenous cells with flattened, but conspicuous scars. Deighton placed species with conspicuously geniculate conidiogenous cells and thickened, darkened scars in Passalora Fr., whereas taxa with quite inconspicuous conidiogenous loci were reallocated to Pseudocercospora Speg. Von Arx (1983) and Braun (1992, 1995a, b) preferred to maintain Phaeoisariopsis, based on synnematous conidiomata, but confined it to species with conspicuous (slightly thickened, not darkened) conidiogenous loci.

The primary aim of the present study was to resolve the generic status of Phaeoisariopsis within Mycosphaerella Johanson, for which a subset of isolates were subjected to DNA sequence analysis of the SSU region. A further aim was to compare isolates of the Andean and Middle-American groups to address the question if they represent two groups or species. For this purpose isolates were compared by means of morphology, cultural characteristics, and DNA sequence analysis of their internal transcribed spacer region (ITS-1, ITS-2 and 5.8S), calmodulin, and actin regions.

MATERIALS AND METHODS

Isolates

Phaseolus leaves exhibiting ALS symptoms, collected in Africa and South America, were studied (Table 1). Single-conidial cultures were established on 2 % malt extract agar (MEA) (Biolab, Midrand, South Africa) as outlined by Crous (1998). Colonies were subcultured onto 2 % potato-dextrose agar (PDA; Gams et al. 1998) and incubated at 25 °C under continuous near-ultraviolet light to promote sporulation.

Table 1.
Isolates used for sequence analysis.

DNA phylogeny

Genomic DNA was isolated from fungal mycelium grown on MEA in Petri dishes and the ITS, actin (ACT) and calmodulin (CAL) regions were amplified and sequenced using the protocols and primers as described by Crous et al. (2004). The 5' end of the 18S rRNA gene (SSU) was amplified and sequenced as described by Braun et al. (2003).

The nucleotide sequences generated in this study were added to other sequences obtained from GenBank (http://www.ncbi.nlm.nih.gov) and the alignment was assembled using Sequence Alignment Editor v. 2.0a11 (Rambaut 2002) with manual improvement of the alignment where necessary. Sequence data were analysed as explained in Braun et al. (2003) using PAUP (Phylogenetic Analysis Using Parsimony) v. 4.0b10 (Swofford 2002) with both neighbour-joining and parsimony algorithms. Neighbour-joining analyses were conducted with the uncorrected (“p”), the Kimura 2-parameter and the HKY85 substitution models in PAUP. When they were encountered, ties were broken randomly. For parsimony analysis, alignment gaps were treated as new character states and all characters were unordered and of equal weight. Heuristic searches were performed with 10 random taxon additions. A partition homogeneity test (Farris et al. 1994) was conducted in PAUP to consider the feasibility of combining the ITS, actin and calmodulin data sets Sequence data were deposited in GenBank (Table 1) and the alignments in TreeBASE (S1507, M2709-10).

Determination of virulence phenotypes

The monoconidial isolates studied (Table 1) have previously been subjected to virulence phenotype characterisation on ALS differential lines from both the large- and small-seeded gene pools, as published previously (Liebenberg 1996, Mahuku et al. 2002).

Morphology and cultural characteristics

Wherever possible, thirty measurements (× 1000 magnification) were made of structures mounted in lactic acid, and the extremes of spore measurements given in parentheses. Colony colours (surface and reverse) were assessed after 14 d on PDA at 25 °C in the dark, using the colour charts of Rayner (1970). Cardinal temperatures for growth (from 9–33 °C, in 3° intervals) were determined on PDA plates as explained in Crous (1998). All cultures obtained in this study are maintained in the culture collection of the Centraalbureau voor Schimmelcultures (CBS) in Utrecht, the Netherlands (Table 1).

RESULTS

DNA phylogeny

The manually adjusted SSU sequence alignment contains 29 isolates (including the two outgroups) and 1029 characters including alignment gaps; of these characters 38 are parsimony-informative, 57 are variable and parsimony-uninformative, and 934 are constant. Neighbour-joining analysis using the three substitution models on the sequence data yielded trees with identical topologies (data not shown). The same overall topology was also obtained with the parsimony analysis, which yielded 13 most parsimonious trees (TL = 135 steps; CI = 0.807; RI = 0.809; RC = 0.653), one of which is shown in Fig. 1. In this tree, species of Pseudocercospora and Stigmina form a well-defined clade (bootstrap support value of 83 %) within Mycosphaerella.

Fig. 1.
One of 15 most parsimonious trees obtained from a heuristic search with 10 random taxon additions of the 18S rRNA gene sequence alignment. The scale bar shows a single change and bootstrap support values from 1000 replicates are shown at the nodes. ...

The ITS region was sequenced to provide better resolution of the order of the species within the Pseudocercospora clade. The manually adjusted ITS sequence alignment contains 45 isolates (including the two outgroups) and 499 characters including alignment gaps; of these characters 168 are parsimony-informative, 25 are variable and parsimony-uninformative, and 306 are constant. Neighbour-joining analysis using the three substitution models on the sequence data yielded trees with identical topologies (data not shown). Only the order and grouping of the deeper nodes differed between the neighbour-joining and parsimony analyses (data not shown). Parsimony analysis yielded 13 most parsimonious trees (TL = 293 steps; CI = 0.816; RI = 0.918; RC = 0.749), one of which is shown in Fig. 2. In this tree, isolates of Ps. griseola are grouped together with a bootstrap support value of 100 %, with the Middle-American isolates (Ps. griseola f. mesoamericana) grouping together with a bootstrap support value of 84 %. Also in the tree are other Pseudocercospora species (89 % bootstrap support), two strains of Ps. vitis (type species of Pseudocercospora, 95 % bootstrap support) and a basal well-defined clade (bootstrap support value of 100 %) of two GenBank sequences of Stigmina platani.

Fig. 2.
One of 13 most parsimonious trees obtained from a heuristic search with 10 random taxon additions of the ITS sequence alignment. The scale bar shows 10 changes and bootstrap support values from 1000 replicates are shown at the nodes. Thickened lines ...

To determine whether Ps. griseola isolates from Middle-American and Andean origin can be distinguished phylogenetically, the ACT (235 characters) and CAL (316 characters) sequences were combined with the ITS sequences. The partition homogeneity test showed that the three loci were combinable into a single analysis (P = 0.6550). The manually adjusted combined alignment consists of 1050 bases (including alignment gaps) and 30 isolates (including the two outgroups). Of the 1050 characters, 288 are parsimony-informative, 42 were variable and parsimony-uninformative, and 720 were constant. The topologies of the trees obtained from the neighbour-joining analyses were identical to each other and also to that obtained from the parsimony analysis (data not shown). Parsimony analysis of the combined data yielded three most parsimonious trees (TL = 353 steps; CI = 0.994; RI = 0.994; RC = 0.988), one of which is shown in Fig. 3. The tree shows two distinct clades, namely Ps. griseola f. griseola and what we call here the Ps. griseola f. mesoamericana clade. Bootstrapping using parsimony results in support values of 53 % and 71 % for each clade, respectively. These values increase to 62 % and 98 %, respectively, if neighbour-joining with the HKY85 substitution model is used for bootstrapping. The Ps. griseola f. griseola clade is further split into two groups (62 / 95 % and 52 / 71 % bootstrap support, respectively), which is the result of three characters that changed in the CAL sequence of isolates CPC 12238 and CPC 12239 (99.04 % sequence similarity to the other Ps. griseola f. griseola isolates).

Fig. 3.
One of three most parsimonious trees obtained from a heuristic search with 10 random taxon additions of a combined ITS, actin and calmodulin sequence alignment. The scale bar shows 10 changes and bootstrap support values from 1000 replicates are shown ...

Taxonomy

Pseudocercospora griseola (Sacc.) Crous & U. Braun, comb. nov. MycoBank MB500855. Fig. 4.

Fig. 4.
Pseudocercospora griseola. A–C. Leaf disease symptoms. D. Lesions on bean pod. E–G. Fasciculate conidiophores. H–J. Conidiogenous cells giving rise to conidia. K–O. Conidia. Scale bars: F = 8 μm, G = 200 μm, ...

Basionum: Isariopsis griseola Sacc., Michelia 1: 273. 1878.

  • [equivalent] Phaeoisariopsis griseola (Sacc.) Ferraris, Ann. Mycol. 7: 273. 1909.
  • [equivalent] Lindaumyces griseolus (Sacc.) Gonz. Frag. (as “g riseola”), Mem. R. Acad. Ci. Exact. Madrid, Ser. 2, 6: 339. 1927.
  • [equivalent] Cercospora griseola (Sacc.) Ragunath. & K. Ramakr., J. Madras Univ. 35–36: 11. (1965–1966) 1968.
  • = Cylindrosporium phaseoli (Cylindrospora) Rabenh., Klotzschii Herbarium vivum mycologicum, Editio nova, Series Prima, Centuria 4, No. 327, Dresden 1856, nom. nud., also Bot. Zeitung 15(6): 94. 1857, nom. nud. and Flora 15(9): 134. 1857, nom. nud.
  • = Graphium laxum Ellis, Bull. Torrey Bot. Club 8: 64. 1881.
  • [equivalent] Isariopsis laxa (Ellis) Sacc., Syll. Fung. 4: 631. 1886.
  • [equivalent] Phaeoisariopsis laxa (Ellis) S.C. Jong & E.F. Morris, Mycopathol. Mycol. Appl. 34: 269. 1968.
  • = Cercospora solimanii Speg. (solimani), Anales Soc. Ci. Argent. 16: 167. 1883.
  • = Cercospora columnaris Ellis & Everh. (as “columnare”), Proc. Acad. Nat. Sci. Philadelphia 46: 380. 1894.
  • [equivalent] Pseudocercospora columnaris (Ellis & Everh.) J.M. Yen, in Yen & Lim, Gard. Bull., Singapore 33: 172. 1980.
  • = Arthrobotryum puttemansii Henn., Hedwigia 41: 309. 1902.
  • = Cercospora stuhlmannii Henn., Bot. Jahrb. Syst. 33: 40. 1904.

Syntypes: on Phaseolus vulgaris, Italy, Selva, Aug. 1877, Saccardo, Mycotheca Veneta 1247 (e.g., B, HAL, PAD).

Formae novae: Pseudocercospora griseola (Sacc.) Crous & U. Braun, f. griseola

Specimen examined: Tanzania, on Phaseolus vulgaris, F.S. Ngulu & C. Mushi, CBS H-19683, epitype designated here, CBS 119906 = CPC 10468. culture ex-epitype. The epithet “griseola” was based on European material, and from our analysis, it appears that European material is representative of P. griseola f. griseola.

Pseudocercospora griseola (Sacc.) Crous & U. Braun, f. mesoamericana Crous & U. Braun f. nov. MycoBank MB500856.

Differt a f. griseola variatione virulentiae majore, culturis crescentibus ad ≥ 30 °C.

Morphologically similar to P. griseola f. griseola, but distinct by having a broader range of virulence on different bean types, and being able to grow at or above 30 °C, which is not the case for f. griseola.

Specimen examined: South Africa, on Phaseolus vulgaris, M.M. Liebenberg, CBS H-19684, holotype, culture ex-type CBS 119113 = CPC 10463.

Descriptions (selection): Gonzáles Fragoso (1927: 339), Chupp (1954: 295, as Cercospora columnaris), Ellis (1971: 269), Shin & Kim (2001: 151–153).

Illustrations (selection): Saccardo, Fungi italici, Pl. 838, Padova 1881; Briosi & Cavara, Funghi parassiti delle piante coltivate od utili, Fasc. I, No. 17, figs 1–2, Pavia 1888; Gonzáles Fragoso (1927: 340, fig. 79); Ellis (1971: 269, fig. 183); Deighton (1990: 1098, figs 2–3); Shin & Kim (2001: 153, fig. 65).

Description in vivo: On leaves, petioles, stems and pods; leaf spots amphigenous, angular–irregular, rarely subcircular–elliptical, mostly vein-limited, 1–8 mm wide, finally sometimes confluent, forming larger patches, brown, ranging from pale olivaceous, olivaceous-brown, yellowish brown, greyish brown to dark brown, on pods often reddish brown and more regular, subcircular–elliptical, margin indefinite, only delimited by veins, or surrounded by a narrow, dark brown border or marginal line. Caespituli on petioles, pods, stems and leaves, amphigenous, mostly hypophyllous, usually scattered, occasionally aggregated, conspicuous, punctiform, dark brown to blackish grey. Mycelium internal. Stromata almost lacking to well-developed, subglobose, depressed to lacrimoid, up to 70 μm diam, brown. Conidiophores numerous, up to approx. 40, in dense fascicles, often forming synnematous conidiomata, erumpent, 100–500 × 20–70 μm, rarely longer, olivaceous-brown, composed of a more or less firm stipe of closely appressed conidiophores and a terminal, loose capitulum, i.e. conidiophores splaying out at the end of the conidiomata, free ends usually up to 100 μm long, individual conidiophores filiform, appressed threads 2–5 μm wide, up to 7 μm wide towards the apex, pluriseptate, subhyaline to olivaceous-brown, thin-walled, occasionally becoming rough-walled with age. Conidiogenous cells integrated, terminal, 20–100 μm long, subcylindrical to subclavate, usually not or only barely geniculate, but moderately geniculate in some collections; conidiogenous loci terminal and lateral, quite inconspicuous to subconspicuous, i.e. unthickened or almost so, but slightly darkened-refractive, in surface view visible as minute circles, 1.5–2.5 μm diam, usually flat, non-protruding. Conidia solitary, obclavate-cylindrical, broadly subfusiform, short conidia sometimes ellipsoid-ovoid to short cylindrical, straight to curved, 20–75(–85) × 4–9 μm, (0–)1–5(–6)-septate, usually not constricted at the septa, rarely with slight constrictions, subhyaline to pale olivaceous or olivaceous-brown, thin-walled, smooth, sometimes rough-walled, with obtuse apex, and obconically truncate to rounded base, 1.5–2.5(–3) μm wide, hila unthickened or almost so, at most somewhat refractive.

Cultural characteristics: Forma griseola; on OA colonies flat to slightly erumpent, spreading with moderate aerial mycelium; margins smooth, regular, surface with patches of olivaceous-grey and smoke-grey to dirty-white; on PDA erumpent with moderate aerial mycelium, surface pale olivaceous-grey to olivaceous-grey in the central part; margin iron-grey, and also iron-grey in reverse. Cardinal temperature requirements for growth: minimum 6 > °C, optimum = 24 °C, maximum < 30 °C. Forma mesoamericana; on OA flat to slightly erumpent, spreading, with moderate aerial mycelium; margins irregular, feathery to smooth, even; surface with the central part dirty-white to pale or darker olivaceous-grey, outer region iron-grey; on PDA spreading, erumpent, with moderate aerial mycelium; surface olivaceous-grey in the central part; outer region and reverse iron-grey, margins feathery, irregular. Cardinal temperature requirements for growth: minimum 6 > °C, optimum 24 °C, maximum > 30 °C.

Herbarium specimens examined: On Lablab niger, Japan, Tokyo, Toyoda, Itino-machi, Minamitama-gun, 9 Aug. 1962, S. Takamoto (IMI 96372). On Phaseolus vulgaris, Italy, Selva, Aug. 1877, Sacc., Mycoth. Ven. 1247 (HAL), type of Isariopsis griseola; Italy, Pavia, Casatisma e Albaredo Arnaboldi, 1888, Briosi & Cavara, Funghi parass. 17 (HAL); Russia, Czernigov, Borzova, Aug. 1914, G. Nevodovsky, Petr. Mycoth. gen. 249 (B); South Korea, Chunchon, 7 Oct. 2003, H.D. Shin (HAL). On Phaseolus sp., Brazil, São Paulo, Botanical Garden, 26 Dec. 1901, Puttemans, No. 413 (B), type of Arthrobotryum puttemansii; Italy, Bugellae et Vercellis, Cesati, Rabenh., Herb. mycol., Ed. 2, No. 327 (HAL), type of Cylindrosporium phaseoli; USA, N.J., Newfield, 27 Sep. 1894, J.B. Ellis (NY), type of Cercospora columnaris. Unidentified host (Phaseolus sp.?), Paraguay, Caá-guazú, Jan. 1882, B. Balansa, No. 3492 (LSP 918), type of Cercospora solimanii.

Hosts and distribution: Lablab niger, ?L. purpureus, ?Lathyrus odoratus, ?Macroptilium atropurpureum, Phaseolus acutifolius, P. aureus, P. coccineus, P. lunatus, P. pubescens, P. vulgaris, Vigna angularis, V. mungo, V. radiata, V. sinensis, V. unguiculata (Leguminosae), worldwide, including Angola, Argentina, Armenia, Australia, Austria, Bhutan, Brazil, Bulgaria, Burundi, Cameroon, Canada, China, Colombia, Congo, Costa Rica, Croatia, Cuba, Dominican Republ., Ecuador, El Salvador, Ethiopia, Fiji, France, Georgia, Germany, Ghana, Great Britain, Greece, Guatemala, Haiti, Hungary, Jamaica, Japan, India, Indonesia, Iran, Ireland, Israel, Italy, Ivory Coast, Jamaica, Japan, Kenya, Korea, Laos, Latvia, Malawi, Madagascar, Malaysia, Mauritius, Mexico, Mozambique, Nepal, Netherlands, Netherlands Antilles, New Caledonia, New Zealand, Nicaragua, Nigeria, Norfolk Island, Panama, Papua New Guinea, Paraguay, Peru, Philippines, Poland, Portugal, Puerto Rico, Réunion, Romania, Russia, Rwanda, Saint Helena, Senegal, Sierra Leone, Singapore, Slovenia, Solomon Islands, Somalia, South Africa, Spain, Sudan, Suriname, Swaziland, Switzerland, Taiwan, Tanzania, Thailand, Trinidad and Tobago, Turkey, Uganda, Ukraine, U.S.A. (CT, DE, Eastern states, FL, HI, IN, MA, MD, ME, MI, MS, NC, NH, NJ, NY, OK, PA, SC, TX, VA, WI), Vanuatu, Venezuela, Virgin Islands, Yugoslavia, Zambia, Zimbabwe (Crous & Braun 2003).

Notes: As a consequence of molecular sequence analyses (Figs (Figs1, 1, ,2, 2, ,3),3), and re-examination and reassessments of the synnematous conidiomata and scar and hilum structures (Fig. 4, see Discussion), Phaeoisariopsis griseola proved to be congeneric with Pseudocercospora. The proposed assignment of this species to Pseudocercospora presupposes acceptance of a formal proposal to conserve the latter genus against the older names Phaeoisariopsis and Stigmina (Braun & Crous 2006). All other taxa formerly placed in Phaeoisariopsis have already been treated and reallocated elsewhere (Crous & Braun 2003).

Cylindrosporium phaseoli Rabenh. is the oldest name coined for this species, which appeared first on the printed label of `Rabenh., Herb. mycol. 327, 1856'. This name was repeated in Fürnrohr (1857), Schlechtendal (1857) and Saccardo (1884), but in all cases without any description (nom. nud.). Gonzáles Fragoso (1927: 339) was the first author who correctly cited this name as synonym of Phaeoisariopsis griseola, which we confirm after having re-examined Rabenhorst's original material.

Deighton (1990) reduced Cercospora solimanii Speg. to synonymy with Ph. griseola, but without any comments and references. Braun (2000) examined type material of this species and confirmed Deighton's (1990) synonymy.

Although there are two clear entities associated with the angular leaf spot disease of bean on pathological or molecular grounds, we were unable to find enough morphological, cultural or phylogenetic support to separate these as two species. Because isolates can readily be classed as either one or the other type based on their host reaction on differential cultivars, we have chosen to designate them as formae of the same species.

DISCUSSION

A primary aim of the present study was to determine the species status of the Andean and Middle-American groups of the angular leaf spot pathogen of beans. Because we have been unable to obtain good morphological differences between the two groups (other than cardinal temperatures for growth), nor clear phylogenetic support for the separation based on various gene loci, we have chosen to recognise these two operational units as formae of the same species, namely f. griseola and f. mesoamericana.

Two basic characters have in the past been used for the discrimination of Phaeoisariopsis and Pseudocercospora, namely the structure of the conidiomata and the type of conidiogenous loci and conidial hila. In molecular studies, the conidiomatal structures were shown to be unreliable at the genus level for anamorphs of Mycosphaerella. This is aptly illustrated by the examples of Septoria Sacc. (pycnidia) and Phloeospora Wallr. (acervuli) (Verkley et al. 2004), Colletogloeopsis Crous & M.J. Wingf. (acervuli) and Phaeophleospora-like species with aseptate conidia and pycnidia (Cortinas et al. 2005), Ramularia Unger (normal fascicles) and Phacellium Bonord. (synnemata) (Crous et al., unpubl. data), which are all irregularly scattered among the cladogrames. The coelomycete genus Septoria (pycnidia) always clusters basal to Cercospora Fresen. (fasciculate hyphomycete) (Crous et al. 2000, 2001). The presence of synnemata is thus insufficient to separate Phaeoisariopsis from Pseudocercospora (Crous et al. 2001, Crous & Braun 2003). Furthermore, Pseudocercospora already includes some synnematous species [e.g. the type species, P. vitis (Lév.) Speg.]. Several species originally placed in Phaeoisariopsis, but with inconspicuous conidial scars, have already been reallocated in Pseudocercospora (Deighton 1990). There are also some other genera of hyphomycetes with synnematous as well as non-synnematous species, e.g., Spiropes Cif. (Ellis 1971).

The structure of the conidiogenous loci and conidial hila represent another important character used for the distinction of Phaeoisariopsis and Pseudocercospora. Prior to the introduction of the scar structure as basic feature in the taxonomy of cercosporoid genera (Deighton 1967, 1973, 1974, 1976), Phaeoisariopsis was mainly or even solely based on the synnematous arrangement of the conidiophores, combined with pigmented conidia formed singly. Therefore, it was hardly surprising that Sawada (1922) transferred Septonema vitis Lév., the type species of Pseudocercospora, to Phaeoisariopsis, and thus reduced Pseudocercospora to synonymy with Phaeoisariopsis. The heterogeneity of Phaeoisariopsis is also reflected by the exclusion of all species, except for the type species, I. griseola, originally placed in this genus by Ferraris (1909): Isariopsis grayiana Ellis ([equivalent] Fusicladium grayianum (Ellis) Deighton & M.B. Ellis), I. mexicana Ellis & Everh. ([equivalent] Exosporium mexicanum (Ellis & Everh.) M.B. Ellis) and I. pilosa Earle (= Morrisographium persicae (Schwein.) Deighton) (see Deighton 1990). Von Arx (1983), Deighton (1990) and Braun (1992, 1995a,1995b, 1998) considered the conidiogenous loci and conidial hila in Phaeoisariopsis to be conspicuous or at least subconspicuous, i.e., barely to slightly thickened and darkened. However, Yen (Yen & Lim 1980) already placed the ALS pathogen in Pseudocercospora (conidiogenous loci inconspicuous), although the wrong combination [Pseudocercospora columnaris (Ellis & Everh.) J.M. Yen] was introduced, and the correct basionym, Isariopsis griseola, cited as synonym. The inclusion of Phaeoisariopsis griseola in Pseudocercospora (Sawada 1922) thus reduces Pseudocercospora to synonymy with Phaeoisariopsis. We have re-examined the scars and hila in Ph. griseola in detail, based on a wide range of samples in vivo and in vitro, including type material of Isariopsis griseola, Cylindrosporium phaseoli, Cercospora columnaris and C. solimanii. The conidiogenous cells are usually not or barely geniculate, the conidiogenous cells are terminal to lateral, non-protruding, quite inconspicuous to subconspicuous, i.e. unthickened or almost so, but slightly darkened-refractive. There are collections with completely inconspicuous conidiogenous loci, e.g. the types of Isariopsis griseola and Cercospora solimanii. In other samples, the loci range from being quite inconspicuous to subconspicuous. The African collection illustrated by Deighton (1990) is an example of subconspicuous loci. However, as demonstrated earlier by molecular examinations, taxa with subconspicuous loci and hila (unthicked or almost so, but slightly darkened-refractive or only the ultimate rim slightly thickened and darkened) clustered together with Pseudocercospora species, so that further segregate-genera like Paracercospora Deighton and Pseudophaeoramularia U. Braun had to be reduced to synonymy with Pseudocercospora (Crous et al. 2000, 2001; Crous & Braun 2003).

Based on the molecular data presented here, the type species of Pseudocercospora (P. vitis) clusters with the type of Phaeoisariopsis (P. griseola), and the type of Stigmina Sacc. [S. platani (Fuckel) Sacc.]. The close affinity of these three genera underlines earlier suspicions of mycologists that criteria such as 1) slightly thickened conidial hila and scars, 2) synnematous to fasciculate to sporodochial conidiomata, 3) transverse to muriformly septate conidia, 4) euseptate to distoseptate conidia, 5) smooth percurrent proliferations and sympodial proliferation, versus irregular, rough percurrent proliferations on conidiogenous cells, are an insufficient basis to separate anamorph genera in Mycosphaerella.

Given the fact that these three genera represent anamorph forms of Mycosphaerella, and that they phylogenetically reside in the same clade, the next predicament arises as to what name should be applied: Pseudocercospora (1910; 1171 names), Phaeoisariopsis (1909, 65 names), or Stigmina (1880, 161 names). Although Stigmina is the oldest name, Pseudocercospora is the most commonly used, and many species of Stigmina in fact represent other fungi. Phaeoisariopsis, which also is older than Pseudocercospora, has been reduced to its type species, with most other species being placed in either Passalora or Pseudocercospora. Stigmina predates Phaeoisariopsis. If the Code of Botanical Nomenclature were to be strictly applied, all species in this complex should be transferred to Stigmina. As the latter is a poorly resolved, still heterogeneous genus, we choose to avoid this upheaval, and support conservation of the commonly used and accepted generic name, Pseudocercospora (Braun & Crous 2006). The latter genus should be used for the whole complex of hyphomycetes formerly placed in Phaeoisariopsis and some of Stigmina. A formal conservation proposal to this extent has been prepared for Taxon (Braun & Crous 2006).

Acknowledgments

Mrs Marizeth Groenewald (CBS) is thanked for conducting the growth study on isolates of P. griseola, Miss Malou van der Horst (CBS) for sequencing some isolates, Dr Walter Gams (CBS) for his comments on nomenclatural issues, and Miss Marjan Vermaas (CBS) for preparing the photo plate. Dr George Mahuku of the Centro Internacional de Agricultura Tropical (CIAT), Colombia, is thanked for providing isolates from south and central America.

Notes

Taxonomic novelties: Pseudocercospora griseola (Sacc.) Crous & U. Braun comb. nov., P. griseola f. mesoamericana Crous & U. Braun f. nov.

References

  • Araya CM, Steadman JR (1998). Variation in virulence, telia production, and RAPD patterns among isolates of the bean rust pathogen from the Americas. Annual Report of the Bean Improvement Cooperation 41: 23–24.
  • Araya CM, Alleyne AT, Steadman JR, Eskridge KM, Coyne DP (2004). Phenotypic and genotypic characterization of Uromyces appendiculatus from Phaseolus vulgaris in the Americas. Plant Disease 88: 830–836.
  • Arx JA von (1983). Mycosphaerella and its anamorphs. Proceedings van de Koninklijke Nederlandse Akademie van Wetenschappen, Series C, 86(1): 15–54.
  • Boshoff WPH, Swart WJ, Pretorius ZA, Liebenberg MM, Crous PW (1996). Isozyme variability among isolates of Phaeoisariopsis griseola in southern Africa. Plant Pathology 45: 344–349.
  • Busogoro JP, Jijakli MH, Lepoivre P (1999). Virulence variation and RAPD polymorphism in African isolates of Phaeoisariopsis griseola (Sacc.) Ferr., the causal agent of angular leaf spot of common bean. European Journal of Plant Pathology 105: 559–569.
  • Brown JWS, McFerson JR, Bliss FA, Hall TC (1982). Genetic divergence among commercial classes of Phaseolus vulgaris in relation to phaseolin pattern. HortScience 17: 752–754.
  • Braun U (1992). Taxonomic notes on some species of the Cercospora complex. Nova Hedwigia 55: 211–221.
  • Braun U (1995a). A monograph of Cercosporella, Ramularia and allied genera (Phytopathogenic Hyphomycetes). Vol. 1. IHW-Verlag, Eching.
  • Braun U (1995b). Miscellaneous notes on phytopathogenic hyphomycetes (II). Mycotaxon 55: 223–241.
  • Braun U (2000). Annotated list of Cercospora spp. described by C. Spegazzini. Schlechtendalia 5: 57–79.
  • Braun U, Crous PW (2006). Proposal to conserve the name Pseudocercospora against Stigmina and Phaeoisariopsis. Taxon: In press.
  • Braun U, Crous PW, Dugan F, Groenewald JZ, Hoog SG de (2003). Phylogeny and taxonomy of Cladosporium-like hyphomycetes, including Davidiella gen. nov., the telemorph of Cladosporium s.str. Mycological Progress 2: 3–18.
  • Buruchara RA 1983. Determination of pathogenic variation in Isariopsis griseola Sacc. and Pseudomonas syringae pv. phaseolicola (Burk., 1926) Young, Dye & Wilkie 1978. Ph.D. thesis. University of Nairobi.
  • Chacón MI, Jara C, Castellanos G, Posso CE, Buruchara R, Cuasquer JB, Pastor-Corrales MA (1997). Genetic diversity and relation between common bean angular leaf spot fungus isolates from Africa and South America: genetic improvement implications. Annual Report of the Bean Improvement Cooperation 40: 122–123.
  • Chacón MI, Pickersgill SB, Debouck DG (2005). Domestication patterns on common bean (Phaseolus vulgaris L.) and the origin of the Mesoamerican and Andean cultivated races. Theoretical and Applied Genetics 110: 432–444. [PubMed]
  • Chupp C (1954). A monograph of the fungus genus Cercospora. Ithaca, New York. Published by the author.
  • CIAT (1996). Bean program annual report for 1994. Working document no. 162. CIAT, Cali, Colombia.
  • Correa-Victoria FJ (1987). Pathogenic variation, production of toxic metabolites, and isoenzyme analysis in Phaeoisariopsis griseola (Sacc.) Ferr. Ph.D. thesis, Michigan State University.
  • Correa-Victoria FJ, Pastor-Corrales MA, Saettler AW (1989). Angular leaf spot. In: Bean Production Problems in the Tropics (Schwarts HF, Pastor-Corrales MA, eds). Centro Internacional de Agricultura Tropical, Cali, Colombia: 59–75.
  • Cortinas MN, Burgess T, Dell B, Xu D, Crous PW, Wingfield BD, Wingfield MJ (2005). First record of Colletogloeopsis zuluensis comb. nov., causing a stem canker of Eucalyptus spp. in China. Mycological Research 110: 229–236. [PubMed]
  • Crous PW (1998). Mycosphaerella spp. and their anamorphs associated with leaf spot diseases of Eucalyptus. Mycologia Memoir 21: 1–170.
  • Crous PW, Aptroot A, Kang JC, Braun U, Wingfield MJ (2000). The genus Mycosphaerella and its anamorphs. Studies in Mycology 45: 107–121.
  • Crous PW, Braun U (2003). Mycosphaerella and its anamorphs. 1. Names published in Cercospora and Passalora. CBS Biodiversity Series 1: 1–571.
  • Crous PW, Groenewald JZ, Pongpanich K, Himaman W, Arzanlou M, Wingfield MJ (2004). Cryptic speciation and host specificity among Mycosphaerella spp. occurring on Australian Acacia species grown as exotics in the tropics. Studies in Mycology 50: 457–469.
  • Crous PW, Kang JC, Braun U (2001). A phylogenetic redefinition of anamorph genera in Mycosphaerella based on ITS rDNA sequence and morphology. Mycologia 93: 1081–1101.
  • Deighton FC (1967). Studies on Cercospora and allied genera. II. Passalora, Cercosporidium and some species of Fusicladium on Euphorbia. Mycological Papers 112: 1–80.
  • Deighton FC (1973). Studies on Cercospora and allied genera. IV. Cercosporella Sacc., Pseudocercosporella gen. nov. and Pseudocercosporidium gen. nov. Mycological Papers 133: 1–62.
  • Deighton FC (1974). Studies on Cercospora and allied genera. V. Mycovellosiella Rangel, and a new species of Ramulariopsis. Mycological Papers 137: 1–75.
  • Deighton FC (1976). Studies on Cercospora and allied genera. VI. Pseudocercospora Speg., Pantospora Cif. and Cercoseptoria Petr. Mycological Papers 140: 1–168.
  • Deighton FC (1990). Observations on Phaeoisariopsis. Mycological Research 94: 1096–1102.
  • Ellis MB (1971). Dematiaceous hyphomycetes. Commonwealth Mycological Institute, Kew.
  • Ellis MB (1976). More dematiaceous hyphomycetes. Commonwealth Mycological Institute, Kew.
  • Farris JS, Kallersjo M, Kluge AG, Bult C (1994). Testing significance of incongruence. Cladistics 10: 315–320.
  • Ferraris T (1909). Osservazioni micologiche. Su specie del gruppo Hyphales (Hyphomycetae). Annales Mycologici 7: 273–286.
  • Führnrohr AE (1857). Klotzschii Herbarium vivum mycologicum sistens Fungorum per totam Germaniam crescentium collectionem perfectam. Editio nova. Centuria IV. et V. cura L. Rabenhorst. Dresdae 1856 et 1857. 4. Flora 15(9): 132–139.
  • Gams W, Hoekstra ES, Aptroot A, eds. (1998). CBS Course of Mycology. 4th edn. Centraalbureau voor Schimmelcultures, Baarn.
  • Gonzáles Fragoso DR (1927). Estiudio sistemático de los Hifales de la Flora Española. Memorias de la Real Academia de Ciencias de Madrid, Series 2, 6: 1–377.
  • Guzmán P, Gilbertson RL, Nodari R, Johnson WC, Temple SR, Mandala D, Mkandawire ABC, Gepts P (1995). Characterization of variability in the fungus Phaeoisariopsis griseola suggests coevolution with the common bean (Phaseolus vulgaris). Phytopathology 85: 600–607.
  • Gepts P, Bliss FA (1985). F1 hybrid weakness in the common bean: Differential geographic origin suggests two bean pools in cultivated bean germplasm. Journal of Heredity 76: 447–450.
  • Gepts P, Bliss FA (1986). Phaseolin variability among wild and cultivated common beans from Colombia. Economic Botany 40: 469–478.
  • Gepts P, Osborn, TC, Rashka, K & Bliss, FA (1986). Electrophoretic analysis of phaseolin protein variability in wild forms and landraces of the common bean, Phaseolus vulgaris: Evidence for multiple centres of domestication. Economic Botany 40: 451-468.
  • Koenig R, Gepts P (1989). Allozyme diversity in wild Phaseolus vulgaris: further evidence for two major centres of genetic diversity. Theoretical and Applied Genetics 44: 809–817. [PubMed]
  • Koenig R, Singh SP, Gepts P (1990). Novel phaseolin types in wild and cultivated common bean (Phaseolus vulgaris, Fabaceae). Economic Botany 44: 50–60.
  • Liebenberg MM (1996). Pathogenic variation in Phaeoisariopsis griseola (Sacc.) Ferraris on dry beans in Southern Africa. M.Sc. Agric. thesis, University of the Orange Free State, Bloemfontein, South Africa.
  • Liebenberg MM, Pretorius ZA (1997). A review of angular leaf spot of common bean (Phaseolus vulgaris L.). African Plant Protection 3: 81–106.
  • Liebenberg MM, Pretorius ZA, Swart WJ (1996). Differences in lesion size caused by Andean-specific and Andean-non-specific isolates of Phaeoisariopsis griseola. Annual Report of the Bean Improvement Cooperation 39: 290–291.
  • Mahuku GS, Henríquez MA, Muñoz J, Buruchara RA (2002). Molecular markers dispute the existence of the Afro-Andean group of the bean angular leaf spot pathogen, Phaeoisariopsis griseola. Phytopathology 92: 580–589. [PubMed]
  • Miklas P, Kelly J (1992). Identifying bean DNA polymorphisms using the polymerase chain reaction. Annual Report of the Bean Improvement Cooperation 35: 21–22.
  • Pastor-Corrales MA (1996). Traditional and molecular confirmation of the coevolution of beans and pathogens in Latin America. Annual Report of the Bean Improvement Cooperation 39: 46–47.
  • Pastor-Corrales MA, Jara C, Singh S (1998). Pathogenic variation in, source of, and breeding for resistance to Phaeoisariopsis griseola causing angular leaf spot in common bean. Euphytica 103: 161–171.
  • Rambaut A (2002). Sequence Alignment Editor. Version 2.0. Department of Zoology, University of Oxford, Oxford.
  • Rayner RW (1970). A mycological colour chart. CMI and British Mycological Society. Kew, Surrey, England.
  • Saccardo PA (1878). Fungi veneti novi vel critici. 8. Michelia 1: 239–275.
  • Saccardo PA (1884). Sylloge fungorum, Vol. 3. Padua.
  • Saettler AW (1991). Angular Leaf Spot. In: Compendium of Bean Diseases (Hall R, ed.). APS Press, St. Paul, U.S.A.: 15–16.
  • Sandlin CM, Steadman JR, Araya CM, Coyne DP (1999). Isolates of Uromyces appendiculatus with specific virulence to landraces of Phaseolus vulgaris of Andean origin. Plant Disease 83: 108–113.
  • Sawada K. (1922). Descriptive catalogue of the Formosan fungi II. Report of the Department of Agriculture, Government Research Institute, Taiwan 2: 139–164.
  • Schlechtendal DFL von (1857). Klotzschii Herbarium vivum mycologicum sistens Fungorum per totam Germaniam crescentium collectionem perfectam. Ed. nova. Centuria IV., cura Dr. L. Rabenhorst. Dresdae 1856. Typis Car. Heinrich. 4. Botanische Zeitung 15: 93–96.
  • Schwartz HF, Correa VF, Pineda DDA, Otoya MM, Katherman MJ (1981). Ascochyta, angular and white fly leaf spots in Colombia. Plant Disease 65: 494–496.
  • Shin HD, Kim JD 2001. Cercospora and allied genera from Korea. Plant Pathogens of Korea 7: 1–302.
  • Singh SP, Nodari R, Gepts P (1991a). Genetic diversity in cultivated common bean: I. Allozymes. Crop Science 31: 19–23.
  • Singh SP, Gutiérrez MA, Urrea C, Gepts P (1991b). Genetic diversity in cultivated common bean: II. Marker-based analysis of morphological and agronomic traits. Crop Science 31: 23–29.
  • Skroch, PW, Santos JB dos, Nienhuis J (1992). Genetic relationships among Phaseolus vulgaris genotypes based on RAPD markers data. Annual Report of the Bean Improvement Cooperation 35: 23–24.
  • Sprecher SL, Isleib TG (1989). Morphological and phenological diversity between bean gene pools. Annual Report of the Bean Improvement Cooperation 32: 54–55.
  • Steadman JR (ed.) (1995). Progress reported at the 2nd International Bean Rust Workshop. Annual Report of the Bean Improvement Cooperation 38: 1–10.
  • Swofford DL (2002). PAUP 4.0b10: Phylogenetic analysis using parsimony. Sinauer Associates, Sunderland, MA, U.S.A.
  • Verkley JM, Starink-Willemse M, Iperen A van, Abeln ECA (2004). Phylogenetic analyses of Septoria species based on the ITS and LSU-D2 regions of nuclear ribosomal DNA. Mycologia 96: 558–571. [PubMed]
  • Verma BR, Sharma SL (1984). Variability in Phaeoisariopsis griseola, the cause of angular leaf spot of beans. Indian Phytopathology 37: 580–581.
  • Wagara IN, Mwang'ombe AW, Kimenju, JW, Buruchara RA, Jamnadass R, Majiwa PAO (2004). Genetic diversity of Phaeoisariopsis griseola in Kenya as revealed by AFLP and group-specific primers. Journal of Phytopathology 152: 235–242.
  • Wortmann CS, Kirkby RA, Elude CA, Allen DJ (1998). Atlas of Common Bean (Phaseolus vulgaris L.) Production in Africa. CIAT, Cali, Colombia.
  • Yen JM, Lim G (1980). Cercospora and allied genera of Singapore and the Malay Peninsula. Gardens' Bulletin Singapore 33: 151–263.

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