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Two new species of Fusarium section Liseola associated with mango malformation

     Department of Microbiology and Plant Pathology, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0002, South Africa

Brenda D. Wingfield

     Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0002, South Africa

Walter F. O. Marasas

     Programme on Mycotoxins and Experimental Carcinogenesis (PROMEC), Medical Research Council (MRC), P.O. Box 19070, Tygerberg, South Africa

Michael J. Wingfield

     Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0002, South Africa

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Mango malformation is an economically important disease of Mangifera indica globally. A recent DNA-based study indicated that two distinct, phylogenetic lineages previously identified as Fusarium subglutinans are associated with this disease in South Africa. The objective of this study was to characterize Fusarium isolates associated with mango malformation, including the two different F. subglutinans groups, based on morphological characteristics. For this purpose we examined Fusarium strains isolated from diseased mango inflorescences from diverse geographical origins. We also used sexual compatibility tests to determine whether sexual reproduction among the strains was possible. The morphological characters considered were shape of the conidia, presence of mono- and/or polyphialides, origin of the conidiophores from the substrate, presence of chlamydospores and the presence of sterile coiled hyphae. Three unique Fusarium spp. were identified. In this paper, we provide formal descriptions for the two new taxa in the section Liseola that we have named F. mangiferae and F. sterilihyphosum. Fusarium mangiferae is conspecific with strains that were previously identified as F. subglutinans and reported to be the causal agent of malformation in mango growing areas throughout the world. Fusarium sterilihyphosum, on the other hand, has been isolated only from malformed mango tissue in South Africa.

Key words: Gibberella fujikuroi complex, mango, taxonomy

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Mango (Mangifera indica L.) malformation is an economically important disease in mango-growing areas of the world including India, Pakistan, Egypt, South Africa, Brazil, Israel, Florida, and Mexico (Kumar et al 1993, Freeman et al 1999). This disease causes abnormal development of vegetative shoots and inflorescences (Kumar et al 1993). Floral malformation is the most prominent symptom and is characterized by abnormal, thick and fleshy panicles (Varma 1983, Kumar et al 1993). Affected panicles bear no fruit, resulting in significant economic losses (Varma et al 1974, Varma 1983, Kumar et al 1993).

The etiology of mango malformation disease is controversial. Physiological abnormality, virus infections, mite infestations and fungal pathogens have been reported as the causal agents of this disease (Kumar et al 1993). Summanwar et al (1966) identified the fungal pathogen commonly associated with the disease as Fusarium subglutinans (Wollenweber & Reinking) Nelson, Toussoun & Marasas (= F. moniliforme Sheldon var. subglutinans Wollenweber & Reinking), residing in section Liseola. Freeman et al (1999) recently demonstrated that isolates identified as F. subglutinans induced typical mango malformation symptoms on mango trees using the isolate MRC 7559 (506/2) originally collected from mango inflorescences in Israel.

Fusarium subglutinans forms part of the Gibberella fujikuroi (Sawada) Ito in Ito & K. Kimura species complex (Leslie 1995, Britz et al 1999). Fusarium subglutinans sensu lato is, however, a polyphyletic taxon that has been associated with various plant hosts, each of which represents a distinct lineage in the G. fujikuroi complex (Leslie 1995, O'Donnell et al 1998, Britz et al 1999, Steenkamp et al 1999, 2000a, O'Donnell et al 2000). These lineages are difficult to distinguish using conventional morphological characters such as those proposed by Nelson et al (1983). Until relatively recently, these fungi were distinguished from each other using pathogenicity and mating studies (Leslie 1995). The different lineages representing F. subglutinans sensu lato are, however, readily distinguishable using DNA sequences of genes for ß-tubulin, translation elongation factor EF-1, histone H3, and calmodulin (O'Donnell et al 1998, Steenkamp et al 1999, 2000a, O'Donnell et al 2000).

Mango malformation in South Africa is associated with two phylogenetically distinct groups of isolates until recently referred to as F. subglutinans (Steenkamp et al 1999, 2000a). Based on the histone H3 and ß-tubulin gene sequences, one group of isolates represents a previously undescribed lineage in the G. fujikuroi complex. The second group of isolates is conspecific with isolates that were previously reported to be the causal agent of mango malformation (Steenkamp et al 2000a). The results presented by these authors also confirmed those of Viljoen et al (1997), O'Donnell et al (2000) and Leslie (pers comm), who have shown using random amplified polymorphic DNA (RAPDs), DNA sequence of several genes, and isozymes, respectively, that mango malformation is associated with two distinct species, both with morphological characters typical of F. subglutinans.

The aim of this study was to characterize, using morphology, Fusarium spp. isolated from malformation mango tissue from diverse geographical origins. For this purpose the morphological characteristics proposed by Nirenberg and O'Donnell (1998) were used. Sexual compatibility tests were also used to verify the identity of some of these Fusarium spp.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Morphological and cultural studies – Fusarium spp. associated with mango malformation in South Africa were isolated from mango trees in Tzaneen (Northern Province), which included the areas Letsitele (LS) and Deer Park (DP). Isolates were also collected from Nelspruit (NS), Fredenheim (FH), Malelane (ML) and Hazyview (HZ) (Mpumalanga). Other isolates used in this study were isolated from malformed mango tissue by other collectors in Florida, Egypt, Israel, Malaysia and South Africa (Table I). Mating tester strains (MRC 6213 and MRC 7488) for F. circinatum Nirenberg & O'Donnell (mating population H of the G. fujikuroi complex) were used in sexual compatibility tests (Britz et al 1999). All the isolates were stored in 15% glycerol at -70 C in the Fusarium culture collection of the Tree Pathology Co-operative Programme (TPCP), Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, South Africa and the culture collection of the Medical Research Council (MRC), P.O. Box 19070, Tygerberg, South Africa.

To stimulate culture and conidial development, Fusarium isolates (Table I) were transferred to CLA (Fisher et al 1982) and KCl agar (Nelson et al 1983). Cultures were incubated at 23 C under fluorescent and cool-white light with a 12 h photoperiod. After 10 to 14 d of incubation, the following morphological characters were examined: shape of the conidia, presence of mono- and/or polyphialides, origin of the conidiophores from the substrate, presence of chlamydospores and sterile coiled hyphae (Nirenberg and O'Donnell 1998). Secondary characteristics such as growth rate and colony color (Rayner 1970) were determined on potato dextrose agar (PDA) after incubation at 25 C in the dark (Nelson et al 1983) for the two newly described species. Each isolate was plated onto three different PDA plates and the growth rate was determined over a period of 10 d. This entire procedure was repeated once more. A one way ANOVA was done to determine whether growth rate differed significantly for the different isolates. Colony color was determined after 14 d using the color coding system of Rayner (1970). Fifty measurements were made of all diagnostic morphological characters. The measurements are indicated as minimum, mean, and maximum.

Mating type and sexual compatibility tests – The mating type (MAT-1 or MAT-2) of all the isolates included in this study were determined using the PCR-based method described by Steenkamp et al (2000b). Only isolates with opposite mating types were crossed within each species and between species using the method described by Klittich and Leslie (1988) with some modifications (Britz et al 1999). Covert et al (1999) found that a lower incubation temperature increased sexual fertility among isolates of G. circinata Nirenberg & O'Donnell. We, therefore, used an incubation temperature of 17 C for our crosses. Sexual crosses have already been performed within and between isolates of the two new species in a previous study (Steenkamp et al 2000a), and the present study served to confirm those results using a more clearly defined collection of isolates. Since the morphological characteristics of some isolates were similar to those of F. circinatum, all isolates (Table I) were crossed with the standard tester strains (MRC 6213 and MRC 7488) for G. circinata (anamorph: F. circinatum).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Morphological and cultural studies – Fusarium isolates from trees suffering from mango malformation in Malaysia, Egypt, Israel, South Africa, and Florida were separated into three different groups based on morphological characters defined by Nirenberg and O'Donnell (1998). Based on the morphological characters that were used, each of these groups represented new species in the G. fujikuroi complex. Of the three groups, two are clearly discrete taxa based both on morphological characteristics and sequencing data (Steenkamp et al 2000a). These two taxa are represented by an extensive group of isolates and we elect to describe them as new species in Fusarium section Liseola.

Fusarium mangiferae

Britz, Wingfield et Marasas sp. nov. Figs. 1–5

View larger version (155K): [in this window] [in a new window]    FIGS. 1–4. Fusarium mangiferae. 1. Branched conidiophores bearing polyphialides with 3 conidiogenous openings (scale bar: 5 µm). 2. Branched conidiophores bearing mono- and polyphialides (scale bar: 5 µm). 3. Microconidia (scale bar: 15 µm). 4. Macrocondium (scale bars: 15 µm)

HOLOTYPUS. Cultura exsiccata in agaro CLA ex MRC 7559, sejuncta a inflorescentis malformatis Mangifera indica, Volcani Center, Bet Dagan, Israel, 1993, S. Freeman (PREM 57299).

Colonies on PDA with average growth rate of 3.4 mm/d at 25 C. Aerial mycelium white, floccose. Reverse of colonies sometimes rosy buff (17''f) to dark purple (65k). Conidiophores on aerial mycelium originating erect and prostrate from substrate. Conidiophores sympodially branched bearing mono- and polyphialides (Figs. 1, 2). Polyphialides have 2–5 conidiogenous openings (Figs. 1, 2). Phialides on the aerial conidiophores mono- and polyphialidic, up to 30.0 µm long and 3 µm wide. Sterile hyphae absent. Microconidia variable in shape, obovoid conidia the most abundant type, oval to allantoid conidia occurring occasionally (Fig. 3). Microconidia mostly 0-septate with 1-septate conidia occurring less abundantly, 0-sepate: 4.3–9.0–14.4 x 1.7–2.4–3.3 µm. Sporodochia present, cream (19'f) and orange (15b). Macroconidia long and slender, usually 3–5 septate (Fig. 4): 43.1–51.8–61.4 x 1.9–2.3–3.4 µm. Chlamydospores absent.

Etymology. Mangiferae (L. gen) indicating the species association with the genus Mangifera L.

Specimens examined. ISRAEL. Bet Dagan, Volcani center plantation, Mango malformation inflorescence on M. indica, 1993, S. Freeman 506/2 (PREM 57299, HOLOTYPE; MRC 7559, ex-holotype); Ginosar, inflorescence malformation of M. indica cultivar Kent, 1998, S. Freeman 34 (MRC 7560); Sde Nitzar, inflorescence malformation of M. indica, 1998, S. Freeman 41 (MRC 7561); Bene Dror, inflorescence malformation of M. indica cultivar Keitt, 1998, S. Freeman 86 (MRC 7562). SOUTH AFRICA. MPUMALANGA: Nelspruit, inflorescence malformation of M. indica, 1982, F. Wehner MRC 2730 (PREM 57300, PARATYPE; KSU 3873, ex-paratype); inflorescence malformation of M. indica, 1998, H. Britz NS1–1 (MRC 8080); inflorescence malformation of M. indica, 1998, H. Britz NS1–9 (MRC 8081); Nelspruit, Fredenheim, inflorescence malformation of M. indica, 1998, H. Britz FH1–6 (MRC 8085); inflorescence malformation of M. indica, 1998, H. Britz FCC 1537-FH1–8 (MRC 8078); inflorescence malformation of M. indica, 1998, H. Britz FCC 1542-FH1–16 (MRC 8079); inflorescence malformation of M. indica, 1998, H. Britz FH1–73 (MRC 8084). MPUMALANGA: Malelane, inflorescence malformation of M. indica, 1998, H. Britz FCC 1551 = ML3–1 (MRC 8077); inflorescence malformation of M. indica, 1998, H. Britz FCC 1547 = ML1–9 (MRC 8082); inflorescence malformation of M. indica, 1998, H. Britz FCC 1545 = ML1–6 (MRC 8086); inflorescence malformation of M. indica, 1998, H. Britz FCC 1546 = ML1–8 (MRC 8087); inflorescence malformation of M. indica, 1998, H. Britz FCC 1548 = ML2–1 (MRC 8083); KWAZULU-NATAL: inflorescence malformation of M. indica, 1984, C. Crookes MRC 3477 (PREM 57301, PARATYPE; KSU-X3875, ex-paratype); inflorescence malformation of M. indica, 1984, C. Crookes MRC 3478 (KSU-X 3876); inflorescence malformation of M. indica, 1984, C. Crookes MRC 3479 (KSU-X 3877). EGYPT. Inflorescence malformation of M. indica, Ibrahim Mausour, KSU-X4706 (MRC 8089); inflorescence malformation of M. indica, Ibrahim Mausour KSU-X4702 (MRC 8090); inflorescence malformation of M. indica, Ibrahim Mausour KSU-X4700 (MRC 8091). USA. FLORIDA: Dade County, Miami, inflorescence malformation of M. indica cultivar Keitt, 1994, R. Ploetz FS16 (MRC 7034); inflorescence malformation of M. indica cultivar Keitt, 1994, R. Ploetz FS23 (MRC 7035); inflorescence malformation of M. indica cultivar Keitt, 1994, R. Ploetz FS55 (MRC 7038); inflorescence malformation of M. indica cultivar Keitt, 1994, R. Ploetz MRC 7039; inflorescence malformation of M. indica, R. Ploetz KSU-X4079 = FRC-M3622 (MRC 8088). MALAYSIA. Inflorescence malformation of M. indica, Baharuddin Salleh KSU-X4382 (MRC 8092); inflorescence malformation of M. indica, Baharuddin Salleh KSU-X4384 (MRC 8093).

Commentary. A dried culture to serve as holotype has been deposited at the Plant Protection Research Institute, Pretoria, South Africa (PREM 57299). Ex-holotype cultures have been deposited in the culture collection of the South African Medical Research Council, Tygerberg, South Africa (MRC 7559) and the department of Plant Pathology, Kansas State University, Manhattan, Kansas, USA (KSU 11781). Ex-paratype cultures have been deposited as MRC 2730 (KSU-X3873) and MRC 3477 (KSU-X3875).

Fusarium sterilihyphosum

Britz, Marasas & Wingfield, sp. nov. Figs. 5–8

View larger version (182K): [in this window] [in a new window]    FIGS. 5–8. Fusarium sterilihyphosum. 5. Conidiophores bearing polyphialides with 3 conidiogenous openings (scale bar: 5 µm). 6. Sterile coiled hyphae (scale bar: 10 µm). 7. Microconidia (scale bar: 10 µm). 8. Microconidia with 0–1 septa and 3-septate macroconidia (scale bars: 20 µm)

HOLOTYPUS. Cultura exsiccata in agaro CLA ex MRC 2802, sejuncta a inflorescentis malformatis Mangifera indica, Letsitele, Tzaneen, Northern Province, South Africa, 1982, J. M. Darvas (PREM 57302).

Colonies on PDA with average growth rate of 4.8 mm/d at 25 C. Aerial mycelium almost white ('1). Reverse of colonies straw to grayish rose (3''f) and light purple (63i). Conidiophores on aerial mycelium erect, occasionally prostrate. Conidiophores sympodially branched bearing mono- and polyphialides (Fig. 5). Phialides on aerial conidiophores mono- and polyphialidic, up to 30.0 µm long and 3 µm wide. Sterile hyphae present (Fig. 6). Microconidia obovoid, oval to allantoid, 0-septate conidia abundant, 1-septate conidia less common (Fig. 7, 8): 0-septate: 4.5–8.8–14.2 x 1.6–2.6–3.5 µm. Sporodochia seldom present, cream (19'f) to orange (15b). Macroconidia slightly beaked apical cells, a footlike basal cell, 3–5 septate (Fig. 8), 28.4–37.1–47.1 x 2.4–3.2–4.1 µm. Chlamydospores absent.

Etymology. Sterilihyphosum (L. adj) refers to the presence of sterile hyphae in mycelium.

Specimens examined. SOUTH AFRICA. NORTHERN PROVINCE: Tzaneen, Letsitele area, Mango malformation inflorescence on M. indica, 1982, J. M. Darvas MRC 2802 = NRRL 25623 (PREM 57302, HOLOTYPE; KSU-X3874, ex-holotype); mango malformation of M. indica, 1997, H. Britz A33–1 (MRC 7606); mango malformation of M. indica, 1997, H. Britz FCC 1315 = A40–1 (PREM 57303, PARATYPE; MRC 8095, ex-paratype); mango malformation of M. indica, 1997, H. Britz FCC 1367 = A1–2 (MRC 7602); mango malformation of M. indica, 1997, H. Britz FCC 1398 = A20–1 (MRC 7605); mango malformation of M. indica, 1997, H. Britz FCC 1286 = A26–1 (PREM 57304, PARATYPE; MRC 8101, ex-paratype); mango malformation of M. indica, 1997, H. Britz FCC 1478 = B12–1 (MRC 8102); mango malformation of M. indica, 1997, H. Britz C6–1 (MRC 8103); mango malformation of M. indica, 1997, H. Britz D2–1 (MRC 8094); mango malformation of M. indica, 1997, H. Britz FCC 1146 = E6–1 (MRC 8096); Deer Park, Mango malformation of M. indica, 1998, H. Britz DP3–5 (MRC 8100); mango malformation of M. indica, 1998, H. Britz DP3–7 (MRC 8106); mango malformation of M. indica, 1998, H. Britz FCC 1632 = DP3–9 (MRC 8107). MPUMALANGA: Hazyview, mango malformation of M. indica, 1998, H. Britz FCC 1563 = HZ1–9 (MRC 8099); mango malformation of M. indica, 1998, H. Britz FCC 1555 = HZ1–1 (MRC 8104); mango malformation of M. indica, 1998, H. Britz FCC 1557 = HZ1–3 (MRC 8108); Malelane, mango malformation of M. indica, 1998, H. Britz ML2–10 (MRC 8105).

Commentary. A dried culture to serve as holotype has been deposited at the Plant Protection Research Institute, Pretoria, South Africa (PREM 57302). Ex-holotype culture specimens have been deposited in the culture collection of the South African Medical Research Council, Tygerberg, South Africa (MRC 2802) and the department of Plant Pathology, Kansas State University, Manhattan, Kansas, USA (KSU-X3874). Ex-paratype cultures have been deposited as MRC 8095 (KSU 11783) and MRC 8101 (KSU 11782).

Of the three Fusarium species found associated with mango malformation, two have been described as new in this study. The third group included three isolates (KSU-X4379, KSU-X4381 and KSU-X2330) that were collected from malformed mango tissue in Malaysia. These isolates have sparse aerial mycelium. Their aerial mycelial conidiophores emerge directly from the substrate hyphae (referred to as erect). The polyphialides of these isolates have fewer than 3 conidiogenous openings. Microconidia are borne in false heads and are obovoid in shape, predominantly without septa but with 1-septate conidia occurring occasionally. Macroconidia are short 19.3–24.8–29.5 x 1.3–2.0–3.0 µm and 3–5 septate. Chlamydospores and sterile coiled hyphae are absent (Table II). We believe that this fungus also represents a new taxon, but the collections are insufficient in number to justify a formal description at this stage.

Mating type and sexual compatibility tests – All isolates of the undescribed Fusarium species represented by only 3 isolates were of the MAT-1 mating type. Most of the F. mangiferae isolates were MAT-2 except for two isolates from Malaysia (MRC 8092 and MRC 8093) that were MAT-1. Both mating types were identified amongst F. sterilihyphosum isolate. The majority of these isolates were MAT-1 and isolates MRC 8101, MRC 8104 and MRC 8105 were MAT-2.

Isolates of F. mangiferae and F. sterilihyphosum of opposite mating type were sexually incompatible when crossed within each species and between the two species. None of the other Fusarium isolates (Table I) were sexually compatible with the tester strains of mating population H.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In this study we have shown that at least two distinct Fusarium spp. are associated with mango malformation symptoms, namely F. mangiferae and F. sterilihyphosum. Fusarium mangiferae was previously shown to be the causal agent of mango malformation and F. sterilihyphosum is associated with similar disease symptoms in South Africa. A third taxon was also identified, but our collections are insufficient in number to justify describing the fungus. Furthermore, the fungus does not occur in South Africa, and has not been a primary focus of our investigation.

The results of this study, together with those of Steenkamp et al (2000a), have shown that mango malformation in South Africa is associated with two distinct species, F. mangiferae and F. sterilihyphosum. Fusarium sterilihyphosum has only been isolated from malformed mango tissue in South Africa. The histone H3 and ß-tubulin gene sequences for isolates of F. mangiferae are similar to those of F. subglutinans strains NRRL 25226 and MRC 7559 (Steenkamp et al 2000a), which were previously reported to be the causal agent of mango malformation (Freeman et al 1999).

Fusarium mangiferae has been isolated from mango malformation symptoms in various geographical areas, such as South Africa, Florida, Egypt, India, Israel and Malaysia. Fusarium mangiferae is morphologically most similar to F. concentricum Nirenberg & O'Donnell and F. guttiforme Nirenberg & O'Donnell. Fusarium concentricum has long, slender, 3–4 septate macroconidia similar to those produced by F. mangiferae, which has sympodially branched conidiophores in contrast to the branched conidiophores of F. concentricum. Fusarium guttiforme can be distinguished from F. mangiferae based on the presence of the uniformly obovoid microconidia and 3-septate macroconidia that are shorter in length than those of F. mangiferae. The occasional production of 3-septate macroconidia in F. guttiforme isolates (MRC 7539, MRC 6784, and MRC 6785) found in the present study was also observed by Viljoen et al (1997). Nirenberg and O'Donnell (1998) did not refer to the macroconidial characteristics in their description of F. guttiforme.

Fusarium sterilihyphosum has been isolated only from South Africa. This species is morphologically similar to F. mangiferae, but can be distinguished from F. mangiferae. Fusarium sterilihyphosum has shorter 3–5 septate macroconidia, faster growth rate on PDA at 25 C than F. mangiferae and produces sterile coiled hyphae. Fusarium sterilihyphosum is most closely related to F. guttiforme based on histone gene sequence (Steenkamp et al 2000a). Morphologically, F. sterilihyphosum resembles F. circinatum and F. pseudocircinatum O'Donnell & Nirenberg. These three species all produce sterile coiled hyphae. However, macroconidia are long, slender, and 3–5 septate in F. sterilihyphosum, while shorter 3-sepate macroconidia are produced in both F. circinatum and F. pseudocircinatum.

Both F. sterilihyphosum and F. mangiferae are morphologically distinct from species belonging to F. subglutinans sensu lato occurring on various host plants, including F. begoniae Nirenberg & O'Donnell, F. bulbicola Nirenberg & O'Donnell, F. circinatum (MP-H), F. guttiforme, F. concentricum, F. pseudocircinatum, F. sacchari (Butler) W. Gams (MP-B), and F. subglutinans sensu stricto (MP-E) (Table II). Fusarium mangiferae and F. sterilihyphosum can also be distinguished from each other based on morphological characteristics. Sterile coiled hyphae and shorter 3–5 septate macroconidia produced by F. sterilihyphosum distinguish it from F. mangiferae. Fusarium mangiferae had a slower growth rate than F. sterilihyphosum on PDA at 25 C. However, growth rate is a secondary morphological characteristic and no significant difference (P > 0.001) among isolates of both species was observed. Furthermore, secondary characteristics are generally not used in species descriptions in view of the variability within populations and/or the instability of these characters (Gerlach and Nirenberg 1982, Nelson et al 1983, Nirenberg and O'Donnell 1998).

Both mating types (MAT-1 and MAT-2) were identified in F. mangiferae and F. sterilihyphosum isolates. This is in contrast to the Steenkamp et al (2000a) study, where isolates of each of these species included only a single mating type. In the present study, isolates of the two species having opposite mating type were sexually incompatible.

Likewise, isolates of F. mangiferae and F. sterilihyphosum of different mating types failed to cross with each other. This failure to produce sexual crosses could be explained by sterility, female-sterility of isolates or unfavorable conditions for crosses to occur (Perkins 1994, Leslie 1995). At this stage, there is thus no evidence to suggest that sexual outcrossing is occurring within or between these two fungi from mango.

Mango is native to Asia, eastern India, Burma, and the Andaman Islands, and mango malformation was first reported over a century ago in India (Kumar et al 1993). Fusarium mangiferae isolates from South Africa, United States, Israel, Malaysia, and Egypt grouped into the so-called ‘Asian clade’ of O'Donnell et al (1998) based on histone H3 and ß-tubulin gene sequences (O'Donnell et al 1998, 2000, Steenkamp et al 2000a). Fusarium mangiferae from different geographical areas was most probably introduced from India (Zheng & Ploetz 2002), which would explain the presence of F. mangiferae isolates grouping in the ‘Asian clade’. Fusarium sterilihyphosum isolates from mango malformation symptoms in South Africa grouped into the so-called ‘American clade’ (O'Donnell et al 2000, Steenkamp et al 2000a). Based on its phylogenetic position, O'Donnell et al (2000) speculated that F. sterilihyphosum (MRC 2802 = NRRL 25623) originated from mango that was imported into South Africa from South America. Vegetative malformation has been reported in Mexico (Noriega-Cantu et al 1999). These Mexican isolates produced sterile coiled hyphae and grouped also in the ‘American clade’ based on ß-tubulin gene sequences, like F. sterilihyphosum isolates from South Africa (David M. Geiser pers comm). Clearly, further investigations with strains from South America would be required to test the hypothesis that F. sterilihyphosum isolates in South Africa originated in South America.

The fact that three distinct taxa are found associated with mango malformation symptoms emphasizes a serious problem regarding the etiology of mango malformation disease. Fusarium mangiferae has been unequivocally indicated as the causal agent of mango malformation (Freeman et al 1999). It is, however, not known whether F. sterilihyphosum or the undescribed Fusarium sp. are also able to cause disease on mango trees. Their role in the etiology of mango malformation disease clearly requires further intensive study.

	ACKNOWLEDGMENTS

 
We thank the National Research Foundation (NRF) and members of the Tree Pathology Co-operative Programme (TPCP) for financial support. We are grateful to Dr. John F. Leslie for supplying Fusarium isolates from mango malformation in Malaysia and Egypt, Dr. Randy Ploetz for supplying Fusarium isolates from mango malformation in Florida, and Dr. S. Freeman for supplying Fusarium isolates from mango malformation in Israel. We also appreciate the suggestions of two anonymous reviewers and the associate editor that were useful in improving our manuscript.

	FOOTNOTES

 
¹ Present address: Onderstepoort Veterinary Institute, Private Bag X5, Onderstepoort, 0110, South Africa

² Corresponding author, mailing address: Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0002, South Africa. Phone (27 12) 420 3938, Fax (27 12) 420 3960. Teresa.coutinho{at}fabi.up.ac.za

Accepted for publication December 15, 2001.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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Covert SF, Briley A, Wallace MM, McKinney VT., 1999 Partial MAT-2 gene structure and the influence on mating success in Gibberella circinata. Fungal Genet Biol 28:43-54[Medline]

Fisher NL, Burgess LW, Toussoun TA, Nelson PE., 1982 Carnation leaves as a substrate and for preserving cultures of Fusarium species. Phytopathology 72:151-153

Freeman S, Maimon M, Pinkas Y., 1999 Use of GUS transformants of Fusarium subglutinans for determining etiology of mango malformation disease. Phytopathology 89:456-461

Gerlach W, Nirenberg HI., 1982 The genus Fusarium—a pictorial atlas. Mitt Biol Bundesanst Land-Forstw Berlin-Dahlem 209:1-406

Klittich CJR, Leslie JF., 1988 Nitrate reduction mutants of Fusarium moniliforme (Gibberella fujikuroi). Genetics 118:417-423[Abstract/Free Full Text]

Kumar J, Singh US, Beniwal SPS., 1993 Mango malformation: one hundred years of research. Annu Rev Phytopathol 31:217-232

Leslie JF., 1995 Gibberella fujikuroi: available populations and variable traits. Can J Bot 73:S282-S291

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