Ceratocystis wilt of cacao-a disease of increasing importance

ABSTRACT Ceratocystis cacaofunesta (formerly C. fimbriata) causes a lethal wilt disease of cacao (Theobroma cacao) in the Caribbean and Central and South America. Recent studies employing phylogenetics, intersterility, and host range separate the cacao pathogen from other strains of the C. fimbriata complex. Ceratocystis wilt has been managed through genetic resistance, but the disease is an emerging problem in Bahia, Brazil, where it was recently introduced. Genetic studies indicate that populations of the fungus in Costa Rica, Colombia, and Bahia may have been introduced on cacao cuttings; whereas populations in Rond?nia, Brazil, and western Ecuador appear to be native. The fungal genotype present in Bahia is similar to those found in Rond?nia and may have been introduced on propagative material with witches' broom resistance.     

Genetic Variability and Host Specialization in the Latin American Clade of Ceratocystis fimbriata

ABSTRACT The Ceratocystis fimbriata complex includes many undescribed species that cause wilt and canker diseases of many economically important plants. Phylogenetic analyses of DNA sequences have delineated three geographic clades within Ceratocystis fimbriata. This study examined host specialization in the Latin American clade, in which a number of lineages were identified using sequences of the internal transcribed spacer (ITS) region of the rDNA. Three host-associated lineages were identified from cacao (Theobroma cacao), sweet potato (Ipomoea batatas), and sycamore (Platanus spp.), respectively. Isolates from these three lineages showed strong host specialization in reciprocal inoculation experiments on these three hosts. Six cacao isolates from Ecuador, Trinidad, and Columbia differed genetically from other cacao isolates and were not pathogenic to cacao in inoculation tests. Further evidence of host specialization within the Latin American clade of Ceratocystis fimbriata was demonstrated in inoculation experiments in growth chambers using sweet potato, sycamore, Colocasia esculenta, coffee (Coffea arabica), and mango (Mangifera indica) plants; inoculation experiments in Brazil using Brazilian isolates from cacao, Eucalyptus spp., mango, and Gmelina arborea; and inoculation experiments in Costa Rica using Costa Rican isolates from cacao, coffee, and Xantho-soma sp. Hosts native to the Americas appeared to be colonized by only select pathogen genotypes, whereas nonnative hosts were colonized by several genotypes. We hypothesize that local populations of Ceratocystis fimbriata have specialized to different hosts; some of these populations are nascent species, and some host-specialized genotypes have been moved to new areas by humans.     

Comparison of populations of the wilt pathogen Ceratocystis albifundus in South Africa and Uganda

Ceratocystis albifundus is an important fungal pathogen of Acacia mearnsii trees in South Africa. In a previous study, a high level of gene diversity was demonstrated in a South African population of C. albifundus . This, together with the occurrence of the pathogen on native Protea species and its exclusive occurrence in South Africa, led to the hypothesis that C. albifundus is probably native to that country. More recently, C. albifundus has been reported from A. mearnsii in south-western Uganda. The aim of this study was to compare the populations of C. albifundus from Uganda and South Africa based on genetic diversity, population structure and possible gene flow. This was achieved using codominant microsatellite markers developed for the closely related species Ceratocystis fimbriata . Available isolates for comparison were from six different areas of South Africa and six jungle stands in Uganda. Eight of the 11 available markers amplified loci in C. albifundus . Gene diversity was higher in the Ugandan population, but genotypic diversity was greater for the South African isolates. There were no common genotypes between the two populations and they shared only 22% of the total alleles. The populations were genetically isolated from each other and highly substructured within. There was no association between isolates collected from the same geographic locations, and gene flow between the two populations was low. Results suggest that C. albifundus was probably not introduced into Uganda from South Africa but rather that an ancestral population, yet to be discovered, is the source of both populations.     

Cacao diseases: important threats to chocolate production worldwide

ABSTRACT Theobroma cacao, cacao, is an ancient, neotropical domesticate. It is now grown throughout the humid, lowland tropics and is the basis of a multibillion dollar confectionary trade. Diverse diseases impact production of the crop. They reduce yields by ca. 20%, but could cause far greater losses if certain highly damaging diseases were to become more widely distributed. Among the most potentially dangerous of these diseases are frosty pod, caused by Moniliophthora roreri, and witches' broom, caused by M. perniciosa (previously Crinipellis perniciosa). These two diseases occur only in the Western Hemisphere, and severe losses would follow their introduction to West Africa and Asia, where ca. 86% of all cacao production occurs. Elsewhere, Cacao swollen shoot virus and the damaging black pod agent, Phytophthora megakarya, are found in Western Africa; whereas vascular streak dieback, caused by Oncobasidium theobromae, is present only in Asia. Breeding programs are challenged by minimal resistance to some of the diseases. Progress that has been made is threatened by the "emergence" of other serious diseases, such as Ceratocystis wilt (Ceratocystis cacaofunesta). During this symposium, new insights are discussed on the biology, origins, pathology and phylogeny of the pathogens; as well as the biological, chemical and genetic management of the diseases that they cause.     

Spatial genetic structure and dispersal of the cacao pathogen Moniliophthora perniciosa in the Brazilian Amazon

Moniliophthora perniciosa is the causal agent of witches’ broom in Theobroma cacao (cacao). Three biotypes of M. perniciosa are recognized, differing in host specificity, with two causing symptoms on cacao or Solanaceae species (C‐ and S‐biotypes), and the third found growing endophytically on lianas (L‐biotype). The objectives of this study were to clarify the genetic relationship between the three biotypes, and to identify those regions in the Brazilian Amazon with the greatest genetic diversity for the C‐biotype. Phylogenetic reconstruction based on the rRNA ITS regions showed that the C‐ and S‐biotypes formed a well‐supported clade separated from the L‐biotype. Analysis of 131 isolates genotyped at 11 microsatellite loci found that S‐ and especially L‐biotypes showed a higher genetic diversity. A significant spatial genetic structure was detected for the C‐biotype populations in Amazonia for up to 137 km, suggesting ‘isolation by distance’ mode of dispersal. However, in regions containing extensive cacao plantings, C‐biotype populations were essentially ‘clonal’, as evidenced by high frequency of repeated multilocus genotypes. Among the Amazonian C‐biotype populations, Acre and West Amazon displayed the largest genotypic diversity and might be part of the centre of diversity of the fungus. The pathogen dispersal may have followed the direction of river flow downstream from Acre, Rondônia and West Amazon eastward to the rest of the Amazon valley, where cacao is not endemic. The Bahia population exhibited the lowest genotypic diversity, but high allele richness, suggesting multiple invasions, with origin assigned to Rondônia and West Amazon, possibly through isolates from the Lower Amazon population.     

Genetic Variability and Structure of Canadian Populations of the Sapstain Fungus Ceratocystis resinifera

ABSTRACT Genomic DNA was extracted from 129 isolates of Ceratocystis resinifera, a species belonging to the C. coerulescens complex, and 19 polymorphic random amplified polymorphic DNA markers were used to study the population genetic structure of this fungus. The analysis suggested a moderate value for genetic diversity (H(S) = 0.209). However, when monomorphic markers and rare alleles, representing 89 markers, also were included in the calculation, the genetic diversity of Canadian populations of C. resinifera appeared to be much lower (H(S) = 0.045). This could be explained by two hypotheses: (i) recent introduction of this species into North America and (ii) clonal reproduction (by selfing). No specialization by C. resinifera for coniferous tree species was observed based on genetic differentiation index between isolates sampled from Pinus and Picea spp. and on phylogenetic analysis using Dice coefficient of association. In spite of a low genetic diversity, a very high genetic differentiation was observed among the nine geographical populations studied (F(ST) = 20.8%). The genetic differences were especially striking when populations from Eastern Canada were compared with populations from Western Canada (phiST = 0.27%; P < 0.001), suggesting that a geographic reproductive barrier occurs in Central Canada. This barrier may be the consequence of a weak migration of insect vectors of C. resinifera due to reduced presence of hosts in the Canadian Great Plains, where extensive agriculture occurs. However, results from pairwise F(ST) matrix and phylogeny of haplotypes suggest that the barrier is not totally impenetrable because some gene flow occurred from the west and from the east in the Big River (Saskatchewan) population located in the middle of the Great Plains.     

Genetic diversity and interfertility among highly differentiated populations of Ceratocystis fimbriata in Brazil

Mating studies showed that isolates of the insect‐associated wilt pathogen Ceratocystis fimbriata from Eucalyptus spp., mango, fig, inhame (Colocasia esculenta), Gmelina arborea and sweet potato were interfertile, and progeny from those crosses showed normal segregation for microsatellite markers. Genetic diversity was compared among 13 populations of C. fimbriata collected from six states in Brazil using 15 highly polymorphic microsatellite markers. The gene diversity values of most eucalyptus and mango populations from Minas Gerais, Bahia, Rio de Janeiro and São Paulo states were similar to putatively native populations of Ceratocystis platani and C. cacaofunesta, two other species in the C. fimbriata complex that are homothallic. Index of association values indicated substantial asexual reproduction or selfing in populations on mango and eucalyptus. Most of these eucalyptus and mango populations were not highly differentiated from each other, and these populations and genotypes appeared to be more closely related to each other than to other populations by upgma analyses. By contrast, the G. arborea population from Pará and the fig and inhame populations from São Paulo had relatively low levels of diversity and were highly differentiated from each other and all other studied populations, suggesting that they were from different origins and had gone through genetic bottlenecks. One of the eucalyptus populations in Bahia consisted of a single genotype and may have been introduced to the site in infected cuttings from another Bahia location. Similarly, a mango population from Mato Grosso do Sul consisted of a single genotype, which was identical to one of the genotypes found on mango in São Paulo. Aside from introductions by humans, mating studies and genetic analyses suggest that limited dispersal distance and a high degree of selfing or asexual reproduction lead to local populations of C. fimbriata that have limited diversity but are highly differentiated from other populations.     

Biodiversity and biogeography of the cacao (Theobroma cacao) pathogen Moniliophthora roreri in tropical America

Moniliophthora roreri , the cause of moniliasis or frosty pod rot, occurs on the neotropical rainforest genera Theobroma and Herrania . While this basidiomycete has had devastating effects on the cacao tree ( T. cacao ) in tropical America, where it is confined, little is known of its biogeography and intraspecific genetic variability. Here, AFLP and ISSR profiles of 94 isolates of M. roreri from across its geographic range in Central/South America were analyzed. The study provided limited evidence to support the hypothesis that M. roreri is capable of sexual reproduction. The highest levels of genetic diversity occurred in Colombia and not in Ecuador as originally believed. The fungus was broadly divided into five genetic groups. Two of these have a wide geographic range: Bolívar group (north of Santander in Colombia, eastern Venezuela, peripheral Ecuador, Peru), and Co-West group (western Colombia, central Ecuador, Central America). The other groups are all apparently endemic to Colombia (Co-East and Co-Central groups) or north-western Ecuador ( Gileri group). We speculate that central/north-eastern Colombia may represent the centre of origin for M. roreri . Sequence data from the internal transcribed spacer region of the nuclear rDNA repeat were congruent with the AFLP/ISSR results, dividing M. roreri into two broad groups: the Orientalis group, comprising most isolates from the Co-East, Co-Central and Bolívar groups, and the Occidentalis group, comprising isolates from the Co-West and Gileri groups. The spread of M. roreri into new areas and countries mediated by human activity is discussed.     

Populations of Ceratocystis fimbriata on Colocasia esculenta and other hosts in the Mata Atlântica region in Brazil

Ceratocystis fimbriata is native to Brazil, where it is able to cause serious diseases on numerous hosts, especially on non‐native plants. Because C. fimbriata is soilborne and not wind dispersed, highly differentiated populations are found in different regions of Brazil. The present study compared populations of C. fimbriata on taro, mango, eucalyptus and kiwifruit from the coastal Mata Atlântica region with native populations of the fungus from the Cerrado‐transition region in Brazil by using 14 SSR markers and DNA sequences of ITS and mating type genes. Microsatellite and phylogenetic analyses were performed to test the hypothesis that populations on different hosts from the Mata Atlântica region are related to each other and are native to the region. The ITS sequences varied greatly among the taro isolates, with six sequences identified, from which two had not been previously reported. For mating type genes, four sequences were identified among the isolates on taro, mango, eucalyptus and kiwifruit. Phylogenetic analyses showed that Mata Atlântica populations formed a monophyletic group distinct from Cerrado‐transition region populations, although earlier studies had shown that isolates from the two regions are interfertile and are considered as a single biological species. Microsatellite analysis revealed low gene diversity for each of the three Mata Atlântica populations on taro, mango and kiwifruit, suggesting that these populations had gone through genetic bottlenecks, probably by dispersal of select genotypes in vegetative propagation material. Also, microsatellite markers showed that two microsatellite genotypes from taro are widely spread in Brazil, probably by infected corms.     

Evidence for Natural Selection in the Mitochondrial Genome of Mycosphaerella graminicola

ABSTRACT Pathogenicity assays were combined with restriction fragment length polymorphism (RFLP) markers in the mitochondrial and nuclear genomes to compare Mycosphaerella graminicola populations adapted to bread wheat (Triticum aestivum) and durum wheat (T. turgidum) in the Mediterranean Basin. The majority of isolates had unique nuclear DNA fingerprints and multilocus haplotypes. Only six mitochondrial DNA (mtDNA) haplotypes were identified among 108 isolates assayed. There were minor differences in frequencies of alleles at nuclear RFLP loci between the two host-adapted populations, but differences in the frequencies of mtDNA haplotypes were highly significant (P < 0.0001). mtDNA haplotype 1 dominated on the isolates adapted to bread wheat, and its frequency was twice as high as for the isolates adapted to durum wheat. mtDNA haplotype 4, which contained a unique approximately 3-kb insertion, was detected only in isolates showing specificity toward durum wheat and was the dominant haplotype on this species. We propose that the low mitochondrial diversity in this pathogenic fungus is due to a selective sweep and that differences in the frequencies of mtDNA haplotypes between the two host-adapted populations were due to natural selection according to host species.     

石榴枯萎病菌的遗传多样性研究

 利用随机扩增多态性DNA(RAPD)和小卫星标记(DAMD)技术分析中国石榴枯萎病菌 (Ceratocystis fimbriata)的遗传多样性,并对来自印度、巴西、美国、新西兰、巴布新几内亚等多种寄主的C. fimbriata样品进行扩增,结果显示：所有C. fimbriata在遗传相似系数0.70 (RAPD)和0.60(DAMD)时均被分为4个相似的组群,其中中国石榴枯萎病菌间及其与芋头黑腐病菌(中国)具有一致的同源性(similarity index：1.00),而其它不同寄主来源的分离菌显示出一定的多态性。来自印度的石榴枯萎病菌与来自中国的有明显差异(RAPD,DAMD遗传相似系数分别为0.83和0.71),中国芋头寄主分离菌与巴西芋头分离菌间也存在一定差异。相对于来自印度石榴C. fimbriata来说,来自中国石榴与巴西芋头的分离菌有更近的系统发育关系。所有来自甘薯寄主的C. fimbriata 聚类为一个单独的分支。这表明供试的所有中国石榴菌株间及与中国芋头菌株间均具有一样的基因型,与印度同寄主(石榴)的C. fimbriata基因型存在明显差异。     

Armillaria root rot spreading into a natural woody ecosystem in South Africa

Signs and symptoms of a disease similar to those of armillaria root rot have recently been observed on various native woody plants on the foothills of Table Mountain in South Africa, one of the most botanically diverse natural environments globally. This is of concern because the root rot fungus Armillaria mellea has previously been shown to be an alien pathogen of European origin in planted gardens in the City of Cape Town. An aim of this study was to identify the cause of the root rot disease on infected plants. Based on DNA‐sequence phylogeny, it was shown that isolates collected from at least 16 native tree and woody shrub species represented the non‐native A. mellea. Microsatellite markers were then used to determine the genetic diversity and population structure of the A. mellea isolates from Table Mountain and two planted gardens where the pathogen has previously been found. Population genetic analyses revealed low levels of gene diversity and no population differentiation amongst the three populations. The results provide the first firm evidence that A. mellea has escaped the planted environment and invaded a sensitive and ecologically important natural woody environment in South Africa. This is only the second definitive case of a non‐native tree pathogen invading a natural ecosystem in the country.     

Population Genetic Structure of Septoria passerinii in Northern Great Plains Barley

ABSTRACT The genetic structure of Septoria passerinii from nine field populations was examined at several scales (within lesions, among lesions in a leaf, among leaves in a field, and among fields in North Dakota and western Minnesota) by using amplified fragment length polymorphism (AFLP) markers. A total of 390 isolates were sampled from seven barley fields located in North Dakota and two barley fields located nearby in western Minnesota in 2003 and 2004. Based on 57 polymorphic AFLP markers, AFLP DNA fingerprints identified 176 different genotypes among 390 (non-clone-corrected) isolates in nine different fields. In two intensively sampled sites, ND16 (Williston, ND) and ND17 (Langdon, ND), only one to four different genotypes were found within a lesion. A higher level of genetic and genotypic diversity was found within a leaf in which six to nine different genotypes were found from lesions on a leaf. The genetic diversity within a leaf was similar to the genetic diversity within a field. The average genetic diversity (H) within a field across all AFLP loci was approximately 0.3, except at site ND12 (Carrington, ND) where it was 0.16. Genotypic diversity was high in all populations, and with the exception of ND15 (Rothsay, MN), very low multilocus linkage disequilibrium values ( r(d)) were found in all populations. The population differentiation, G(ST), was relatively high (G(ST) = 0.238) among the nine populations due to the high G(ST) in ND12, ND14 (Twin Valley, MN), and ND15. Population differentiation without those three populations was 0.09. A lack of correlation between geographical distance and genetic distance was found, suggesting the potential for a high level of gene flow between different geographical regions. The population genetic structure described in this study for S. passerinii in North Dakota and western Minnesota is consistent with that of a sexually reproducing fungus.     

Comparison of Crinipellis perniciosa isolates from Brazil by ERIC repetitive element sequence-based PCR genomic fingerprinting

Fifty isolates of Crinipellis perniciosa originating from Theobroma cacao , Heteropterys acutifolia and Solanum lycocarpum , from six states within Brazil, were characterized through ERIC-PCR, representing the first application of this method for molecular characterization within C. perniciosa . Phenetic analysis of banding patterns revealed a separation of isolates on the basis of host of origin, with T. cacao -derived isolates showing only a 0·2 similarity level to a cluster comprising the isolates from H. acutifolia and S. lycocarpum . Considerable intraspecific variability was observed within C. perniciosa isolates from T. cacao , with distinct groups observed correlating with geographical origin. Given that a number of isolates from T. cacao from the Amazon region grouped with isolates from Bahia state, this work discusses the possibility that current C. perniciosa populations pathogenic on T. cacao in Bahia originated from the Amazon region, rather than from alternative host plants.     

Molecular characterization of fungal endophytic morphospecies isolated from stems and pods of Theobroma cacao

Endophytic fungi were isolated from healthy stems and pods of cacao ( Theobroma cacao ) trees in natural forest ecosystems and agroecosystems in Latin America and West Africa. These fungi were collected for screening as a potential source of biocontrol agents for the basidiomycetous pathogens of cacao in South and Central America, Moniliophthora roreri (frosty pod rot) and Moniliophthora perniciosa (witches' broom). Many of these isolates were morphologically unidentifiable as they failed to form fruiting structures in culture, or only produced arthrosporic stages. Affinities with basidiomycetes were suspected for many of these based on colony morphology. Fifty-nine of these morphologically unidentifiable isolates were selected for molecular identification by DNA extraction and sequence analysis of nuclear ribosomal DNA (rDNA). The large subunit (LSU) was chosen for initial sequencing because this region has been used most often for molecular systematics of basidiomycete fungi, and comprehensive LSU datasets were already available for sequence analyses. Results confirmed that the majority of the isolates tested belonged to the Basidiomycota, particularly to corticoid and polyporoid taxa. With LSU data alone, identification of the isolates was resolved at varying taxonomic levels (all to order, most to family, and many to genus). Some of the isolates came from rarely isolated genera, such as Byssomerulius , whilst the most commonly isolated basidiomycetous endophyte was a member of the cosmopolitan genus Coprinellus (Agaricales). The role of these fungi within the host and their potential as biological control agents are discussed.     

Variations in the Phytophthora infestans Population in Nepal as Revealed by Nuclear and Mitochondrial DNA Polymorphisms

ABSTRACT Phytophthora infestans isolates collected from potato and tomato crops from various parts of Nepal during the 1999 and 2000 crop seasons were characterized for nuclear and mitochondrial DNA polymorphisms using restriction fragment length polymorphism markers. The nuclear DNA probe RG57 detected 11 multilocus genotypes among 280 isolates. Three genotypes were detected 21 times or more, constituting 94% of the total population, whereas frequencies of other genotypes ranged from 0.004 to 0.014. The overall genotypic diversity as estimated by the Gleason index was 1.78. Most of the overall diversity was present at the highest level (i.e., interregional, 46%), indicating limited gene flow among regions. Cluster analysis of multilocus genotypes derived from RG57 and mating type data for Nepalese isolates and representative isolates worldwide showed Nepalese isolates grouping into four clusters. Characterization of 67 isolates for mitochondrial DNA polymorphisms revealed the presence of two mt-haplotypes, Ia and Ib with the proportions of 0.88 and 0.12, respectively. Polymorphisms in nuclear and mitochondrial DNA revealed a moderate level of diversity in this population. Genotype NP3 had an identical RG57 fingerprint to US1 and had mt-haplotype Ib, confirming the presence of an old population in Nepal. Most of the genotypes had a different RG57 fingerprint than that of US1 and mt-haplotype Ia, the common characteristics of new populations. The presence of a new population at high proportions in Nepal was consistent with the global trend of mt-haplotype distribution, and suggests the displacement of old populations. This study indicates at least three possible introductions of P. infestans to Nepal.     

Genetic Diversity in Colletotrichum gloeosporioides from Stylosanthes spp. at Centers of Origin and Utilization

ABSTRACT Using molecular markers, this work compares the genetic diversity in Colletotrichum gloeosporioides infecting species of the tropical forage legume Stylosanthes at the center of origin in Brazil and Colombia with that of Australia, China, and India, where Stylosanthes spp. have been introduced for commercial use. There was extensive diversity in the pathogen population from Brazil, Colombia, China, and India. The Australian pathogen population was least diverse probably due to its geographical isolation and effective quarantine. The extensive diversity in China and India means that threats from exotic pathogen races to Stylosanthes pastures can potentially come from countries outside the South American center of origin. In Brazil and India, both with native Stylosanthes populations, a high level of genetic differentiation in the pathogen population was associated with sites where native or naturalized host population was widely distributed. There was limited genetic diversity at germplasm evaluation sites, with a large proportion of isolates having identical haplotypes. This contrasts recent pathogenicity results for 78 of the Brazilian isolates that show hot spots of complex races are more common around research stations where host germplasm are tested, but few are found at sites containing wild host populations. For a pathogen in which the same races arise convergently from different genetic backgrounds, this study highlights the importance of using both virulence and selectively neutral markers to understand pathogen population structure.     

New Populations of Sclerotinia sclerotiorum from Lettuce in California and Peas and Lentils in Washington

ABSTRACT Four populations of Sclerotinia sclerotiorum in North America were inferred previously, based on analyses of both rapidly evolving markers (DNA fingerprint and mycelial compatiblity), and multilocus DNA sequence spanning the range between fast and slow evolution. Each population was defined as an interbreeding unit of conspecific individuals sharing a common recent ancestor and arising in a unique evolutionary event. The present study applies this standard to extend characterization of S. sclerotiorum populations to the Western United States. Isolates of S. sclerotiorum (N = 294) were determined to represent three genetically differentiated populations: California (CA, lettuce), Washington (WA, pea/lentil), and Ontario (ON, lettuce). CA was the most diverse population yet sampled in North America. Clonality was detected in ON and WA. No DNA fingerprints were common among the populations. The index of association (I(A)), based on fingerprint, was closer to zero (0) for CA than it was for the other populations. High diversity and lack of association of markers in California are consistent either with genetic exchange and recombination, or with large population size and high standing genetic variation. Intra- and interlocus conflict among three DNA sequence loci was consistent with recombination. The coalescent IGS genealogy confirmed subdivision and showed CA to be older than WA or ON. The Nearest Neighbor statistic on combined data confirmed subdivision among all present and previously defined populations. All isolates had both MAT1-1 and MAT1-2, consistent with uniform homothallism.     

Phylogeny of the frosty pod rot pathogen of cocoa

Morphological, cytological and molecular evidence is presented which confirms that the frosty pod rot pathogen of cocoa, formerly classified as the mitosporic fungus Moniliophthora roreri (Deuteromycota), belongs to the hymenomycetous genus Crinipellis (Basidiomycota) and that two varieties should now be recognized: Crinipellis roreri var. roreri and the new variety C. roreri var. gileri . The latter was collected on Theobroma gileri , an endemic tree of submontane forests in north-west Ecuador, and can be distinguished from Ecuadorian and Peruvian isolates from cocoa ( T. cacao ) on the basis of spore morphology, incompatibility and nucleotide sequence data. As with var. roreri , meiosis is shown to occur within the dispersive and infective spore stage of var. gileri and these meiospores are interpreted to represent a much modified probasidium. In addition, in a field inoculation experiment, an isolate from T. gileri proved to be noninfective to cocoa pods when compared with positive control strains isolated from T. cacao in western Ecuador and T. bicolor in eastern Ecuador. It is concluded that var. gileri is the vestigial progenitor of the frosty pod rot pathogen of cocoa, with a host range and distribution restricted to T. gileri in the mesic forests of north-west South America.     

Gene Flow and Sexual Reproduction in the Wheat Glume Blotch Pathogen Phaeosphaeria nodorum (Anamorph Stagonospora nodorum)

ABSTRACT Restriction fragment length polymorphisms (RFLPs) were used to characterize the genetic structures of three field populations of Phaeosphaeria nodorum from Texas, Oregon, and Switzerland. Data from seven nuclear RFLP loci were used to estimate gene diversity and genetic distances and to make indirect measures of gene flow between populations. Three of the seven RFLP loci differed significantly in allele frequencies across populations. On average, 96% of the total gene diversity was found within populations. There was little evidence for population subdivision, suggesting that gene flow was not restricted among populations. Based on an average population differentiation of 0.04, we estimated that the exchange of 11 migrants among populations per generation would be needed to account for the present level of population subdivision. Genotype diversity based on DNA fingerprints was at a maximum for the Swiss population, whereas populations in Texas and Oregon had lower genotype diversities. Many multilocus haplotypes were found in each population. Ninety-five percent of RFLP allele pairs were in gametic equilibrium. The data were consistent with random mating within each population.