Epidemic spread of smut fungi (Quambalaria) by sexual reproduction in a native pathosystem

Quambalaria are fungal pathogens of Corymbia, Eucalyptus and related genera of Myrtaceae. They are smut fungi (Ustilaginomycota) described from structures that resemble conidia and conidiophores. Whether these spore forms have asexual or sexual roles in life cycles of Quambalaria is unknown. An epidemic of Q. pitereka destroyed plantations of Corymbia in New South Wales and Queensland (Australia) in 2008. We sampled 177 individuals from three plantations of C. variegata and used AFLPs to test hypotheses that the epidemic was spread by asexual reproduction and dominated by a single genotype. There was high genotypic diversity across ≥600 AFLP loci in the pathogen populations at each plantation, and evidence of sexual reproduction based on neighbour-net analyses and rejection of linkage disequilibrium. The populations were not structured by host or location. Our data did not support a hypothesis of asexual reproduction but instead that Q. pitereka spreads exclusively by sexual reproduction, similar to life cycles of other smut fungi. Epidemics were exacerbated by monocultures of Corymbia established from seed collected from a single provenance. This study showcases an example of an endemic pathogen, Q. pitereka, with a strictly outbreeding life cycle that has caused epidemics when susceptible hosts were planted in large monoculture plantations

Aggressiveness of Phytophthora medicaginis on chickpea: Phenotyping method determines isolate ranking and host genotype-by-isolate interactions

Phytophthora medicaginis causing Phytophthora root rot of chickpea (Cicer arietinum) is an important disease, with genetic resistance using C. arietinum × Cicer echinospermum crosses as the main disease management strategy. We evaluated pathogenic variation in P. medicaginis populations with the aim of improving phenotyping methods for disease resistance. We addressed the question of individual isolate aggressiveness across four different seedling-based phenotyping methods conducted in glasshouses and one field-based phenotyping method. Our results revealed that a seedling media surface inoculation method used on a susceptible C. arietinum variety and a moderately resistant C. arietinum × C. echinospermum backcross detected the greatest variability in aggressiveness among 37 P. medicaginis isolates. Evaluations of different components of resistance, using our different phenotyping methods, revealed that differential pathogen–isolate reactions occur with some phenotyping methods. We found support for our hypotheses that the level of aggressiveness of P. medicaginis isolates depends on the phenotyping method, and that phenotyping methods interact with both isolate and host genotype reactions. Our cup-based root inoculation method showed promise as a non-field-based phenotyping method, as it provided significant correlations with genotype–isolate rankings in the field experiment for a number of disease parameters

The Vulnerability of Bananas to Globally Emerging Disease Threats

Banana cultivation has increased significantly over the last century to meet the growing demand for this popular fruit. Originating in Southeast Asia, bananas are now produced in >135 different countries in tropical and subtropical regions of the world. Most of this expansion of production is based on a single banana variety, Cavendish, which makes up almost all the export trade grown in large-scale monocultures and a large part of the local trade and represents >40% of all bananas grown globally. Over the last century several major diseases of the banana have emerged and widely expanded their geographic ranges. Cultivars within the Cavendish variety are highly susceptible to these diseases, including yellow Sigatoka, black leaf streak, Eumusae leaf spot, freckle, Fusarium wilt tropical race 4, banana bunchy top, and the bacterial wilts Moko, Xanthomonas wilt, and banana Blood disease. This review graphically illustrates the emergence and rapid intercontinental spread of these diseases and discusses several major disease epidemics in bananas. Evidently, the large-scale monoculture based on the single variety Cavendish has resulted in an extreme level of genetic vulnerability. The resistance to diversification in the Cavendish production chain and the lack of investment in genetics and plant breeding in the recent past means that currently limited genetic solutions are available to replace the Cavendish banana with a set of market acceptable resistant varieties from a range of different genetic backgrounds

Pathogenic variation of Alternaria species associated with leaf blotch and fruit spot of apple in Australia

Four Alternaria species groups (A. longipes, A. arborescens, A. alternata/A. tenuissima and A. tenuissima/A. mali) are associated with leaf blotch and fruit spot of apple in Australia. There is no information on the variability of pathogenicity among the species and isolates within each species causing leaf blotch or fruit spot. We used a detached leaf assay and an in planta fruit inoculation assay to determine the pathogenicity and virulence of the four Alternaria species. Our results showed that isolates within the same species were not specific to either leaf or fruit tissue and showed great variability in pathogenicity and virulence, indicating cross-pathogenicity, which may be isolate dependent rather than species dependent. Generally, virulence of A. tenuissima and A. alternata isolates on leaf and fruit was higher than other species. Isolates of all species groups were pathogenic on leaves of different cultivars, but pathogenicity on fruit of different cultivars varied among isolates and species. Implications of our findings on prevalence of the diseases in different apple-producing regions in Australia and the development of targeted disease management of the diseases are discusse

Fungal clones win the battle, but recombination wins the war

Clonal reproduction is common in fungi and fungal-like organisms during epidemics and invasion events. The success of clonal fungi shaped systems for their classification and some pathogens are tacitly treated as asexual. We argue that genetic recombination driven by sexual reproduction must be a starting hypothesis when dealing with fungi for two reasons: (1) Clones eventually crash because they lack adaptability; and (2) fungi find a way to exchange genetic material through recombination, whether sexual, parasexual, or hybridisation. Successful clones may prevail over space and time, but they are the product of recombination and the next successful clone will inevitably appear. Fungal pathogen populations are dynamic rather than static, and they need genetic recombination to adapt to a changing environment

AFLP linkage map of the Oomycete Phytophthora infestans

Here we present the first comprehensive genetic linkage map of the heterothallic oomycetous plant pathogen Phytophthora infestans. The map is based on polymorphic DNA markers generated by the DNA fingerprinting technique AFLP (Vos et al., 1995, Nucleic Acids Res. 23: 4407-4414). AFLP fingerprints were made from single zoospore progeny and 73 F1 progeny from two field isolates of P. infestans. The parental isolates appeared to be homokaryotic and diploid, their AFLP patterns were mitotically stable, and segregation ratios in the F1 progeny were largely Mendelian. In addition to 183 AFLP markers, 7 RFLP markers and the mating type locus were mapped. The linkage map comprises 10 major and 7 minor linkage groups covering a total of 827 cM. The major linkage groups are composed of markers derived from both parents, whereas the minor linkage groups contain markers from either the A1 or the A2 mating type parent. Non-Mendelian segregation ratios were found for the mating type locus and for A3 AFLP markers, all of which are located on the same linkage group as the mating type locus. (C) 1997 Academic Press