Differential Susceptibility of Tree Tomato (Solanum betaceum) Cultivars to Late Blight Caused by Phytophthora betacei.

Host-pathogen interactions of a new species of Phytophthora, causal agent of late blight of tree tomato (Solanum betaceum Cav.), identified as Phytophthora betacei, were investigated with four different cultivars. Thirty-six P. betacei isolates, collected from southern Colombia between 2008 and 2009, were used to inoculate common tree tomato cultivars, Común, Híbrido, Injerto, and Holandés. Data on incubation and latent periods as well as infection efficiency, lesion development, and total sporulation were collected via detached leaf assays. Significant differences in susceptibility, based on the parameters measured, were observed. Común was the most susceptible cultivar, followed by Injerto, Híbrido, and Holandés. The mean incubation period was lowest for Común at 125.6 h post-inoculation (hpi) and highest for Híbrido at 139.4 hpi. No significant differences in latent period were observed. All 36 isolates produced necrotic lesions on Común, and 33, 24, and 21 caused infection on Injerto, Híbrido, and Holandés, respectively. Two isolates were able to cause infection only on Común, and 13 isolates were able to infect all four cultivars. Infection efficiency was significantly higher for the cultivar Común, followed by Injerto, Híbrido, and Holandés. Average lesion size was larger on Común than on any other cultivar. An inverse relationship of lesion size and total sporulation was observed. Común had significantly lower total sporulation than Híbrido and Holandés, which had the smallest average lesion sizes. These data show variation in pathogenicity of P. betacei isolates, under controlled conditions, and differential susceptibility of four distinct S. betaceum cultivars.

Geographic Differentiation and Population Genetic Structure of Moniliophthora roreri in the Principal Cocoa Production Areas in Colombia.

Frosty pod rot (FPR) disease on cocoa, caused by Moniliophthora roreri, is one of the most devastating cocoa disease in the Western Hemisphere. In Colombia, the disease is particularly severe in the Magdalena Valley, which is considered the possible center of origin for the pathogen species. We analyzed the genetic diversity of isolates from the departments of Santander, Antioquia, Tolima, and Huila in Colombia using 23 simple-sequence repeats (SSR) markers. In total, 117 different multilocus genotypes were found among 120 isolates, each one representing a unique haplotype. High mutation rates in the SSR and gene flow can explain the high levels of diversity. Also, the observed and standardized indexes of association (IA and rd) indicate that the populations of M. roreri are clonal. Furthermore, given the high haplotype diversity and the significant linkage disequilibrium observed, we hypothesize that M. roreri could be a primarily asexual species undergoing sporadic recombination or partial recombination through parasexuality. A Bayesian clustering analysis implemented by STRUCTURE showed that the most probable number of genetic groups in the data was three, confirming the geographical differentiation among isolates. Similar results were obtained by a discriminant analysis of principal components, a principal coordinate analysis, and a neighbor-joining tree from microsatellite loci base on Nei distance. Cacao genotypes and environmental variables did contribute to the genetic differentiation of the groups. We discuss how this information could be used to improve the management of FPR at the regional level.

Speciation in fungal and oomycete plant pathogens.

The process of speciation, by definition, involves evolution of one or more reproductive isolating mechanisms that split a single species into two that can no longer interbreed. Determination of which processes are responsible for speciation is important yet challenging. Several studies have proposed that speciation in pathogens is heavily influenced by host-pathogen dynamics and that traits that mediate such interactions (e.g., host mobility, reproductive mode of the pathogen, complexity of the life cycle, and host specificity) must lead to reproductive isolation and ultimately affect speciation rates. In this review, we summarize the main evolutionary processes that lead to speciation of fungal and oomycete plant pathogens and provide an outline of how speciation can be studied rigorously, including novel genetic/genomic developments.

An ephemeral sexual population of Phytophthora infestans in the Northeastern United States and Canada.

Phytophthora infestans, the causal agent of late blight disease, has been reported in North America since the mid-nineteenth century. In the United States the lack of or very limited sexual reproduction has resulted in largely clonal populations of P. infestans. In 2010 and 2011, but not in 2012 or 2013, 20 rare and diverse genotypes of P. infestans were detected in a region that centered around central New York State. The ratio of A1 to A2 mating types among these genotypes was close to the 50∶50 ratio expected for sexual recombination. These genotypes were diverse at the glucose-6-phosphate isomerase locus, differed in their microsatellite profiles, showed different banding patterns in a restriction fragment length polymorphism assay using a moderately repetitive and highly polymorphic probe (RG57), were polymorphic for four different nuclear genes and differed in their sensitivity to the systemic fungicide mefenoxam. The null hypothesis of linkage equilibrium was not rejected, which suggests the population could be sexual. These new genotypes were monomorphic in their mitochondrial haplotype that was the same as US-22. Through parentage exclusion testing using microsatellite data and sequences of four nuclear genes, recent dominant lineages US-8, US-11, US-23, and US-24 were excluded as possible parents for these genotypes. Further analyses indicated that US-22 could not be eliminated as a possible parent for 14 of the 20 genotypes. We conclude that US-22 could be a parent of some, but not all, of the new genotypes found in 2010 and 2011. There were at least two other parents for this population and the genotypic characteristics of the other parents were identified.