Over the past few years, symptoms akin to late blight disease have been reported on a variety of crop plants in South America. Despite the economic importance of these crops, the causal agents of the diseases belonging to the genus Phytophthora have not been completely characterized. In this study, a new Phytophthora species was described in Colombia from tree tomato (Solanum betaceum), a semi-domesticated fruit grown in northern South America. Comprehensive phylogenetic, morphological, population genetic analyses, and infection assays to characterize this new species, were conducted. All data support the description of the new species, Phytophthora betacei sp. nov. Phylogenetic analyses suggest that this new species belongs to clade 1c of the genus Phytophthora and is a close relative of the potato late blight pathogen, P. infestans. Furthermore, it appeared as the sister group of the P. andina strains collected from wild Solanaceae (clonal lineage EC-2). Analyses of morphological and physiological characters as well as host specificity showed high support for the differentiation of these species. Based on these results, a complete description of the new species is provided and the species boundaries within Phytophthora clade 1c in northern South America are discussed.

BackgroundPhytophthora infestans (Mont.) de Bary, the causal agent of potato late blight, is responsible for tremendous crop losses worldwide. Countries in the northern part of the Andes dedicate a large proportion of the highlands to the production of potato, and more recently, solanaceous fruits such as cape gooseberry (Physalis peruviana) and tree tomato (Solanum betaceum), all of which are hosts of this oomycete. In the Andean region, P. infestans populations have been well characterized in Ecuador and Peru, but are poorly understood in Colombia and Venezuela. To understand the P. infestans population structure in the Northern part of the Andes, four nuclear regions (ITS, Ras, β-tubulin and Avr3a) and one mitochondrial (Cox1) region were analyzed in isolates of P. infestans sampled from different hosts in Colombia and Venezuela.ResultsLow genetic diversity was found within this sample of P. infestans isolates from crops within several regions of Colombia and Venezuela, revealing the presence of clonal populations of the pathogen in this region. We detected low frequency heterozygotes, and their distribution patterns might be a consequence of a high migration rate among populations with poor effective gene flow. Consistent genetic differentiation exists among isolates from different regions.ConclusionsThe results here suggest that in the Northern Andean region P. infestans is a clonal population with some within-clone variation. P. infestans populations in Venezuela reflect historic isolation that is being reinforced by a recent self-sufficiency of potato seeds. In summary, the P. infestans population is mainly shaped by migration and probably by the appearance of variants of key effectors such as Avr3a.

Foliar virulence of Phytophthora infestans isolates to two Solanum betaceum cultivars, Red and Yellow, was characterized using a detached leaf assay. Six P. infestans sensu lato isolates from S. betaceum belonging to at least two different populations were included (representing the EC-3 clonal lineage and a population that was not previously reported). Three isolates from potato (representing the EC-1 clonal lineage) were included in order to determine their ability to attack S. betaceum cultivars and whether there was difference in virulence between these and isolates from S. betaceum. Significant variation for virulence parameters was found between isolates, isolateorigins, and cultivars. Infection frequency, sporulation intensity, and incubation period allowed differentiation of EC-1 isolates from S. betaceum isolates. Isolates infecting both cultivars exhibited significantly higher values of infection frequency and sporulation intensity and lower latent period values. Cv. Red showed significantly higher values for area under the lesion expansion curve (AULEC), and final lesion size (FLS) and lower values for incubation period (IP) in comparison with cv. Yellow. These results demonstrated that there are two separate populations of P. infestans attacking S. betaceum each with specific virulence towards two different cultivars of Solanum betaceum. Key words: detached leaves assay, EC-1 clonal lineage, EC-3 clonal lineage, late blight, tree tomato. RESUMO Virulência foliar de isolados de Phytophthora infestans sensu lato em folhas destacadas de duas cultivares de Solanum betaceumA agressividade dos isolados de Phytophthora infestans sensu lato obtidos de duas cultivares de Solanum betaceum, Vermelha e Amarela, foi caracterizada em bioensio com folhas destacadas. Seis isolados de S. betaceum pertecendo a pelo menos duas populações diferentes foram incluídos (representando a linhagem clone EC-3 e uma população ainda não reportada). Três isolados obtidos de batata também foram incluídos para determinar se estes poderiam atacar cultivares de S. betaceum e se haveria diferença na agressividade entre isolados, origem dos isolados e cultivares. Freqüência de infecção, densidade populacional e o período de incubação permitiram a diferenciação do isolado EC-1 a partir dos isolados de S. betaceum. Isolados capazes de infectar os dois cultivares exibiram maior frequencia de infecção e densidade populacional e o menor período de latência. Um dos três isolados da população EC-1 não infectou folhas de S. betaceum cv. Vermelha, apresentando o maior valor de freqüência de infecção, maior área abaixo da curva de progresso da expansão de lesão (AACPEL) e tamanho final da lesão e o menor período de incubação. Estes resultados demonstraram a existência de duas populações distintas de P. infestans atacando S. betaceum com agressividades especificas para cada cultivar de Solanum betaceum. Palavras-chave: agressividade foliar, bioensaio com folhas destacadas, linhagem clone EC-1, linhagem clone EC-3, requeima, tamarillo.

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.

Phytophthora infestans, es el agente causal de una de las enfermedades más limitantes de cultivos de papa (Solanum tuberosum) en el departamento de Nariño causando que los agricultores utilicen de forma intensiva diferentes fungicidas, llegando a representar el 30% de los costos de producción, siendo un grave problema ambiental y de salud pública. El propósito de esta investigación es apoyar los programas de manejo integrado de enfermedades a partir de la generación de una línea base de sensibilidad de 20 aislamientos de P. infestans del departamento de Nariño frente a tres fungicidas sistémicos comerciales compuestos por Metalaxil 8% + Mancozeb 64%, Cimoxanil 6% + Propineb 70% y Fluopicolide 6% + Propineb 66,7%. Este estudio se llevó a cabo siguiendo la metodología in vitro utilizando cinco dosis basadas en una escala exponencial. Los resultados mostraron un comportamiento heterogéneo donde algunos aislamientos presentaron resistencia al fungicida compuesto por Metalaxil + Mancozeb, alcanzando valores promedio de EC50 de 6,67 µg mL-1, mientras que para los fungicidas compuestos por Cimoxanil + Propineb y Fluopicolide + Propineb se presentaron niveles de sensibilidad intermedia y sensibles con valores de EC50 promedio de 1,57 µg mL-1 y 2,61 µg mL-1. Por otra parte, la tasa de esporulación se redujo en su totalidad para los tres fungicidas cuando se evaluaron las dosis de 100, 1.000, 10.000 µg mL-1.