Recent advances in cucurbit downy mildew research and their contribution to the development of integrated protection of cucurbits

Knowledge of the recent distribution, host range, virulence, DNA profiles, and resistance to fungicides of Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev, and sources of host resistance to this pathogen are reviewed (Lebeda and Cohen, 2011; 2012; Cohen et al., 2015). P. cubensis is widely distributed in all continents, with a high capacity to adapt to changing environmental conditions and new regions. Its host-plant range includes more than 60 species and 20 genera of Cucurbitaceae (Lebeda and Schwinn, 1994; Lebeda and Cohen, 2011). P. cubensis exhibits clear host specialization, with shifts in host range and an increasing spectrum of new host species (Lebeda et al., 2011)

Response of Cucumis melo accessions to isolates of Pseudoperonospora cubensis with different levels of virulence

Melon (Cucumis melo) is seriously affected by Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew. Within C. melo, several genotypes resistant to P. cubensis have been described; however, we lack detailed experimental studies focused on deciphering variation in this host-pathogen interaction. The response of 115 accessions of C. melo to 8 isolates of P. cubensis was studied under controlled conditions. Pathogen isolates with low, medium and high virulence represented 8 distinct pathotypes. The virulence of isolates expressed by their published pathotypes conformed poorly to their virulence as observed on the C. melo accessions in this experiment. Collectively, 45 different reaction patterns were recorded. Most C. melo accessions (67) were highly susceptible to all isolates. Two accessions of C. melo subsp. agrestis (PI 614174 and PI 614442) were incompletely resistant to all isolates. The reaction of the remaining 46 accessions was race-specific. None of screened C. melo accessions expressed complete resistance to all isolates. Accession PI 315410 from India was resistant to 5 isolates and incompletely resistant to 2 others. The reduced development of at least one P. cubensis isolate was recorded on leaf discs of 17 accessions. Accessions with incomplete resistance to at least one isolate originated most frequently from India, Zimbabwe and Zambia. The study: a) confirmed the race-specificity of interactions between C. melo and P. cubensis, b) revealed C. melo subsp. agrestis as a taxon with potential field resistance to a number of races P. cubensis, and c) showed that within C. melo populations from India and southern Africa there are accessions completely or incompletely resistant to European races of P. cubensis

Genetic variability and distance between Lactuca serriola L. populations from Sweden and Slovenia assessed by SSR and AFLP markers

The study involved 121 samples of the common weed, Lactuca serriola L. (prickly lettuce), representing 53 populations from Sweden and Slovenia. The seed materials, originating from different habitats, were regenerated and taxonomically validated at the Department of Botany, Palacký University in Olomouc, Czech Republic. The morphological characterizations of the collected plant materials classified all 121 samples as L. serriola f. serriola; one sample was heterogeneous, and also present was L. serriola f. integrifolia. Differences in the amount and distribution of the genetic variations between the two regions were analyzed using 257 amplified fragment length polymorphism (AFLP) and 7 microsatellite (SSRs) markers. Bayesian clustering and Neighbor- Network were used for visualization of the differences among the samples by country. Under the Bayesian approach, the best partitioning (according to the most frequent signals) was resolved into three groups. While the absence of an admixture or low admixture was detected in the Slovenian samples, and the majority of the Swedish samples, a significant admixture was detected in the profiles of five Swedish samples collected near Malmö, which bore unique morphological features of their rosette leaves. The Neighbor-Network analysis divided the samples into 6 groups, each consisting of samples coming from a particular country. Reflection of morphology and eco-geographical conditions in genetic variation are also discussed

Genetic variability and distance between Lactuca serriola L. populations from Sweden and Slovenia assessed by SSR and AFLP markers - Online supplement

The study involved 121 samples of the common weed, Lactuca serriola L. (prickly lettuce), representing 53 populations from Sweden and Slovenia. The seed materials, originating from different habitats, were regenerated and taxonomically validated at the Department of Botany, Palacký University in Olomouc, Czech Republic. The morphological characterizations of the collected plant materials classified all 121 samples as L. serriola f. serriola; one sample was heterogeneous, and also present was L. serriola f. integrifolia. Differences in the amount and distribution of the genetic variations between the two regions were analyzed using 257 amplified fragment length polymorphism (AFLP) and 7 microsatellite (SSRs) markers. Bayesian clustering and Neighbor- Network were used for visualization of the differences among the samples by country. Under the Bayesian approach, the best partitioning (according to the most frequent signals) was resolved into three groups. While the absence of an admixture or low admixture was detected in the Slovenian samples, and the majority of the Swedish samples, a significant admixture was detected in the profiles of five Swedish samples collected near Malmö, which bore unique morphological features of their rosette leaves. The Neighbor-Network analysis divided the samples into 6 groups, each consisting of samples coming from a particular country. Reflection of morphology and eco-geographical conditions in genetic variation are also discussed

Distribution and ecology of wild lettuces Lactuca serriola L. and Lactuca virosa L. in central Chile

Wild lettuces (Lactuca L.) may provide valuable genetic resources for crop breeding, but are also significant invasive weeds. Monitoring populations of these species is important, therefore, both to inform genetic resource conservation efforts as well as for non-native plant species control. We explored the occurrences, distributions, and characteristics of populations of wild Lactuca species in central Chile through field surveys in 2016 and 2017, significantly adding to the available information on these species in the region. We documented two species - prickly lettuce (Lactuca serriola L.) and opium (bitter) lettuce (Lactuca virosa L.) in 204 localities. Occurrence only of L. serriola was recorded at 166 sites, only L. virosa at 21 sites, and both at 17 sites. Prickly lettuce was observed equally in urban areas and along transport corridors outside cities. Population sizes varied from a few plants to hundreds of individuals. Opium lettuce was recorded most frequently along roads outside cities, with population sizes of several individuals to a dozen. Species succession in different elevational gradients was observed along three roads: R 60 (Los Andes – Portillo), G-27 (Cajón Maipú), and 115 (Lago Colbún – Lago del Maule). These observations indicate that: i) both allochthonous (Euroasian) wild Lactuca species occur and are able to regenerate in central Chile; ii) L. serriola forms dense populations in urbanized areas; iii) both species can expand along transport corridors to high elevations; iv) the spread of L. virosa and persistence of dense populations in elevations above 2,000 m a.s.l. prove the invasiveness of this species in extreme climates; v) both species should be monitored (and treated) both in urbanized and agricultural areas, as well as in unique local mountainous areas

Research gaps and challenges in the conservation and use of North American wild lettuce germplasm

The North American crop wild relatives (CWR) of lettuce (Lactuca L.) represent an underexplored pool of genetic diversity of potential value to breeding programs. The 10 species belong to three different groups: a native clade including at least six allotetraploid species [L. biennis (Moench) Fernald, L. canadensis L., L. floridana (L.) Gaertn., L. graminifolia Michx., L. hirsuta Muhl. ex Nutt., and L. ludoviciana (Nutt.) Riddell], a diploid clade with one species [L. tatarica (L.) C. A. Mey. subsp. pulchella (Pursh) Stebbins], and a clade related to the cultivated taxon (L. sativa L.) with three non‐native species (L. saligna L., L. serriola L., and L. virosa L.). In this review, we examine the role of herbarium and genebank holdings in taxonomic and other foundational studies, as well as for germplasm exploration and use. We compile the state of knowledge on the ranges of lettuce CWR in North America, modeling the potential distributions of the species and assessing their ex situ and (for native species) in situ conservation status. We categorize seven of the species as high priority for further conservation and three as medium priority, with none currently considered low priority or sufficiently conserved. Further, we review morphological, phenological, genetic diversity, and pest and disease information with regard to North American species. We conclude by outlining the critical gaps and describing a way forward for addressing challenges in the conservation and use of North American wild lettuce germplasm

Recent advances in cucurbit downy mildew research and their contribution to the development of integrated protection of cucurbits

Knowledge of the recent distribution, host range, virulence, DNA profiles, and resistance to fungicides of Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev, and sources of host resistance to this pathogen are reviewed (Lebeda and Cohen, 2011; 2012; Cohen et al., 2015). P. cubensis is widely distributed in all continents, with a high capacity to adapt to changing environmental conditions and new regions. Its host-plant range includes more than 60 species and 20 genera of Cucurbitaceae (Lebeda and Schwinn, 1994; Lebeda and Cohen, 2011). P. cubensis exhibits clear host specialization, with shifts in host range and an increasing spectrum of new host species (Lebeda et al., 2011).This is a proceeding published as Aleš Lebeda, Eva Křístková, Božena Sedláková, Mark P. Widrlechner. Recent advances in cucurbit downy mildew research and their contribution to the development of integrated protection of cucurbits. IOBC‐WPRS Bulletin Vol. 142, 2019, pp. 1‐4. From Proceedings of the Meeting at Wädenswil/Arenenberg (Switzerland), 02-06 October, 2017. Posted with permission.</p

Response of Cucumis melo accessions to isolates of Pseudoperonospora cubensis with different levels of virulence

Melon (Cucumis melo) is seriously affected by Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew. Within C. melo, several genotypes resistant to P. cubensis have been described; however, we lack detailed experimental studies focused on deciphering variation in this host-pathogen interaction. The response of 115 accessions of C. melo to 8 isolates of P. cubensis was studied under controlled conditions. Pathogen isolates with low, medium and high virulence represented 8 distinct pathotypes. The virulence of isolates expressed by their published pathotypes conformed poorly to their virulence as observed on the C. melo accessions in this experiment. Collectively, 45 different reaction patterns were recorded. Most C. melo accessions (67) were highly susceptible to all isolates. Two accessions of C. melo subsp. agrestis (PI 614174 and PI 614442) were incompletely resistant to all isolates. The reaction of the remaining 46 accessions was race-specific. None of screened C. melo accessions expressed complete resistance to all isolates. Accession PI 315410 from India was resistant to 5 isolates and incompletely resistant to 2 others. The reduced development of at least one P. cubensis isolate was recorded on leaf discs of 17 accessions. Accessions with incomplete resistance to at least one isolate originated most frequently from India, Zimbabwe and Zambia. The study: a) confirmed the race-specificity of interactions between C. melo and P. cubensis, b) revealed C. melo subsp. agrestis as a taxon with potential field resistance to a number of races P. cubensis, and c) showed that within C. melo populations from India and southern Africa there are accessions completely or incompletely resistant to European races of P. cubensis.This is a manuscript of an article published as Lebeda, Aleš, Eva Křístková, Jana Štěpánková, Božena Sedláková, and Mark P. Widrlechner. "Response of Cucumis melo accessions to isolates of Pseudoperonospora cubensis with different levels of virulence." Scientia horticulturae 200 (2016): 45-54. doi: 10.1016/j.scienta.2015.09.028. Posted with permission.</p

Razširjenost in ekologija divjih vrst solate Lactuca serriola L. in Lactuca virosa L. v centralnem Čilu

Wild lettuces (Lactuca L.) may provide valuable genetic resources for crop breeding, but are also significant invasive weeds. Monitoring populations of these species is important, therefore, both to inform genetic resource conservation efforts as well as for non-native plant species control. We explored the occurrences, distributions, and characteristics of populations of wild Lactuca species in central Chile through field surveys in 2016 and 2017, significantly adding to the available information on these species in the region. We documented two species - prickly lettuce (Lactuca serriola L.) and opium (bitter) lettuce (Lactuca virosa L.) in 204 localities. Occurrence only of L. serriola was recorded at 166 sites, only L. virosa at 21 sites, and both at 17 sites. Prickly lettuce was observed equally in urban areas and along transport corridors outside cities. Population sizes varied from a few plants to hundreds of individuals. Opium lettuce was recorded most frequently along roads outside cities, with population sizes of several individuals to a dozen. Species succession in different elevational gradients was observed along three roads: R 60 (Los Andes – Portillo), G-27 (Cajón Maipú), and 115 (Lago Colbún – Lago del Maule). These observations indicate that: i) both allochthonous (Euroasian) wild Lactuca species occur and are able to regenerate in central Chile; ii) L. serriola forms dense populations in urbanized areas; iii) both species can expand along transport corridors to high elevations; iv) the spread of L. virosa and persistence of dense populations in elevations above 2,000 m a.s.l. prove the invasiveness of this species in extreme climates; v) both species should be monitored (and treated) both in urbanized and agricultural areas, as well as in unique local mountainous areas