No abstract

Cucurbit downy mildew, caused by the oomycete Pseudoperonospora cubensis, is a devastating, worldwide-distributed disease of cucurbit crops in the open field and under cover. This review provides recent data on the taxonomy, biology, ecology, host range, geographic distribution and epidemiology of P. cubensis. Special attention is given to host-pathogen interactions between P. cubensis and its economicallyimportant cucurbit hosts (Cucumis sativus, C. melo, Cucurbita pepo, C. maxima, and Citrullus lanatus); pathogenic variability in P. cubensis at the species, genus, and population levels; and, differentiation of pathotypes and races. Genetics and variability of host resistance and cellular and molecular aspects of such resistance are considered. A focus is given to methods of crop protection, including prevention and agrotechnical aspects, breeding for resistance-classical and transgenic approaches, chemical control and fungicide resistance. Novel technologies in biological and integrated control are also discussed. This review also summarizes the most important topics for future research and international collaboration.

No abstract

Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past *50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance. Keywords

No abstract

In this review, we present a critical analysis of the current status of wild Lactuca L. germplasm in relation to its utility for lettuce breeding. We discuss wild Lactuca germplasm in ex situ collections from the perspectives of taxonomy, biogeography, biology and ecology, gene pools, field exploration and acquisition, descriptor development, characterization and evaluation, and enhancement. Future research and other activities related to wild Lactuca germplasm and their continued exploitation in lettuce breeding are considered.

The downy mildew pathogen, Pseudoperonospora cubensis, which infects plant species in the family Cucurbitaceae, has undergone major changes during the last decade. Disease severity and epidemics are far more destructive than previously reported, and new genotypes, races, pathotypes, and mating types of the pathogen have been discovered in populations from around the globe as a result of the resurgence of the disease. Consequently, disease control through host plant resistance and fungicide applications has become more complex. This resurgence of P. cubensis offers challenges to scientists in many research areas including pathogen biology, epidemiology and dispersal, population structure and population genetics, host preference, host-pathogen interactions and gene expression, genetic host plant resistance, inheritance of host and fungicide resistance, and chemical disease control. This review serves to summarize the current status of this major pathogen and to guide future management and research efforts within this pathosystem.

Changes in the genetic structure of the lettuce crop ( Lactuca sativa ) and populations of the lettuce downy mildew pathogen ( Bremia lactucae ) in Germany from 1974 to 1997 were studied and the genetic and population genetic results of race surveys of B. lactucae were reinterpreted. Data on 176 B. lactucae isolates collected from L. sativa cultivars at 48 sites in 11 German federal states were analysed. The host population contained 12 well-defined race-specific resistance genes ( Dm 1, Dm 2, Dm 3, Dm 4, Dm 5/8, Dm 6, Dm 7, Dm 11, R12, Dm 13, Dm 16 and R18). However, only some of these genes (e.g. Dm 3, Dm 7, Dm 11, Dm 16 and R18) played an important role in management of resistance in the host crop. Substantial changes in the frequencies of these R-genes were recorded over the period as a reaction to the occurrence of new virulent phenotypes in the pathogen population. Generally, the durability of resistance was very short. However, distinct regional patterns were observed for the frequencies of different virulence (v) factors. The data on changes in the virulence of B. lactucae population exemplified well the process of host-pathogen coevolution. During the period studied, there was a considerable increase in the frequency of some v-factors (v2, v3, v5/8, v6, v7, v11 and v16) and in the complexity of v-phenotypes, while the diversity in the pathogen population in general decreased. In 1996/97, 12 of the 14 v-factors evaluated reached frequencies higher than 0·90 in the pathogen population. Relatively low frequencies were observed only for factors v14 (0·17) and v18 (0·24). The presence of 23 v-factors was studied in six German isolates virulent on lettuce cultivar Titan, possessing the resistance gene R36 derived from Lactuca saligna and introduced in German lettuce crops during the mid-1990s. Isolate DEG2 showed the highest complexity of v-phenotype; it overcame the resistance in cv. Mariska (R18) and line CS-RL (R18+?), but some sporulation occurred also on cv. Titan ( Dm 6 + R36). Conversely, the isolates SR2 and SAW1 overcame resistance based on R36, but were avirulent to R18. Future utilization of R-genes in commercial lettuce cultivars is proposed; R-genes R18 and R36 could be considered most efficient for this purpose. However, they may be effective only in certain regions.