Phytophthora infestans populations can differ in composition as a result of host-specialisation on tomato and potato hosts. In Great Britain many amateur gardeners grow outdoor tomatoes but there is little or no commercial tomato production outdoors. This study analysed isolates of P. infestans from British gardens with twelve multiplexed SSR markers that are used to monitor the disease on commercial potato crops. Samples of P. infestans from tomato hosts were collected in three years and from potato in one year from across Great Britain. Seven previously unreported genotypes were detected in garden populations and higher frequencies of unique clonal lineages (28% to 40%) were present compared with populations from British commercial potato crops reported elsewhere. Garden populations had a lower proportion (11% to 48% less) of the most common lineages (13_A2 and 6_A1) which together made up at least 86% of the commercial potato populations during the sampling Accepted ArticleThis article is protected by copyright. All rights reserved.period. Host species accounted for only 2.6% of molecular variance detected between garden potatoand tomato-hosted samples. No significant difference in clonal lineage composition was found between host species in Great Britain and this could be due to the whole P. infestans population overwintering on potato. British garden populations on both hosts were much more diverse than those on commercial potato crops; this finding may be influenced by less frequent fungicide use by gardeners and a higher diversity of un-sprayed susceptible potato cultivars enabling metalaxyl-sensitive and less virulent genotypes to survive in gardens.

Tomatoes are a major global food staple but Phytophthora infestans (an Oomycete) causes lateblight, a devastating disease that precludes commercial tomato production from moist temperate areas such as the United Kingdom and Northern Europe. We dissected the genetic architecture of resistance to lateblight as well as traits that improve yield and fruit quality in a tomato cross between a popular breeding, line NC 2 CELBR, which produces large fruits, and an heirloom cultivar called 'Koralik' which produces small, sweet fruits. We used an F 2 mapping population to identify quantitative trait loci (QTL) for phenotypes including number of fruits, size of fruits, total crop yield, and soluble solids content in two different environments. Surprisingly, we found very few QTLs shared between the two environments, underscoring the importance of the local environment and genotypeby-environment interactions. We also assayed the virulence of three different isolates of P. infestans to identify QTLs that confer some resistance to the pathogen. We found nine crop-related QTLs and two QTLs for late-blight resistance-related phenotypes. One late-blight resistance QTL was inherited from Koralik (Chromosome 11, 70.2-83.5 cM) and it probably represents an undiscovered source of late-blight resistance. Yield QTLs were also located on chromosome 11 where Koralik alleles increase fruit number and yield, and adjacent regions decrease fruit size. On Chromosome 9, Koralik alleles increase fruit sweetness (Brix) by 25%. These results indicate that Koralik is a valuable donor parent that can be used by tomato breeders in targeted breeding strategies for fresh market tomatoes.