The cultivation and production of cucumber are seriously affected by downy mildew caused by Pseudoperonospora cubensis. Downy mildew damages leaves, stems and inflorescences, and then reduces the yield and quality of cucumber. This review summarized the research advances in cucumber downy mildew, including pathogen detection and defense pathways, regulatory factors, mining of pathogens-resistant candidate genes, proteomic and genomic analysis, and development of QTL remarks. This review may facilitate clarifying the resistance mechanisms of cucumber to downy mildew.

Cucurbits (Cucurbitaceae family) include many economically important fruit vegetable crops such as watermelon, pumpkin/squash, cucumber, and melon. Seed size (SS) is an important trait in cucurbits breeding, which is controlled by quantitative trait loci (QTL). Recent advances have deciphered several signaling pathways underlying seed size variation in model plants such as Arabidopsis and rice, but little is known on the genetic basis of SS variation in cucurbits. Here we conducted literature review on seed size QTL identified in watermelon, pumpkin/squash, cucumber and melon, and inferred 14, 9 and 13 consensus SS QTL based on their physical positions in respective draft genomes. Among them, four from watermelon (ClSS2.2, ClSS6.1, ClSS6.2, and ClSS8.2), two from cucumber (CsSS4.1 and CsSS5.1), and one from melon (CmSS11.1) were major-effect, stable QTL for seed size and weight. Whole genome sequence alignment revealed that these major-effect QTL were located in syntenic regions across different genomes suggesting possible structural and functional conservation of some important genes for seed size control in cucurbit crops. Annotation of genes in the four watermelon consensus SS QTL regions identified genes that are known to play important roles in seed size control including members of the zinc finger protein and the E3 ubiquitin-protein ligase families. The present work highlights the utility of comparative analysis in understanding the genetic basis of seed size variation, which may help future mapping and cloning of seed size QTL in cucurbits.

The complete nucleotide sequences of Beet pseudoyellows virus (BPYV)-MI (cucumber isolate; Matsuyama, Idai) genomic RNAs 1 and 2 were determined and compared with the previously sequenced Japanese cucumber strain (BPYV-JC) and a strawberry strain (BPYV-S). The RNA 2 of BPYV-MI showed 99 % nucleotide sequence identity with both BPYV-JC and -S having highly conserved eight ORFs. In contrast, the RNA1 of BPYV-MI showed sequence identities of 98 and 86 % with BPYV-JC and -S, respectively. Phylogenetic analysis of RNA-dependent RNA polymerase (RdRp) coding sequences from three fully sequenced BPYV strains and five partially sequenced cucurbit-infecting BPYV strains from Japan and South Africa has shown that cucurbit-infecting strains are closer to each other than to BPYV-S. In addition, the strawberry strain BPYV-S has an ORF2 in the downstream of RdRp gene in RNA1, but all the cucumber strains, BPYV-JC, -MI, and those from South Africa, lacked the ORF2 of RNA1, highlighting the difference between common BPYV cucumber strains and a unique strawberry strain.