Phytophthora species were surveyed by collecting soil samples and placing bait leaves in selected streams during June-October in the years 2005, 2006 and 2010 at three sites in oak forests in Diqing Tibetan Autonomous Prefecture of NW Yunnan province, China. Seventy-three isolates of Phytophthora spp. were recovered from 135 baited leaf samples and 81 soil samples. Eight Phytophthora species were identified by observation of morphological features and ITS1-5.8S-ITS2 rDNA sequence analysis. The eight taxa included two well-known species P. gonapodyides and P. cryptogea, two recently described species P. gregata and P. plurivora, two named but as yet undescribed taxa, P. taxon PgChlamydo and P. taxon Salixsoil, and two previously unrecognized species, Phytophthora sp.1 and P. sp.2. The most numerous species, P. taxon PgChlamydo, and the second most abundant species, P. taxon Salixsoil, were recovered at all three sites. Phytophthora cryptogea was detected only once at site Nixi. Phytophthora gregata and P. sp.2 were isolated from a stream only at site Bitahai, while the other three species were each found at two sites. Phylogenetic analysis revealed that the isolates belonged to three ITS clades, one species including six isolates in clade 2, six species including 66 isolates in clade 6 and one species in clade 8. There was a relatively rich species and genetic diversity of Phytophthora detected in the investigated regions where the forest biotic and abiotic factors affecting the growth and evolution of Phytophthora populations were diverse.
Acidovorax citrulli causes seedling blight and bacterial fruit blotch (BFB) of cucurbits, a serious disease threat to cucurbit seed and fruit production worldwide. Despite its economic importance, current knowledge on the biology and pathology of A. citrulli is limited. This work studies a mini-Tn5 mutant, MJ22-3, that showed reduced virulence on melon seedlings. The gene disrupted by Tn5 was identified as leuB, which encodes a putative 3-isopropylmalate dehydrogenase, an enzyme that appears to be essential for leucine biosynthesis. The leuB gene in-frame deletion mutant MDleuB, as well as the Tn5-inserted mutant MJ22-3, failed to grow in minimal medium without leucine and exhibited reduced swimming motility; however, the mutations had no effect on biofilm formation or induction of the hypersensitive response on tobacco (Nicotiana tabacum). When inoculated at a low cell density (10 4 CFU mL
À1), the leuB gene mutants showed a significantly slower growth rate and attenuated virulence on melon cotyledons compared to the wildtype MH21. However, inoculations with the mutants at high concentration (10 8 CFU mL À1 ), or low concentration (10 4 CFU mL À1 ) inoculations supplemented with 0Á01% leucine resulted in growth rates and disease severities similar to that of the wildtype MH21. The results suggest that leucine biosynthesis is essential for both in vivo growth and full virulence of A. citrulli MH21 on melon seedlings.
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