Population changes in Phytophthora infestans in Taiwan associated with the appearance of resistance to metalaxyl.

In recent years, late blight, caused by Phytophthora infestans (Mont) De Bary, has increased in severity in many parts of the world, and this has been associated with migrations which have introduced new, arguably more aggressive, populations of the pathogen. In Taiwan, late blight has been endemic on outdoor tomato crops grown in the highlands since the early 1900s, but recent epidemics have been more damaging. To ascertain the present status of the Taiwanese population of P infestans, 139 isolates of the pathogen collected and maintained by the Asian Vegetable Research and Development Center (AVRDC) were characterized using mating type, metalaxyl sensitivity, allozyme genotype, mitochondrial haplotype and RFLP fingerprinting. Up to 1997, all isolates were found to belong to the old clonal lineage of P infestans (US-1 and variants), but in isolates from 1998 a new genotype appeared, and by 2000 this had apparently completely displaced the old population. This new genotype was an A1 mating type and has the dilocus allozyme genotype 100/100/111, 100/100 for the loci coding for glucose-6-phosphate isomerase and peptidase, respectively. These characters, together with RG57 fingerprinting, indicated that these isolates belonged to the US-11 clonal lineage, a minority (11%) being a previously unreported variant of US-11. Whereas metalaxyl-resistant isolates were not detected in the old population, 96% of the new genotypes proved resistant, with the remainder being intermediate in sensitivity. It may be inferred from this sudden, marked change in the characteristics of the Taiwanese P infestans that a new population of the pathogen was introduced around 1997-98 and that this may well have already been metalaxyl-resistant when it arrived, although a role for in situ selection cannot be excluded.

Sensitivity of Vegetative Pathogens to High Hydrostatic Pressure Treatment in Phosphate-Buffered Saline and Foods.

The effect of high hydrostatic pressure (up to 700 MPa) at 20°C on the survival of vegetative pathogens was investigated in 10 mM phosphate buffer (pH 7.0), ultra high-temperature-treated (UHT) milk, and poultry meat. In buffer, Yersinia enterocolitica was most sensitive, with a pressure of 275 MPa for 15 min resulting in more than a 105 reduction in numbers of cells. Treatments of 350 MPa, 375 MPa, 450 MPa, 700 MPa, and 700 MPa for 15 min were needed to achieve a similar reduction in Salmonella typhimurium , Listeria monocytogenes , Salmonella enteritidis , Escherichia coli O157:H7, and Staphylococcus aureus respectively. A significant variation in pressure sensitivity was observed between different strains of both L monocytogenes and E. coli O157:H7. The most resistant strains ( L. monocytogenes NCTC 11994 and E. coli O157:H7 NCTC 12079) were chosen for further studies on the effect of substrate on pressure sensitivity. In both cases the organisms were more resistant to pressure when treated in UHT milk than in poultry meat or buffer. There was evidence, assessed by differential plating using trypticase soy agar with and without additional NaCl, that sublethally injured cells were present at pressures lower than were required for death. This information may be of value if pressure is combined with preservation treatments such as mild heating. The variation in results obtained with different organisms and in different substrates should be recognized when recommendations for the pressure processing of foods are being considered.