Mating type, mefenoxam sensitivity, and pathotype diversity in Phytophthora infestans isolates from tomato in Brazil

The objective of this work was to characterize 79 Phytophthora infestans isolates collected in tomato (Solanum lycopersicum) fields, as to mating type, mefenoxam sensitivity, and pathotype composition. The isolates were sampled in 2006 and 2007 in seven Brazilian states as well as in the Distrito Federal. They were characterised as to mating type (n=79), sensitivity to fungicide mefenoxam (n=79), and virulence to three major resistance genes Ph-1, Ph-2, and Ph-3/Ph-4 (n=62). All isolates were of the mating type A1. Resistant isolates were detected in all sampled states, and its average frequency was superior to 50%. No difference was detected in pathotype diversity, neither between subpopulations collected in 2006 and 2007 nor between isolates grouped as resistant or intermediately sensitive to mefenoxam. All major resistance genes were overcome at different frequencies: Ph-1, 88.7%; Ph-2, 64.5%; and Ph-3/Ph-4, 25.8%. Isolates with virulence genes able to overcome all major resistance genes were detected at low frequencies. Tomato breeding programs in Brazil must avoid the development of cultivars with resistance based exclusively on major genes

A 2nd Worldwide Migration and Population Displacement of Phytophthora-Infestans

The appearance of the A2 mating type (previously restricted to central Mexico) in Europe during the 1980s prompted an investigation of the genetic make-up of European populations using allozyme loci as genetic markers. The investigation shows that major genetic changes have occurred in populations of Phytophthora infestans in the Netherlands. Poland, and the British Isles. It now appears that a new type of population has been introduced into several locations, and has displaced or is displacing the original populations in these locations. The new and old population types are characterized by unique allozyme alleles and genotypes. The mechanism for displacement of the 'old' by the 'new' population is not yet known