UNIFORMITY AMONG RACES OF UROMYCES-APPENDICULATUS IN RESPONSE TO TOPOGRAPHIC SIGNALING FOR APPRESSORIUM FORMATION

Forty pathogenic races of Uromyces appendiculatus from diverse origins were tested for thigmotropic sensing of surface topography in the formation of appressoria. Defined topographic signals (ridges) ranging from 0 to 1.24-mu-m high were tested. All races responded similarly, with minimal levels of appressorium formation occurring on ridges less than 0.18-mu-m high and greater than 1.24-mu-m high. Development of appressoria occurred optimally (> 80%) on ridges 0.4-0.8-mu-m high. The uniform response for appressorium formation on the different topographies was observed among test races and among repeated tests of a standard race. The similarity in response for appressorium development among these bean rust races has implications for development of thigmotropic-based race nonspecific resistance.