4′,6-diamidino-2-phenylindole Avr gene CF DAPI FITC GISH HR Journal Article NBT Phytophthora infestans R gene RAPD ROS Reactive Oxygen Species Research Support, Non-U.S. Gov't Resistance Solanum tuberosum Solanum villosum Somatic hybrids VT hybrids avirulence gene cultivars culture filtrate cvs fluorescein isothiocyanate genomic in situ hybridization hypersensitive response nitroblue tetrazolium random amplified polymorphic DNA reactive oxygen species