Effects of virus infection of faba bean on subsequent infection by Uromyces viciae‐fabae

Prior infection of faba bean with bean yellow mosaic virus (BYMV) and bean leaf roll virus decreased pustule density on leaves subsequently infected by Uromyces viciae-fabae. Changes were most marked on young leaves showing conspicuous symptoms of systemic virus infection and in plants virus-infected for at least 2-4 weeks. Pustule density progressively decreased when inoculations were made with uredospore generations successively produced on and inoculated to BYMV-infected leaves. Uredospore germination and germ tube length were similar on virus-free and BYMV-infected leaves and on agar seeded with spores produced for one or several generations on BYMV-infected or virus-free plants. Formation of appressoria was slightly reduced on BYMV-infected leaves but changes in the post-penetration development of the rust probably accounted for much of the decreased pustule production encountered.