Small broomrape (Orobanche minor) and Egyptian broomrape (Orobanche aegyptiaca) parasitization of red clover (Trifolium pratense)

Experiments were conducted to investigate the possibility that rhizobacterial nodulation in legumes influences parasitic attack by broomrape. Small broomrape (Orobanche minor), which is commonly associated with swellings of the host root, was studied on red clover (Trifolium pratense) in the presence or absence of Rhizobium leguminosarum by. trifolii inoculum and was compared to Egyptian broomrape (Orobanche aegyptiaca), which is not associated with host root swellings. Small broomrape showed greater percent seed germination and formed a greater number of attachments on red clover inoculated with rhizobacteria in comparison with noninoculated plants. In contrast, inoculum did not appear to enhance Egyptian broomrape seed germination or the number of its attachments on the host roots compared to noninoculated controls. In aseptic culture, the addition of Rhizobium increased the success of small broomrape in parasitizing clover but was not essential for parasitization. No rhizobacteria were detected in cultures derived from tubercles under septic or aseptic conditions. Morphological observations of small broomrape attachments on red clover suggest that parasitic attachments were not situated over the bacterial nodules but may involve parasite-induced protrusion of host plant root cortex. These results indicate that although rhizobacterial nodulation is not required for parasitization, the presence of nodules facilitates small broomrape germination and attachment to red clover.