Temperature effects on the disease reactions of sunflower to infection by Orobanche cumana

Three virulent populations (CU194, SE193, and SE194) of the parasitic plant Orobanche cumana were inoculated onto four lines (KA-41, J-8281, HA-89, and RHA-273) of sunflower (Helianthus annuus L.). Pots were transferred to growth chambers set at 15, 19, 23, and 27 degreesC. Emergence of broomrape plants and infection incidence were determinants of disease reaction. All broomrape populations were pathogenic to the sunflower lines KA-II, HA-89, and RHA-273, although differences in virulence were found. At 15 to 23 degreesC, the populations of broomrape infected these three sunflower lines, but a delay in emergence of broomrape was found at 15 degreesC; whereas, at 27 degreesC, the level of infection was restricted. Only population CU194 infected the resistant line J-8281, with infection occurring mainly at 23 and 27 degreesC, but few broomrape plants emerged. Our results suggest that the effect of temperature on the host-parasite relationship is complex.