Effects of environment and sowing date on the competition between faba bean (Vicia faba) and the parasitic weed Orobanche crenata

In field trials conducted in southern Turkey during two growing seasons, effects of environment and management on the host-parasite association faba bean (Vicia faba)-Orobanche crenata were quantified. Experimental factors in each experiment included two sowing dates one to two months apart, parasite seedbank densities of 0, 25 and 200 seeds kg(-1) soil and two susceptible faba bean cultivars. In all treatments, parasitism had minor effects on faba bean vegetative growth. Reductions of stem dry weight occurred in infected crops during grainfilling and were most likely related to increased assimilate retranslocation. Up to 271 parasites m(-2) attached to the roots of the host, accumulating a maximum biomass of 701 g m(-2). Parasite number per host was a function of seedbank density and host root length density (RLD) in the top 15 cm of soil. Delayed sowing reduced the combined dry weight accumulation of host and parasite, which was not affected by parasite infection. Pod yield of faba bean ranged from 132 to 10 19 g m(-2) and was affected by pedoclimatic conditions, sowing date and parasite infection. Parasitism mainly decreased host yield by reducing pod number. Seed size decreased to a lesser extent, while seed number per pod was not affected by parasite attack. Timing of host and parasite development phases, namely of the critical phase for faba bean pod setting, strongly affected host-parasite interactions and had decisive influence on the assimilate competition between pods and parasites. Via effects on host and parasite phenology, delayed sowing improved the relative competitive ability of pods during the critical phase, leading to lower parasite number and dry weight. As a consequence, reductions in combined host and parasite dry weight associated with delayed sowing mainly occurred at the expense of parasites. These findings may be transferred into specific control measures aimed at minimising the relative competitive ability of O. crenata during faba bean pod setting. (c) 2004 Elsevier B.V. All rights reserved.