REPLACEMENT SERIES ANALYSIS OF THE COMPETITIVE INTERACTION BETWEEN A WEED AND A CROP AS INFLUENCED BY A PLANT-PARASITIC NEMATODE

Methods for evaluating the influence of a plant parasitic nematode on plant competition were developed for a nematode-weed-crop system. The interrelationships between the fiddleneck flower gall nematode [Anguin a amsinckiae (Steiner & Scott, 1935) Thorne, 1961], its weed host, fid dleneck (Amsinckia intermedia Fischer & Meyer) and wheat (Triticum aes tivum L.) were investigated. Replacement series experiments were condu cted in a greenhouse with ratios (weed : crop) of 0 : 1, 3 : 1, 1 : 1, 1 : 3, and 1 : 0. Experiments were replicated both in the presence an d absence of the nematode. Results showed that the nematode reduced th e dry weight of the weed by 25%, but had no significant effect on the dry weight of the crop. Competitive indices implied that the crop was the dominant competitor both in the presence and absence of the nemato de. Measurements of niche differentiation indicated that the two plant species did not fully share the same limiting resources in the absenc e of the nematode, and that competition for limited resources intensif ied when the nematode was present.