Intraspecific competition and fungal diseases as sources of variation in sunflower yield

We propose that sunflower yield is negatively associated with responsivenes s to intraspecific competition but that this relationship can be affected b y fungal diseases. This hypothesis was tested in environments when Scleroti nia head rot (Sclerotinia sclerotiorum (Lib.) de Bary) and Verticillium wil t (Verticillium dahliae Klebahn) were the prevalent diseases. Comparisons i ncluded cultivars released to the market in the last four decades (Experime nts 1 and 2), and current hybrids of varying yield potential (Experiments 3 and 4). In each experiment, yield of undisturbed controls was compared wit h that achieved by reducing inter-row competition. Yield increase in respon se to reduced competition ranged from 0 to 84%, highlighting the variation among cultivars. In Experiments 1 and 3, the percentage of leaves with symp toms of Verticillium wilt accounted for 28-32% of the variation in control plot yield, while response to competition accounted for 60-75% of the resid ual variation; a negative association between control plot yield and respon se to competition was shown. In Experiments 2 and 4, incidence of head rot accounted for most of the variation in yield among cultivars. The lack of a ssociation between yield and response to competition in crops with head rot , and the negative relationship between yield and response to competition i n crops with Verticillium wilt was consistent with our working hypothesis. (C) 2000 Elsevier Science B.V. All rights reserved.