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

We propose that sunflower yield is negatively associated with responsiveness to intraspecific competition but that this relationship can be affected by fungal diseases. This hypothesis was tested in environments when Sclerotinia head rot (Sclerotinia sclerotiorum (Lib.) de Bary) and Verticillium wilt (Verticillium dahliae Klebahn) were the prevalent diseases. Comparisons included cultivars released to the market in the last four decades (Experiments 1 and 2), and current hybrids of varying yield potential (Experiments 3 and 4). In each experiment, yield of undisturbed controls was compared with that achieved by reducing inter-row competition. Yield increase in response to reduced competition ranged from 0 to 84%, highlighting the variation among cultivars. In Experiments 1 and 3, the percentage of leaves with symptoms of Verticillium wilt accounted for 28-32% of the variation in control plot yield, while response to competition accounted for 60-75% of the residual variation; a negative association between control plot yield and response 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 association between yield and response to competition in crops with head rot, and the negative relationship between yield and response to competition in crops with Verticillium wilt was consistent with our working hypothesis. (C) 2000 Elsevier Science B.V. All rights reserved.