The sunflower midge, Contarinia schulzi Gagné, causes economic damage by inducing abnormal growth in infested heads (capitula) of sunflower, Helianthus annuus L. The primary objective of this study was to determine whether sunflower midge damage could be simulated and whether that simulated damage could be used to select midge-tolerant sunflower germplasm. An additional objective was to develop a quantitative alternative to the scoring systems used to visually estimate damage. Sunflower plants were treated by injecting buds with the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), which resulted in a distortion of the head morphology that was similar in appearance to the damage caused by the sunflower midge. The extent of distortion depended not only on the dose of 2,4-D but also on the size and growth stage of the head when injected. Among sunflower hybrids tested, resistance to the sunflower midge was significantly, negatively correlated with 2,4-D damage. Therefore, injection of sunflower heads with 2,4-D appears to be an effective method of screening sunflower germplasm for tolerance to the sunflower midge. Two distortion indices, based on measurements of head shape, were compared with the visual damage system. Although the visual rating system is faster, distortion index 2 gave similar results and is preferred when it is necessary to avoid individual differences in scoring techniques.
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