Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2015
Publication Date: 1/27/2015
Citation: Short, D., Sandoya, G., Vallad, G., Koike, S., Xiao, C., Wu, B., Gurung, S., Hayes, R.J., Subbarao, K. 2015. Dynamics of Verticillium species microsclerotia in field soils in response to fumigation, cropping patterns, and flooding. Phytopathology. 105:638-645. Interpretive Summary: Verticillium dahliae is a soil-inhibiting fungus, causing wilt or collapse of many economically important crops. Control of Verticillium wilt has relied on methods that reduce the pathogen's inoculum to a level at which little or no disease develops. In this study, we monitored the responses of V. dahliae inoculum in soils to soil fumigation with methyl bromide and chloropicrin, crop rotation patterns, and flooding. Fumigation and flooding significantly reduced but did not eliminate V. dahliae. Increases in the recolonization of V. dahliae in soils depended on crops grown after fumigation. These findings will help develop and implement crop-specific management strategies for V. dahliae.
Technical Abstract: Many soil-inhabiting fungi are capable of surviving the dynamic soil microenvironment through the formation of resilient resting structures, such as thick-walled spores, melanized hyphae, and sclerotia. Verticillium dahliae is a soil-inhabiting, economically significant plant pathogenic fungus that persists in the soil for up to 14 years as melanized microsclerotia (ms). Management of Verticillium wilt has relied on methods that reduce ms below crop-specific thresholds at which little or no disease develops. Methyl bromide, a broad-spectrum biocide has been used as a pre-plant soil fumigant for over 50 years to reduce V. dahliae microsclerotia. However, reductions in the number of ms in the vertical and horizontal soil profiles, and the rate at which soil recolonization occurs has not been studied. The dynamics of ms in soil pre- and post- methyl bromide + chloropicrin fumigation were followed over three years in three 8 × 8-m sites each in two fields. In separate fields, the dynamics of ms in the vertical soil profile of 60 cm depth in 5-cm increments pre- and post-fumigation with methyl bromide + chloropicrin followed by various cropping patterns were studied over four years. Finally, ms densities were assessed in six 8 × 8-m sites in a separate field prior to and following a natural 6-week flood. Methyl bromide + chloripicrin significantly reduced but did not eliminate V. dahliae ms either in the vertical or horizontal soil profiles. Densities of ms were highest in the top 5-20 cm of the vertical soil profile but were consistently detected at 60-cm depths. Increases in the recolonization levels of ms highly depended upon the crops grown post-fumigation. Six-weeks of natural flooding significantly reduced, but did not eliminate V. dahliae.