The change will be effective 1 May. They have been working together on the fund for around two years.
Whitman, head of high yield at Threadneedle, will continue to manage other funds including the Strategic Bond Fund, the Credit Opportunities Fund and the High Yield Bond Fund.
The fund, which holds around £796.8m, has significantly outperformed its sector since mid-September last year. The graph shows its performance compared to its sector over the past 12 months.
The largest proportion of assets, 26.4%, is invested in the telecoms sector. Liberty Global, the US cable company that is buying Virgin Media, is one of its top five holdings.