Detached leaf and whole plant assays for soybean aphid resistance: differential responses among resistance sources and biotypes.

The soybean aphid, Aphis glycines Matsumura, is a pest of cultivated soybean, Glycine max (L.) Merr., in North America. Recent developments in host plant resistance studies have identified at least four soybean aphid resistance genes (Rag1-4) and two soybean aphid biotypes (biotype 1 and 2), defined by differential survivability on resistant soybean. Detached soybean leaves were tested as a more rapid and practical assay to assess host plant resistance and virulence. Two susceptible lines ('Wyandot' and 'Williams 82') and two resistant lines (PI 243540 and PI 567301B) were examined. Various life history characteristics were compared among aphids on whole plants and detached leaves. Results indicated that resistance to soybean aphid was lost using detached leaves of PI 567301B but retained with PI 243540. To test for aphid virulence, net fecundities were compared among biotype 1 and biotype 2 after rearing on detached leaves of the resistant 'Jackson' (to which biotype 2 is virulent). A significant difference was detected in net fecundities among biotypes on detached leaves of Jackson and used to predict growth rates and virulence from 30 field-collected individuals of unknown virulence. No field individuals matched biotype 2 predictions, but four individuals had higher net fecundities than biotype 2 predictions (13%) and could be considered moderately virulent. The results indicated that the retention of soybean aphid resistance in detached leaves is dependent on PI and resistant source, but if resistance is retained, detached leaves could be used to determine soybean aphid virulence.