Effectiveness of recurrent selection of white clover (Trifolium repens) for resistance to New Zealand populations of clover cyst nematode (Heterodera trifolii)

Abstract Selections of white clover resistant or susceptible to Heterodera trifolii in earlier rounds of a recurrent selection programme were intercrossed and selected in three further rounds of selection. Resistance and susceptibility were measured by comparisons of cysts per gram of root dry weight (cpg). In the third, fourth, and fifth rounds of recurrent selection, the mean cpg of progenies from resistant parents was 23% (n = 974), 6% (n = 870) and 11% (n = 98) of that of progenies from susceptible parents. In each selection round, the resistant selections had 31, 4 and 7% of the number of cysts on susceptible selections. The percentages of immune plants (i.e. with 0 cysts) in the resistant groups were 1, 39, and 50% after the third, fourth, and fifth rounds of recurrent selection, respectively. Inoculum from 16 populations of H. trifolii from around New Zealand was added to two resistant and two susceptible white clover lines. After one generation of nematode development there were fewer cysts on the resistant lines for each population tested. Two resistant and two susceptible lines were compared in a glasshouse trial for their reaction to parasitism by H. trifolii . Infection reduced the shoot weight of all four lines but resistant lines grew better than either one or both susceptible lines at 109, 126, and 160 days after inoculation. In contrast, the uninfected lines did not differ in shoot weight over five samplings. More cysts were recovered from susceptible lines (means of 370 and 430 cysts per plant) than from resistant lines (means of 41 and 56 cysts per plant). Results indicate that development of a white clover cultivar resistant to H. trifolii is achievable and that the resistance will be effective against the nematode in most of New Zealand. The growth study indicated that advantages will be conferred in pastures where the presence of H. trifolii causes major limitations to white clover growth and persistence. Zusammenfassung- Die Wirksamkeit einer mehrfach wiederholten Selektion von Weissklee (Trifolium repens) auf Resistenz gegen das Kleezystenalchen (Heterodera trifolii)- In einem fruheren Selektionsprogramm gegen Heterodera trifolii resistente oder anfallige Weisskleeselektionen wurden gekreuzt und in weiteren Selektionsrunden selektiert. Resistenz und Anfalligkeit wurden durch Vergleich der Anzahl der Zysten je Gramm Wurzeltrockengewicht (cpg) bestimmt. In der dritten, vierten und funften Runde der Wiederholungsselektion betrugen die c