Studies in the nodulation of Kenya white clover (Trifolium semipilosum) under field conditions in south-east Queensland

Abstract

The nodulation of Kenya white clover (Trifolium semipilosum) was examined under field conditions to evaluate reported establishment problems attributed to poor nodulation. Two isolates of Rhizobium identified as strain CB782 were obtained from nodules of T. semipilosum located in separate field sowings each of which was considered an establishment failure but which had recovered to healthy stands at the time of sampling. The two isolates were compared with a laboratory maintained culture of CB782 for nodulation and nitrogen fixation. Both isolates and the original strain were equally effective in N-fixation and all three increased plant dry weight, per cent plants with effective (pink) nodules and amount of nitrogen fixed compared with an uninoculated control. Increasing the level of seed application of inoculum up to the normal rate increased effective nodulation, but excess inoculum levels resulted only in greater numbers of smaller effective (pink) nodules without any increase in nodule dry weight. High inoculum levels decreased the number of white and green nodules compared with the control. Over 50 per cent effective nodulation was obtained with less than 50 Rhizobium seed-1 at sowing. Serological identification and effectiveness tests of strains in nodules confirmed that most white and green nodules on T. semipilosum resulted from infection by naturally occurring white clover rhizobia. These were ineffective in nitrogen fixation with T. semipilosum but fully effective on T. repens. Serological typing of nodules also indicated that normal levels of inoculation resulted in more than 80 per cent of nodules being formed by the specific inoculum strain. We conclude that field sowings of T. semipilosum can be effectively nodulated by the specific inoculum strain CB782 which is both effective and persistent under south-east Queensland conditions. We suggest that most reported nitrogen deficiency symptoms of establishing T. semipilosum are due to infection by Rugose Leaf Curl Virus.