Relationship of rainfall, cultural practices, soil and plant nutrients, and seedling survival with root disease and parasitic nematode numbers in annual Medicago spp. pastures

Surveys were conducted for annual Medicago spp. (medic) pastures in the grain belt of south-west Western Australia during spring 1996 and winter-spring 1997 to determine the relationship of rainfall, cultural practices, soil and plant nutrients, and seedling survival with severity of root disease and numbers of parasitic nematodes. Medic pasture was sampled on 116 farms. Most pastures consisted of a single medic variety, viz. Serena, Santiago, Cyprus, or Caliph, whereas about 33% of sites had mixed varieties. Regression analyses showed that high rainfall and application of phosphorus fertilisers were correlated with increased severity of rot in medic tap roots. Crop history and medic variety were not related to the level of root rot. Numbers of Pratylenchus in medic roots were not correlated with the level of tap or lateral root rot, medic variety, rainfall, or with the application of insecticide, fertilisers, or herbicides. Soil with relatively high levels of P, NO3-, or Fe was associated with an increased level of tap root rot. Soils with high pH were associated with reduced tap root rot. Soils with relatively high K were related to severe lateral root rot, wheras relatively high levels of P in soil were associated with reduced lateral root rot. Plants with high levels of tap root rot showed low levels of Mg, whilst low levels of Ca and NO3- in tissues were related to high levels of lateral root rot. High levels of tap root rot were associated with relatively high levels of total N, K, and S, Cu, Zn, Mn, and NO3- in plant tissues. Plants with relatively high levels of lateral root rot had relatively high levels of Cu in shoots. Of the 116 annual Medicago pastures sampled, only 1% had adequate Mg content and only 19% had adequate Ca content. However, 83% had higher than adequate levels of Cu, 70% had higher than adequate levels of Mn, and all samples showed more than adequate levels of chloride. Experimental sites of M. polymorpha cv. Serena at 6 farms showed that the percentage survival rate of seedlings was negatively correlated with the severity of tap and lateral root rot in the previous year. These results indicate that in the farms surveyed there is a serious threat to annual medic pastures from root rot fungi. The severity of the disease was partly determined by soil conditions and cultural practices.