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 Pratylenchusin 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, whereas 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.
Inventory and Assessment of Foliar Natural Enemies of the Soybean Aphid (Hemiptera: Aphididae) in South Dakota