Root Disease Constrains in Pasture Productivity

Summary

Damage to roots of subterranean clover is widespread in southern Australia during autumn-winter. A major cause is root rot due to oomycete and fungal pathogens. The damage takes two forms: (i) damage to the hypocotyl, cotyledons and root radicle when the seed is germinating resulting in death before emergence (often referred to as “damping off”) and (ii) damaged roots on seedlings. A survey across southern Australia (17 sites) found the median pre-emergence loss was 21% of germinating seeds (however, one site recorded a 93% loss), with 70% of the surviving plants having substantial damage to their roots. Root damage ranges from minor shortening of lateral roots, through substantial pruning of the primary laterals, to loss of the taproot. Root damage such as this is invariably accompanied by significant reductions in shoot yield. Estimates of the constraint to shoot yield of subterranean clovers during autumn-winter are typically about 20-40% but, occasionally, very high yield losses are also recorded.
It was unclear whether root damage limits yield ‘indirectly’ by restricting nutrient uptake, or as a ‘direct’ restriction of the shoot-root balance of the plant (i.e. a consequence of “root pruning”), or is due to both of these influences acting concurrently. If damaged roots are unable to forage adequately for phosphorus (P) in soil, it is possible that: (a) fertiliser applied at the time roots are being damaged may overcome pasture yield constraints, and (b) poor root growth may be increasing the amounts of P that farmers need to apply to achieve high pasture yield. Although it proved difficult to entirely separate the direct and indirect influences of root pruning on plant growth, the experiments provided evidence that direct restriction to shoot growth by root pruning was a major factor in poor autumn-winter growth.  The impact was analogous to the creation of “bonsai”-like plants. It was concluded that adding additional P-fertiliser at this time of the year would not overcome poor growth associated with damaged roots unless soil P fertility was extremely low. Presently, there is interest in developing pasture legumes with long fine roots and long root hairs that can yield well at lower soil test P concentrations. Legumes such as these would reduce the P fertiliser required for maximum production and should also reduce risks of P loss from farmland to waterways. However, before any major investment is made in plant improvement, it is essential to know whether root damage by fungal pathogens is likely to negate the value of selecting plants with improved (P-efficient) root morphology.  Injections of fungicide into the root zone of subterranean clover growing in low P soil during spring did not result in any improvement in clover growth in these experiments, indicating that access to P in soil had not been improved by application of the fungicide.
In recent years, DNA-based tests for the main root rot pathogens of subterranean clover have been developed by SARDI Plant and Soil Health. These tests enable the profile of root pathogens in pasture paddocks to be determined. In these experiments, the root pathogen DNA tests were used to test the levels of pathogen DNA that were directly associated with damaged root systems.