Queensland blue couch (Digitaria didactyla) is a robust and popular turfgrass for sub-tropical Australia.
Known for its distinctive blue-tinged, fine-textured leaves, this species can be prone to a disorder caused by iron deficiency. The symptoms appear as randomly occurring yellow patches, which can appear rapidly.
Aussiblue (D. didactyla ), a more vigorous and lighter green coloured variety, is also affected by the disorder, but less so than Queensland blue couch.
Unlike yellowing caused by nitrogen deficiency, the problem can sometimes correct itself without treatment.
Closer examination of affected turf shows that the newest leaves or whole shoots on rooted nodes have turned yellow or white. The area between parallel leaf veins becomes yellow and progressively white, resulting in a green-and-yellow or a green-and-white striped effect. Often the bottom part of an individual leaf has a more pronounced symptom than the area towards the tip of the same leaf. In time, the veins also lose their greenness, resulting in a completely yellow or white leaf. In severe cases, patches of turf can die.
This disorder is caused by a deficiency of the essential trace element iron in new leaves. Symptoms are often seen after heavy rainfall or irrigation. Hard water (high in dissolved bicarbonates) and alkaline soils, particularly sands, predispose plants to the problem.
Affected plants are often found to have a poor root system, relative to the vigour seen in above-ground growth. The symptom is believed to occur when the leaves’ requirement for iron exceeds the ability of the root system to supply it.
Iron deficiency should not be confused with a nitrogen deficiency. This is seen as a generalised yellowing. In close-up, the oldest grass blades are affected first and the growing tips die back.
In 2002, Mal Hunter, Wal Scattini and Queensland turf producers Steve Sinclair and Bob Pinkerton published the results of an extensive study on this problem. They found that the best treatment was to apply a foliar spray of iron sulphate at a rate of 3.75 g/L per five square metres or iron EDTA (a chelated form of iron) at a rate of 6 g/L per five square metres at monthly intervals. Where treatment was effective, a noticeable greening response occurred within three days.
Their results also suggested that practices that enhance the development of a large, deeply rooted and healthy root system reduced the symptom. This can be achieved by planting healthy new turf onto a well prepared soil of reasonable depth and by irrigating more heavily but less frequently, to encourage grass roots to penetrate more deeply.
The heaviest applications of nitrogen fertiliser should be restricted to the cooler months and applied on the basis of ‘a little and often’. Fertiliser practice should aim to nurture the root system without generating excessive leaf growth, which will aggravate the problem.
Interestingly, the study also showed that adding sulphur or iron sulphate directly to soil to improve iron availability in the root zone was ineffective. No re-greening effects were produced.
Some general tips for the management of yellowing in Queensland blue couch:
- Check your soil pH – a pH of 7.0 or above can increase the risk of it happening.
- Granular iron treatments can stain paths and carpets. A practical solution for home gardeners is to top-dress with a proprietal lawn-blend fertiliser containing slow release forms of iron.
- Think below the surface – encourage healthy root systems.
Other potential treatment benefits
Research in other turfgrasses has shown that grasses well-supplied with iron had improved vigour, tolerance to and recovery from cold, and better recuperation from some herbicide applications. Although these effects have not been researched in Queensland blue couch, measures that improve overall grass health generally make plants more robust in the face of external stress.
Should problems persist after several treatment cycles, the use of less susceptible species such as buffalo grass (Stenotaphrum secundatum), zoysia (Zoysia spp.) or green couch (Cynodon dactylon) may be considered for use under similar conditions.