Hot and dry summer conditions saw charcoal rot raise its unwanted head in many Darling Downs sorghum crops this year.
Charcoal rot in sorghum is caused by the soil borne fungus Macrophomina phaseolina and is a major stalk rotting disease which can lead to plant lodging.
There’s little data on the extent of charcoal rot yield losses in Australia but overseas yield losses have been estimated at 50%, making it a significant threat to crop profitability when conditions are favourable.
The causal agent, M. phaseolina can infect via the roots of sorghum plants at almost any stage of plant growth, but develops more rapidly in plants closer to maturity.
Extensive colonisation of stem tissue generally occurs post flowering when plants are placed under stress, such as unfavourable environmental conditions - particularly hot, dry weather.
This can be exacerbated by the application of defoliants which also act as a stressor, encouraging further growth and invasion of the stem.
Charcoal rot is recognisable by the stalk’s blackish appearance just above ground level and is easily identifiable when stems are split longitudinally.
It doesn’t only affect sorghum - the fungus is widely distributed throughout Australia infecting the root and stems of over 400 plant species including all major summer field crops and many summer and winter weeds.
Research has shown that the microsclerotes can survive in the soil and on stubble for 4+ years but it isn’t certain what soil conditions are necessary to reduce the survival of microsclerotes in Australian soils, although overseas studies indicate survival can be significantly impeded by wet soil.
All of this raises questions over the best way to manage and prevent charcoal rot. Basically it comes down to practices given that there are no effective foliar fungicides.
A paper presented by University of Southern Queensland pathologist Sue Thompson at last year’s Grains Research and Development Corporation (GRDC) Grains Research Update at Jondaryan outlined some key management considerations to help minimise the risk of infection.
These include: planting into adequate soil moisture and ensuring row spacing and plant populations are suitable for the field and seasonal situation, to minimise possible post flowering moisture stress; application of adequate fertilisers to maintain plant health and vigour and reduce nutrient related stress (particularly avoiding excessive nitrogen and low levels of potassium); and ensuring crop rotations don’t include susceptible crop hosts.
Research suggests application and timing of desiccant and harvest is also important. Early application of a desiccant can increase stress and lodging potential (if applied when <95% seed are at black layer) as much as a desiccant applied too late, particularly if lodging is already occurring and disease incidence is high. It stands to reason then that timely harvest once the desiccant has been applied is essential.
Lastly, the use of lodging resistant, drought tolerant, non-senescent varieties are also showing some promise in helping manage the charcoal rot risk.
For a more detailed insight into the management of charcoal rot, visit the GRDC website for a copy of the Updates paper `Disease control in summer crops and management strategies to minimise financial losses’ by following this link.
Arthur Gearon, GRDC northern panellist, Chinchilla
0427 016 658
Sarah Jeffrey, Senior Consultant Cox Inall Communications
0418 152 859