As the 2018/19 summer cropping season draws to a close, sorghum growers are once again confronted with an unfortunate all too familiar problem with charcoal rot and lodging in their paddocks (Figures 1, 2 and 3). This summer season was a text-book case where conditions were ideal for the disease to develop, hot and dry.
Due to the recent issues experienced by growers with charcoal rot, University of Southern Queensland (USQ) is working with the GRDC to collect data from agronomists and growers via a simple survey that will help understand what factors drive disease development and what the associated yield losses may be. If you are a grower or agronomist and are interested in contributing to this work, please contact Dante Adorada by email: firstname.lastname@example.org or by phone: (07) 4631 1262 for more information or download the survey form and send it back to Dante at the e-mail address provided.
About the disease
Charcoal rot disease, caused by the fungus Macrophomina phaseolina, is common during seasons with prolonged hot, dry weather or when other unfavorable environmental conditions stress the plant. Splitting sorghum stalks longitudinally will show ash grey tissue or microsclerotia, the survival structure of the fungus, giving the internal stalk tissue a peppered look (Figure 4). The disease causes major sorghum stalk rotting, which can lead to plant lodging.
The fungus is soilborne and widespread throughout Australian paddocks. Surviving up to four years in the soil as microsclerotes (that give the stalk the peppered appearance), it can infect more than 400 species of plants including all major summer field crops and many weeds.
Despite the lack of any formal quantification in Australia, significant yield losses have been associated with lodging, where prevailing hot dry conditions have resulted in widespread high incidences of M. phaseolina and subsequent lodging. Losses occur both due to the disease reducing head fill and lodging. Lodging losses vary and are dependent on the ability of individual growers to retrieve lodged heads with harvesting equipment available.
The fungus infects via the roots at any time during the season but symptoms are not normally observed until at, or near harvest when lodging is presented. Predisposing conditions include:
- high plant populations;
- leaf diseases;
- frost or hail damage;
- mechanical damage;
- crop desiccation;
- excess nitrogen fertilization;
- insect feeding, and;
- hot, dry conditions throughout the season.
Unfortunately, little can be done for the current season to correct the problem. Growers and agronomists are encouraged to re-visit guides and resources to understand the disease and options to manage charcoal rot in coming seasons.
Best management practices for managing charcoal rot include:
- Managing soil moisture by planting into adequate moisture and observing proper plant population densities and row spacings.
- Using appropriate fertiliser rates to maintain adequate nutrition, avoid excessive nitrogen or low levels of potassium.
- Planting stay-green hybrids with good scores for charcoal rot, standability, and drought tolerance.
- Using defoliants judiciously as they can act as a stressor and can lead to further infection and harvesting in a timely fashion after a desiccant has been applied.
- Note that foliar fungicide applications are NOT effective in controlling stalk rot diseases and should not be used.
The fungus is soilborne and wide-spread throughout Australian paddocks. Surviving up to four years in the soil as microsclerotes, it can infect more than 400 species of plants including all major summer field crops and many weeds. Despite the lack of any formal quantification in Australia, significant yield losses have been associated with lodging, where prevailing hot dry conditions have resulted in widespread high incidences of M. phaseolina and subsequent lodging. Losses associated with lodging varies and is dependent on the ability of individual growers to retrieve lodged heads with harvesting equipment available.
PREDICTA®B results for the charcoal rot pathogen, Macrophomina phaseolina, as well as other stalk rot pathogens, Fusarium thapsinum and F. andayazi, as tests are under evaluation. Current research indicates that it is important to make sure that host crop stubble is included in PREDICTA®B soil samples. Fifteen (15) small pieces appear to be enough to include when sampling. However, we are still not sure of the exact relationship of Macrophomina soil and stubble population density categories to lodging or the role being played by Fusarium stalk rot. Population density categories enable growers to benchmark levels against the rest of industry, but the relationship to yield loss is still a work in progress.
Further information on charcoal rot
Lisa Kelly, DAFQ
Alan McKay, PIRSA-SARDI