Normally charcoal rot does not reduce lupin crop yields as symptoms only appear after pod set. However, unseasonable heat in August 2014 saw crops in the northern and eastern districts of Western Australia infected earlier, placing stress on the plants during pod set and pod ripening. The disease brought on senescence, causing plants to dry off and die prematurely.
Charcoal rot is a widespread soil-borne fungus that is only a problem in highly stressed crops. Patches that dried off this season were generally on very dry and shallow soils where the plants had limited root systems and few deep roots.
The soil-borne fungus Macrophomina phaseolina causes charcoal rot and although it is relatively widespread, it is usually a weak pathogen. It attacks plants that are moisture stressed, when soil temperatures are warm and can be carried over from one season to the next as resting bodies (sclerotia) in the soil and on infected plant debris. After the resting bodies germinate and the fungal growth invades the plant, fruiting bodies form.
Charcoal rot can infect a wide range of other crops and pastures including lucerne, canola, mung beans, soybeans, sorghum, sunflowers and cotton, and irrigated summer crops are particularly susceptible. Cereal crops are not normally infected.
In lupins, the first symptom is premature senescence of individual or patches of plants within a crop. The stem and taproot of infected plants will have an ash-grey discolouration (when split open) due to masses of tiny black microsclerotia embedded in the tissue (DAFWA Bulletin 4720, Producing Lupins).
The area of infected plants in a lupin crop can range from a small patch to most of a paddock, depending on soil conditions.
Lucerne plants commonly exhibit light brown lesions on the crown and roots which later become spongy. Small black resting bodies (sclerotia) will eventually cover the conducting tissues. By this time the tops of the plant will have wilted and died (Agriculture Victoria Agnote No. AG0730, Diseases of lucerne – 5: Fungal stem diseases).
This disease can not be prevented but does not usually cause significant yield losses in grain crops so specific management is not usually required.
The primary control option is to use crop rotations that include non-host crops such as wheat and barley. This will help to reduce the concentration of soil borne inoculum. In irrigated summer crops adequate water and nutrition can help prevent infection. (NSW DPI Summer crop production guide, 2014).
Pulse Australia have estimated that yield losses to charcoal rot in 2014 will only account for around 10 per cent of the total production of lupins.
However, charcoal rot will affect 80 to 90 per cent of plants in affected patches and paddocks, and individual yield losses will be high.
Fortunately, Pulse Australia industry development manager Alan Meldrum has advised growers that charcoal rot is unlikely to be seen affecting lupins to this extent in the future. Growers should take heart that this is probably a one-off event.