Certified GKH Gardening Expert
Since we are based in the US, I'm not completely familiar with your climate, but would it be safe to assume that you have had a wet season recently? This can cause a fungal infection, which has likely happened.
For now, you can treat with a mixture of wettable sulfur and DOLOMITIC lime. You will have to order this online. For some reason they do not sell it in your country. I am to understand that they sell hydrated lime, which is significantly more toxic for some reason, but not dolomitic lime. This is a safe product that consists of calcium carbonate, and magnesium carbonate. It will buffer the soil to the treatment that you are about to give it, which will kill off the infection but can be harsh on the soil if not treated with a buffer.
Both of these can be used once or twice a year to prevent such an occurrence.