Midday Sunday, a strange mould or fungi, bright yellow, almost chrome yellow, on top of a compost heap. An hour and a half later it had changed, the surface starting to brown. This was more noticeable mid-afternoon. By late afternoon, early evening, the surface was browned as though it was a loaf of bread just out of the oven. The fungi had appeared during the morning or overnight as it was not there the day before.
During the morning there had been heavy, fine rain. The previous two weeks or more, hot dry weather, though the compost heap had been watered with lukewarm water to stop it from drying out.
The compost heap consists of mainly garden weeds, autumn leaves, with vegetable scraps from the kitchen and scrunched up paper to provide fibre.
On at least one other occasion there has been a similar mould or fungi on the top of the compost heap, but not seen on any other compost heap.
What is it?