The Bolete Mould is parasitic on various species of bolete, eventually covering the whole fruitbody.  Initially it is white, becoming pale yellow and then golden yellow.  This is the asexual state, producing large powdery 'aleuriospores', and it is most often recorded in this state.  The sexual state, producing olive-green perithecia, is rarely recorded.

Hypomyces microspermus also attacks Boletes and is identical in the field - but only attacks the Xerocomelloid type such as Red Cracking Bolete; these have dry caps, often velvety in texture and often cracking into a mosaic pattern.   As Hypomyces chrysospermus also attacks Xerocomelloid boletes, mould found on these boletes must be microscopically examined.  Other Hypomyces species may also attack boletes. 

non-Xerocomelloid Xerocomelloid /microscopy

Photograph the mould on the host bolete; if possible, identify the genus of bolete and note this in your comments.  If the bolete is not identifed or is an Xerocomelloid species (e.g Red Cracking bolete) microscopic examination is needed.

On Bolete mushrooms.