Dear friends, thanks for sharing your experience on this topic. I am happy that I can provide something to debate upon. I am posting images of my 'Hortiboletus rubellus' with typical red dots. B013 was found in a cave with Quercus ilex roots emerging through a fissure in the rock and the Hortiboletus growing in complete darkness.
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B211 is probably X. redeuilhii now that I am gaining experience of Xerocomus (additional image of section of stipe included)
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This link provides verified images of X. engelii (the author sequenced them)
MICOLOGIA, di Salvatore Saitta - Hortiboletus engelii
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"I've had collections of H. engelii where only one of several specimen had them."
This is exactly a similar situation. I wonder if slicing the specimens 8 hours after collection may reduce the colour-effect. I often collect and examine calmly at home. Many collections here are in hot hunting sites and operating incognito!
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So guys, if I had to ask you to construct a key to tell apart engelii, rubellus and redeuilhii how would you go? I try my own ...
1a. Stipe deep red (almost brownish) at the base, sometimes reaching up to 3/4 of its length; flesh at the base of stipe bordered deep red (sectioning obligatory), patchy or continuous, sometimes forming large reddish-brown blobs in the centre; context does not change blue, or if so faintly after a long period of time >>> X. redeuilhii
1b. Stipe sulphur yellow merging to light bronze with flushes of light red, sometimes intense towards the central part, rarely so at the base; flesh at the base of the stipe do not show a distinct red border, but tiny orange-red granules or hue is present at the inner part, sometimes lacking completely ; context stains light blue rapidly or after 5 mins. >>> 2
2a. Context turns faint blue within few seconds after cutting; surface of stipe always with obvious reddish colour; flesh under pileus and upper region of stipe remains yellow >>> H. rubellus
2b. Context turns faint blue after a few minutes from cutting; surface of stipe sometimes lacking reddish tone or very faint; flesh under pileus and upper region of stipe usually forms a reddish layer >>> X. engelii
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If you want my ab1 ITS sequences, I am ready to share them. PM yr email and I will send them for yr further studies.