Broomrape Host Plant Mystery

[svenrufus]

I'm trying to get to the bottom of what the broomrape family are looking for in a host plant.

Down the road a few years ago, in someone's front garden, I found Common Broomrape growing prolifically amongst Hebe and Rose of Sharon. Later I found more of it in the front gates of a school, on other horticultural stuff - not sure what that was to be honest. No sign of any clovers or other legumes on either site as I had always been lead to believe were it's host plants. Anyway I nicked a few seed cases from it and not knowing quite what else to do with it, I just chucked it into a raised bed in the back garden thinking I would then go and get some red clover to offer it. I never did that.

However, I have stuck in some teasels from elsewhere in the garden into that area, and the Broomrape seems to love it. I have never seen Broomrape the size before. As thick as - no, thicker than - my thumb, nearly 2 foot tall at it's peak, and dense like a forest of Horsetails. The teasel on the other hand, well, you could be forgiven for thinking it is Small Teasel. Pathetic. One flower head and not much more than the same height as the Broomrape. That's the most impressive one, the others are barely traceable. The year before the teasels got to over 6 foot.

So what's the score with host plants? How specific are these Broomrape? I have seen them on at least 3 different families of host from the same stock. I have always laboured under the impression that the Broomrapes were pretty specific - hence the specific names. I guess I was wrong. Anyone else with similar observations?

[colinaj]

Broomrapes do not photosynthesis so latch onto nitrogen fixing plants like Clovers but if seeds are sown in an alien environment they will still grow

[AlanS]

Most broomrapes are indeed highly host specific, but a small number of species are not and can become serious crop weeds in warmer parts of the world. They are obligate parasites attaching to host root systems and may persist underground for periods of years before they show themselves by sending up spikes of flowers. Being non-photosynthetic, they really have no other reason to appear above ground.

(In this, they are like the non-photosynthetic orchids, though the orchids obtain their nutrition via fungi.)

Orobanche minor is indeed a generalist, parasitising a huge range of hosts in many different families, so its behaviour on Hebe, Rose of Sharon and Teasels is not atypical. O. crenata, O. ramosa and O. aegyptiaca, all rare aliens here, are also generalists.

It seems to me that when there is a large genus of highly host-specific species, then there are usually a small number of species with a wide host range. Arguably they can exploit the unoccupied niche left by isolated populations of rare hosts and perhaps also have evolutionary links back to presumed generalist ancestors. We see this not only in broomrapes but also in groups of fungi such as powdery mildews (e.g. Erysiphe) and Mollisia, a huge genus of small cup fungi. (Not sure this applies to rusts, though?)

Alan

[svenrufus]

Alan, thanks ever so much for that. That was a fuller and more perfectly targetted response than I could have hoped for. You seem very knowledgable on Broomrapes.

Your answers raise other questions in my mind, I wonder if you have any views on this - if as you say, the Common Broomrape is a generalist, why is it so rarely recorded?

I understand that spending much of it's time safely ensconsed in the earthy embrace of the soil, it won't be seen or recorded, but once it is in flower, it is pretty unmistakeable. Is it just down to under-recording, or is it in fact as scarce as the records suggest? If so, what are the other environmental constraints on it's distribution and frequency? Given the ease with which it established in my back yard through a simple broadcast of seeds/spores, it is odd that it doesn't find itself in more places that it can really get going.

Thanks for you help before, and looking forward to any other comments you have on this.

Cheers
Sven

[PeterJL]

Originally Posted by svenrufus

... the Common Broomrape is a generalist, why is it so rarely recorded?

If you go to the BSBI maps section which shows the distribution of recorded species you'll see that O. minor has been widely recorded in England over the years. It doesn't appear much in Scotland but perhaps it's too far north.

More info on Broomrapes from Alan Silverdale's site here:

Orobanche (Broomrapes) (University of Paisley: Biodiversity Reference)

[AlanS]

Originally Posted by PeterJL

More info on Broomrapes from Alan Silverdale's site here:

Orobanche (Broomrapes) (University of Paisley: Biodiversity Reference)

Which is another web resource I must find time to revise ... sooner or later.

Alan Silverside

[AlanS]

By the way, I agree with Peter that O. minor is widely recorded.

It is easily overlooked and the flowering spikes remain fresh only for a short time, and I think it has years when it flowers more prolifically, and years when it doesn't. Overall, though, I don't think it is regarded as a rare plant.

Except here in Scotland!

Alan

[svenrufus]

Thanks again. Appreciate your help and insight.

[PeterJL]

Originally Posted by AlanS

Which is another web resource I must find time to revise ... sooner or later.

Alan Silverside

Sorry about the spelling Alan - I know how to spell your name but am getting more frequent attacks of senility these days.