Mistletoe transpires to be a nuisance…

Or how too much mistletoe will suck your tree dry (and stop it fixing carbon too)…

It’s the last weekend of mistletoe management and harvest work before Christmas and I’ve been reviewing the orchards we’ve worked in this season. All have been over-neglected, most of the apple trees are in dire need of proper management and all have had far too much mistletoe on them – which is, of course, why we were in them…

Too much mistletoe in a small tree can be bad news, stressing the tree and eventually contributing to its death. And, sure enough, in all of the orchards we’ve worked in recently there have been some newly deceased, but still standing, trees – all with lots of (equally dead) mistletoe. Which makes what we are doing – cutting out excess mistletoe, both male and female plants, regardless of their marketability – all the more important. We don’t want to lose any more of them. I’ve covered management issues here before – and am still looking for your management experiences on the survey at www.british.mistletoe.org.uk

But exactly how does mistletoe stress the tree? Well, one simple example is transpiration – the term used for the passage of water and gases through a plant. Mistletoes – including our European mistletoe Viscum album – transpire more freely than the host tree, forcing a passage of water through the tree’s vascular system from the roots faster than the tree wants to.

This raises interesting issues for the tree in winter, when most would, as most are deciduous, be naturally leafless and therefore not transpiring at all. But the parasitic mistletoe is evergreen – forcing ongoing transpiration throughout the year. And summertime is a problem too – the mistletoe’s leaves transpire faster than the host’s leaves – and so in dry summers the tree will become water-stressed much more quickly if it has lots of mistletoe on it.

There are relatively few studies of exactly how this phenomenon works for Viscum album on apple trees despite it being generally accepted as a major issue. But experimental studies from other host trees with Viscum album certainly support the idea.

Most research has, curiously, been on Pine hosts. Which might sound a little odd to most people – as our Viscum album is usually only seen on deciduous hosts. But there are subspecies of this mistletoe that will grow on evergreens, and one of them, Viscum album subspecies austriacum is a common feature on pines in some parts of Europe.

Recent papers from Spain (Sangüesa-Barreda, Linares and Camarero 2013) and from Switzerland (Zweifel, Bangerter, Rigling and Sterck 2012) have documented the transpiration impacts of this mistletoe – and they confirm that this is a significant problem.

The Swiss study showed that the mistletoe’s stomata – the adjustable pores that all plants have on their leaves to regulate water and gas exchange – were ‘barely regulated’ and so water loss through the mistletoe leaves was substantial. In an effort to compensate for this water ‘leak’ the infested pine trees closed their own stomata. This helped reduce water loss, but not sufficiently enough – and it had the side effect of reducing carbon dioxide assimilation – a gas the tree needs to photosynthesise and grow.  CO2 enters the leaves through the stomata – so closed stomata reduce CO2 uptake.

This is, of course, a double whammy for the pine – reduced water and reduced CO2 supplies – so a real problem for the tree, especially in dry periods. The effect is only serious if there is substantial mistletoe growth – but as mistletoe is spreading more rapidly in the area studied the conclusion is that pine mortality will increase in the area, due to mistletoe spread.

The Spanish study looked at the relationship between mistletoe infestation, intrinsic water use efficiency (iWUE) and the increased atmospheric concentrations of CO2. In theory, because higher CO2 levels mean stomata do not need to open so much, the impacts of closure of host stomata due to mistletoe stress could be offset by there being more CO2 anyway. The study confirmed, like the Swiss one, that mistletoe infestation increased drought-stress in infected trees, but also concluded that ‘rising atmospheric CO2 concentrations cannot compensate for the impacts of drought and mistletoe on tree growth’. Which, to be frank, isn’t surprising.

The overall point of reporting all this? To stress that too much mistletoe, whatever the host, will affect the host’s water and gas exchange, and not in a positive way! Management is needed – especially in smaller hosts like apples.

That shouldn’t put people off growing mistletoe of course –a few growths are unlikely to be a problem. And to reach problem levels your mistletoe growths will need to have been growing for several decades. But if your apple tree looks like these (see right) – you need to do some remedial management – and soon!

(PS There’s a video with some impressively overgrown apple trees in the video on this page... and I’m in it too…)

Refs for those who want to know more….

Roman Zweifel, Sara Bangerter, Andreas Rigling and Frank J. Sterck 2012 Pine and mistletoes: how to live with a leak in the water flow and storage system? Journal of Experimental Botany, Vol. 63, No. 7, pp. 2565–2578, 2012

Gabriel Sangüesa-Barreda, Juan Carlos Linares, J. Julio Camarero  2013 Drought and mistletoe reduce growth and water-use efficiency of Scots pine  Forest Ecology and Management 296 (2013) 64–73

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Commercial break:

Try The English Mistletoe Shop for Grow-Kits, Grow-Kit Gift Cards, Books etc, and mistletoe of course.

Or go direct to Amazon to buy A little Book About Mistletoe – in paperback or Kindle formats