NEWS & VIEWS
Germ Warfare
When I taught further education students about pests and diseases of trees, this bacterial canker on a specimen of Sorbus aria ‘Lutescens’, which was growing not far from the back door of the labs, was a godsend. This canker was actively ejecting ‘gunk’ (Erwinia, possibly?) on damp days in May, and the students could take a small amount of inner bark, put it in water, and by next week’s class a large clump of bacterial ooze would have been produced. In hindsight, I could have made this into an experiment and introduced students to Koch’s postulates (by infecting other Sorbus with this bacterial ooze then re-isolating it), but I’d only just started teaching and the idea didn’t occur to me. We could even have used an ELISA test for fireblight (Erwinia), which is commercially available. Sadly (or possibly cheerily) the canker is now all dry and beginning to occlude (see images to right). Lesson learnt: When nature brings you bacterial gunk oozing out of a tree at the back of your laboratory, see it as the opportunity that it is!
2006
2007
2018
The Cantankerous Canker
When one of your friends does not look so good (scruffy, dirty, spot on their nose or getting fat), you have to subtly bring up the subject and perhaps be a bit oblique about it, so as not to offend. That’s what my friends tell me, at least. In fact, I’m not sure why they keep mentioning it. In which case, a subtle word is needed with this Prunus, which has really ‘gone to the dogs’ due to a long-term stem canker (Neonectria galligena, possibly) which is still expanding (the image shows a time lapse of ten years). The tree has persisted for a long time with this problem, but is ‘persisting’ what is wanted from a small ornamental tree in a larger landscaping scheme? Surely, we want it to be pretty - not pretty diseased? Cankers vary quite a lot in terms of their expansion or contraction over time, it would be useful to be able to predict this (prognosis) from causal agent and host (diagnosis).
July 2008
WWW.PROARBMAGAZINE.COM
July 2018
Lesson learnt: If it ain’t pretty, say so. For trees, mostly, I would say that a minor ugly stem canker doesn’t matter that much, but for some small focal ornamental trees, well that’s what they’re for, so the visuals of such a tree can be more important in particular settings. The tree pictured is now no beauty.
Pro Arb | June 2018 19