Lesley Rowe, of Dunedin, asks :-

I have a native clematis, variety Purity, in my garden. It has started to grow some funny looking stalks. (see photos) The growth reminds me of a type of orchid. The stalks grow normal and then it gets this growth on it. Is it some type of disease?

Dave Kelly, a botanist at the University of Canterbury, responded.

I have had a look at the photos and I think what the Clematis stems have is a kind of gall. I can see the resemblance to orchid flowers, but I think it is accidental, these are swollen stems. My prediction is that if you cut one of these swollen bits in half it will be solid with stem tissue, whereas orchid flowers are hollow.

Galls are a term for swellings on plants, and they can occur on almost any tissue (stem, leaf, flower, fruit, etc). They are caused by another organism, but there is a wide range of possible organisms that cause plant galls, including viruses, bacteria, fungi, mites, and insects. In some cases, such as the galls on the leaves of the native scarlet mistletoe (Peraxilla tetrapetala) the organisms causing the galls have not been discovered despite careful searching.

In this case, though the photos are not very clear, there seem to be small white bodies over the stems, which makes me suspect a rust fungus. The fungus gets into plant tissues where it grows, may make a gall, and eventually produces fruiting bodies on the surface that release spores that blow off to infect other plants.

However I must say I have not worked on rust fungi, so this is what would best be called an educated guess rather than a definitive diagnosis.