Mite feeding on Raspberry rust. Coloured scanning electron micrograph (SEM) of a mite feeding on rust. Raspberry rust (sometimes called raspberry yellow rust) is a disease of the foliage of raspberries, caused by the fungus Phragmidium rubi-idaei.. Mites are a highly adaptable group that are related to spiders and have eight legs (four of which are here seen raised). Some mite species are parasites, while soil mites form part of the great diversity of organisms that contribute to the break down of plant material. Magnification: x260 when printed 10 centimetres wide.