The warm, dry summer we enjoyed this year came to an end when August arrived bringing with it cooler, damper weather*. Funnily enough, damp weather favours the development of rusts on plants as well as on metal, although they’re nothing to do with each other!
Some of the older varieities of raspberry are particularly prone to raspberry rust (Phragmidium rubi-idaei). I have some Glen Ample canes – apparently it is the most common/popular summer fruiting variety in the UK – and they have rust (my Autumn fruiting varieities are, so far, rust-free). I class Glen Ample as an “older” variety, although it has only been on sale since the 1990s, so it’s not really so old.
Rusts are fascinatingly complex organisms with a vocabulary all of their own. The yellowing on the upperside of the leaf alerts you that something is going on underneath. Sure enough, if you look at the underside of the leaf the rust spores are plain to see. At this time of year there are two colours: the orange ones that will have been spreading the fungi through the summer, and especially since the rain came; and the dark ones which will overwinter ready to start the cycle again next year.
Happily, raspberry rust isn’t much of a problem and is unlikely to affect fruit production. As with most fungal pathogens, thinning the plants out to ensure good air circulation and practising good hygiene in clearing away dead material will help keep infection levels down. But there are plenty more, and worse, diseases that could affect raspberries – rust isn’t really something to worry about.
*In Welsh, July is Gorffennaf – the end of summer:)