Plant Doctor Archive

Gold turns sneaky silver

I HOPE you can solve this problem for me. I have floribunda roses down my drive in a bark garden. They are probably about 12 years old. They get VIP treatment - they're watered every week, sprayed every two weeks, manured when necessary and dead-headed all the time - but now they are getting silver blight.

I lost one rose three years ago and replaced the dirt where it had been growing. Despite this, 'Aotearoa' and 'Eldorado' are now blight-stricken.

When I prune it is always fine and I spray them on the same day. It appears if I want to grow roses down the drive I am going to have to remove all the dirt before I can plant any more.

Can you tell me why my roses are getting silver blight? My roses out the back of the house are doing fine. I hope you can tell me what I am doing wrong.

IT SOUNDS like the problem is silver leaf disease (Chondrostereum purpureum), a fungal disease which gets in through broken branches or pruning wounds. The first sign of the disease is when leaves on a branch or even on the whole plant turn silvery. It's a relatively common disease in peach or nectarine trees but can also affect apples, pears and even poplars and willows, as well as roses. Perhaps the infection is coming from trees in your garden or nearby.

Unfortunately, there's no easy cure. Changing the soil won't help as the infection is airborne. Badly infected plants usually go into a slow decline. Rose expert Doug Bone suggests pruning out the infected shoots as soon as you see them, cutting back hard to take out the whole branch with the silvered leaves. Doug also reports success using Trichodowels. You insert these into a hole drilled in the base of the affected rose bush, but unfortunately they are now only available in commercial quantities for orchardists.

To reduce the risk of infection, prune your roses and other susceptible trees only on warm sunny days when there has been no rain for at least the previous 24 hours. Sterilise your tools after each plant, by dipping the blades in methylated spirits before wiping them clean or wipe them with a cloth soaked in meths.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.