Plant Doctor Archive

Rust and black spot on roses

My roses are a problem. I did spray with winter oil and copper oxychloride etc, but with wet weather, some varieties now have severe rust and others have black spot. I have been spraying every 7-9 days with super shield and trying to collect the infected leaves and pruning the dead heads. All seem to make no difference. One rose bush I have just severely pruned again it was so infected. I did speak to the gardeners at the Wellington Botanic Gardens and they have a commercial spray you cannot buy but suggested I spray with milk or baking soda?

It sounds like you are doing all the right things regarding sprays. In fact, you might be doing a bit too much. Removing diseased leaves and dead flowers is definitely a good idea — I assume that you are destroying them?

I wouldn't bother with milk or baking soda in addition to your other sprays, they are really just a safer alternative rather than a panacea for your problems.

The bottom line is that roses do not generally grow well in wet climates. Some cultivars are more disease resistant than others, but some of the older varieties are very susceptible to rust and/or black spot, and will keep becoming reinfected. You may have to bite the bullet and replant with more resistant plants.

If, however, your roses are precious, you could look at the part of the garden they are growing in. Do they get enough sun? Have they had enough fertiliser? Is the soil well-drained? Is there enough airflow around the plants to reduce humidity? You could also have a look at neighbouring plants to see if they are harbouring disease and spreading it.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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