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Black Spot on rose

Spotty Leaves

This is the time of year customers arrive at the garden centre with their little bags of plant problems, one of which is spots on leaves.  Usually spots are caused by fungi with really long names that are difficult to remember, and by bacteria, viruses and even insects.  Sometimes it’s easy to tell what the disorder is, a lot of times it isn’t but usually it can be chalked up to Mother Nature dishing out cool, wet weather.

The fungal disease that most gardeners know is black spot on roses.  Black spots appear on the upper leaf surface and usually the leaves yellow then drop, starting on the lower part of the bush.  Often the second flush of leaves is less affected because by then the rainy season has passed but the spores will still be there and will overwinter.

Roses are high maintenance plants so if you’re going to venture into rose gardening for the first time try to pick disease resistant varieties such as rugosas or David Austin types.

Another plant that is prone to leaf spot is Photinia x fraseri, that evergreen shrub often pruned into a hedge, with the brilliant red foliage in spring.  Caused by a fungus with a long name, black spots appear on the leaves in spring or fall.  I’d almost guarantee that if you have Photinia you will have spotty leaves.

Rust is another fungal disease that shows its presence as spots on leaves.  Hawthorne trees, hypericum, and hollyhocks are the most commonly affected plants that I’ve seen.  These spots start out rust coloured and can turn brown or black.  Rust can overwinter on junipers so best not to plant them anywhere close by.   Raspberry rust is also prevalent at this time of year.

While most plants can tolerate a little leaf spot most people can’t stand by and let it happen so here are a few tips to avoid it in the first place.

Fungal spores transfer from plant to plant through splashing water, being carried about by insects or humans or by floating in the wind.  Try to keep foliage as dry as possible.  This means watering at ground level and not by overhead sprinklers.   Provide good air circulation by not overcrowding susceptible plants.

Spores can overwinter in dead canes and leaves so a proper clean up in the fall is essential in controlling the spread.  Rake up and discard fallen leaves and remove any infected plant material.  Apply fresh mulch in the fall to cover up any bits of leaves that were missed.  Do not compost diseased leaves as I don’t think most home composts get hot enough to kill spores effectively.

Start a fungicide spray program early before the problem gets out of control and repeat as necessary every week or two but don’t apply when it’s hot out as it may damage the leaves.

Horticultural oil or sulphur spray is considered organic but that doesn’t mean you can use them liberally.  Too much of a good thing is always too much and could adversely affect your garden.

Keep plants healthy.  Problems always find a weak plant so keep yours strong by top dressing with Sea Soil Original and using a good organic fertilizer if needed.  Don’t over fertilize with synthetic fertilizer as, not only is it bad for the microorganisms in the soil, but it can lead to too much fresh new growth which is prone to attack by disease and pests such as aphids.

You may not always be totally spot free but it’s not necessary or even desirable to have a totally perfect garden.  In fact, I don’t think there is such a thing.  Concentrate on growing healthy soil which in turn will grow healthier plants more resistant to disease.

Shirley Eppler

May 2014