Plant Doctor Archive

Diseased magnolias

THE foliage on some of my magnolias has become more and more discoloured over recent months. It starts as a small blotch and expands to cover large patches which go right through the leaf. Originally on only one tree, it is now spreading to neighbouring trees. They are a group of five magnolias, planted two years ago. Mature magnolias planted a little further away don't seem to have been affected yet and are growing well in the same conditions.

The Magnolia soulangeana leaf you sent looks like it has been affected by a bacterial disease. Generally, magnolias are not susceptible to many pests or diseases, but varieties with Magnolia liliflora in their parentage (as M. soulangeana varieties do) seem susceptible to this leaf spotting at times. Bacterial diseases are more prevalent in cool, wet weather when the leaves remain wet for long periods. They also tend to be more common in nurseries where plants are watered with overhead sprinklers. So it's possible the plants were already infected when you planted them and it's the wet weather this year that's caused such serious symptoms to appear. It's also possible the disease has come from a nearby garden.

Without lab analysis it's difficult to tell exactly which disease is causing the problem, but the good news is that in a "normal" spring with warmer, drier conditions the plants may show no symptoms at all. There are many broad spectrum fungicides available, but they have little effect on the relatively few bacterial diseases we occasionally come across in our gardens. For the home gardener, copper-based sprays are the most practicable method of controlling bacterial diseases, so I recommend you spray your magnolias with a product such as Champion Copper or Copper Oxychloride. Unfortunately, copper sprays can be washed off in heavy rain, so you may have to spray every 10 days or so until the weather improves. Ideally, spray when the foliage is dry, but not on a hot, sunny day, and take care to mix the spray according to the label recommendations, as a stronger-than-recommended dose may itself cause leaf damage.

To reduce the risk of the disease overwintering in the soil, rake up the fallen leaves in autumn and burn them.

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