Plant Doctor Archive

Diseased crab apple

FOR two years now I have had problems with my crab apple 'Jack Humm'. Last year all the fruit became diseased after blossom time and eventually dropped to the ground. I've attached a photo of what it looks like at present. There is not a lot of air flow, as the tree is against a 6ft-high fence. Does this contribute to the situation?

IT looks as if your tree is suffering from a couple of fungus diseases. The symptoms on the fruit look like apple scab, often called black spot. The disease starts early in the season as spots on the leaves, which look a different green from the rest of the leaf. The spots gradually turn black and can get quite large. The fruit also gets black spots that go brown and corky in the centre, sometimes with cracks. The fungus overwinters on fallen leaves, with spores infecting new growth the following spring. Cool, wet weather encourages the disease.

The other disease that could also be present is powdery mildew, which affects both leaves and fruit. Encouraging better air flow around the tree could help, but may not be practicable. Raking up the fallen leaves and fruit in autumn would help reduce the chances of infection, but disease spores could easily blow in from neighbouring gardens.

Spraying with fungicides should help. In winter, after you've cleaned up fallen leaves and fruit, give the tree a thorough spray with a copper-based fungicide, such as Copper Oxychloride or Super Copper, or alternatively use Lime Sulphur. Then, during spring, spray with a fungicide (Fungus Fighter is good for black spot) every 10 days or so, starting from just after the leaf buds open in spring until three weeks or so after flowering has finished. This should protect the foliage and young fruits from infection and, hopefully, lead to lovely clean crab apples for next year's display.

You may find that following this spraying programme for a year or two is enough to break the cycle of infection and that you won't need to spray every year after that. But it will depend a lot on whether there is a ready source of infection from nearby gardens.

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