Plant Doctor Archive

Controlling powdery mildew

I HAVE a problem with my tamarillo tree. The new leaves grow well but as they get bigger they become mildewy and drop off. I keep it well fed with compost, washed seaweed and occasional banana skins. The fruit used to be very gritty but it has improved. Can you help with the mildew?

THE problem is powdery mildew, of which there are several different species, each affecting a specific range of plants. Powdery mildew spores germinate in warm, humid weather, so the disease is at its worst during summer. With tamarillo, it usually starts as white powdery spots which spread, eventually causing black patches, leaf yellowing and leaf drop.

As you can't control the weather, the next best way to control powdery mildew is to spray in spring before the first infection gets established and follow up with regular sprays at 2-3 week intervals. There are many fungicides you can choose from, including Fungus Fighter, Super Sulphur, Bravo and Fungus & Mildew Spray. Some gardeners use a home-made remedy by mixing 1 teaspoonful of baking soda in 1 litre of water and adding 1ml of dishwashing liquid, though you need to apply this at weekly intervals for it to have much effect. There are also reports that spraying regularly with seaweedbased foliar fertiliser may reduce the severity of powdery mildew attack.

With all these sprays you need to get good coverage over the upper surface of the foliage, particularly the new leaves which, with a large tamarillo tree, can be quite difficult. But don't despair, even if you don't spray, you should still get reasonable quantities of fruit, and in some years the effects of the mildew will be much less than others. You're keeping the tree well fed and watered, which will also help reduce the effects of the disease.

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