I have a bluish colour on the leaves of my hydrangea plant which is in a pot near my 20-year-old old Malus crab apple tree. The bluish, powdery colour is on the leaves and seems to be some sort of fungus. The plant is well watered in a pot. However, the Malus is also affected and has a strange fungus like growth on several parts of the bark which has not gone away. The soil isn’t brilliant around the Malus but I do water it.
T O’Regan, Co Cork
It looks like your hydrangea is showing signs of a very common fungal disease (or to be completely accurate, a large group of closely related fungal diseases) known as powdery mildew. Many other kinds of popular ornamental and edible plants found growing in Irish gardens can be similarly affected, including apple trees (Malus). Symptoms vary according to the different species but typically include a grey dusty fungal coating on the leaves and purple or yellow discolouration of the foliage. Left untreated, it reduces vigour, is unsightly and can sometimes eventually kill the afflicted host plant.
Powdery mildew is usually caused by a combination of humid, shady, overcrowded growing conditions, abrupt swings in temperature. and erratic watering. It’s also much more common in late summer-autumn when plant vigour naturally starts to falter.
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Rather than reaching for a conventional fungicide, try spraying the affected plants with a solution of milk (10 per cent milk to 90 per cent water) or bread soda (mix 1L of water, 3-4 teaspoons of bread soda, one teaspoon of vegetable oil, and a small bit of washing-up liquid), both of which are organically acceptable methods. I’d also suggest improving its growing conditions by moving your hydrangea to a more open site with better ventilation.
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You don’t say how long it has been in its pot, but there’s a good chance that the size and design of the latter is another part of the problem. Hydrangeas are naturally large, thirsty, hungry, quick-growing plants which resent restriction of their root systems. It’s very possible that your plant has outgrown its container, in which case it should be repotted into a larger one in late autumn after leaf fall, using a mix of two parts John Innes No.3 compost to one part peat-free multipurpose compost. Make sure the pot has good drainage holes at the base to prevent the compost from becoming waterlogged. Raising it up slightly on “pot legs” or a few flat stones will also help.
Powdery mildew spreads via minuscule fungal spores on infected plants and can overwinter on fallen leaves and plant debris. For this reason, it’s always a good idea to collect these fallen leaves and then burn or bag and bin them (don’t compost them). Also, do this around the base of your apple tree, as well as mulching the soil here with a 5cm deep layer of well-rotted manure or home-made compost in spring. This will help to improve soil health and fertility, as well as prevent the tree from becoming diseased again next year.
