"This has been a year when powdery mildew awareness has been very critical for the blemish-free plants we want to produce," he said. "We've had a relatively mild winter, so we've not had the usual deterioration of powdery mildew's overwintering structures. It was able to develop quite early."
RHS adviser Guy Barter added that a humid spell in May and warm weather has helped the disease develop. It started with apple powdery mildew, which is one of the earliest kinds, and then spread to other trees, shrubs and hardy perennials.
Barter warned that further outbreaks are likely at this time of year because plants are under stress from dry soil. "Despite the rain the deeper soil is dry in many areas and I would expect that to have an effect. There will be an awful lot of inoculant going into the winter and so another mild winter could lead to more outbreaks next year."
ADAS consultant Andrew Hewson said this summer has seen more powdery mildew on key host crops such as amelanchier, Ribes, lonicera, spiraea, Photinia, ornamental vines, roses and Potentilla. The midsummer heat has also meant less opportunity for growers to spray because the heat creates soft growth, increasing the risk of spray scorch to crop foliage.
Various herbaceous perennials including aster, aquilegia, Delphinium, phlox, scabious, hardy geranium and Sedum, and tree crops such as acer, Crataegus, Malus, Prunus, Quercus and sorbus are also prone and so at risk, added Hewson.
Adlam warned that growers should be aware that not all plants have the same type of powdery mildew and not all products work on every variety. Also, some products are eradicants while others are protectants.
There are five kinds of powdery mildew, each of which affect different types of plant and need the right control products for treatment. "They need to know the plant, the powdery mildew species and the control product," said Adlam.
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