One of the by-products of having great new petunias is that some of them are produced vegetatively (from cuttings) rather than seed. If you’re buying some of the fancier, new plants – one plant to a pot with promises of massive growth – the odds are you’re getting a vegetative plant and not a seed-grown one.
The problem here is that crowded growing spaces can quickly lead to powdery mildew issues on this plant.
There are three forms of the disease that home gardeners are likely to see and none are seed related but are either carried on the plant or blown into the greenhouses on the wind.
The disease can completely cover the leaf – this usually happens starting on the lower leaves but will eventually cover the entire plant with a white dusting effect.
The problem is that this isn’t the only symptom. Often the problem resembles a nutrient deficiency with yellowing lower leaves. To identify whether this is powdery mildew or indeed a nutrient deficiency, check the leaves immediately above the yellowing ones. If pm, they will have slight whitening of the leaves where the fungus is establishing itself but hasn’t damaged the leaf yet.
Cultural control of planting your petunias in well-ventilated areas in the full sun. If you’re trying to save these (sometimes expensive) plants from one year to the next make sure they don’t have powdery mildew to start with or you won’t get rid of it.
Spray controls include copper based products and potassium bicarbonates.