Do your sweet peas always end up looking like this, or similar, but maybe not so bad. This is powdery mildew which is a real problem with Sweet Peas, and probably, at some point, in the season they will become infected, it is almost inevitable. What we can do is fight it off until, hopefully, towards the end of the growing season when we have had time to enjoy the wonderful colour, scent and blooms before the mildew gets the edge.
Illustrated is a bad attack of mildew, and by this stage there is not much to be done, but dig them out.
The key to preventing, ameliorating, or reducing Mildew is planting into organically rich soil, holding moisture, good air circulation and not to let the plants dry out.
The mildew attacks the plants when they become too dry or otherwise stressed. Sweet peas are hungry feeders, and they like plenty of water. Also, when we plant sweet peas, we tend to pack them in, for mass colour effect and we plant too many too close.
So what to do?
First, plant sweet peas in a part of your garden which is the least dry. Before planting add lots of organic matter so the soil holds moisture. Once planted water, water, water, if the days are dry. Mulch the soil to retain the water. Sweet Peas are very susceptible to becoming over dry and this sets of the mildew.
Given the watering requirements, especially in drier areas of the country, it may be time to consider if Sweet Peas are the best annual to grow.