Sweet peas have a waxy coating on their leaves which has two main effects. The first is to offer a significant level of protection against pests and diseases, the second is to make it more difficult to apply chemical sprays effectively.
Plants are most susceptible to attack either when young, before the waxy coating has developed properly, or when old and the the plants' natural defences are waning. Equally, these are the times when the plant will most readily absorb protective chemicals through the leaves and spraying will be most effective. Most problems can be avoided by keeping the crop well ventilated with plenty of air movement through the canopy.
Because of the waxy coating which repels water, high volume sprays are likely to need additional wetter. Alternatively, ULV sprays will minimise this problem while giving better penetration of the crop canopy.
© 2006 P E M Rowland