2 possible causes- 1, sweet pea leaf scorch, remove any badely affected leaves and spray with a foliar feed. 2, viral disease, many plants are affected by viruses, sweet pease are particularly prone, often the leaves may become yellow and blottchy and the plante may become distorted and stunted, aphids can carry viruses, afected plants should be removed and destroyed and the remainder sprayed with aphid insecticde.