Sweet peas (Lathyrus odoratus) are elegant cool-season annuals with brightly colored flowers. They grow best in moist, well-drained soil and full sun. Sweet peas are members of the legume family and thrive in early spring before summer's heat wilts them. While they are generally easy to grow, sweet peas are susceptible to some diseases that can cause their leaves to turn yellow.
The pea mosaic virus can cause yellowing and mottling of the sweet pea's leaves. Symptoms include foliage with dark green areas combined with yellow-green areas. The mosaic virus is spread by aphids, which can be treated with an insecticide and by controlling nearby weeds.
Powdery mildew (Erysiphe polygoni) is a grayish-white growth on the stems and leaves of infected sweet pea plants. As the fungus progresses, infected leaves will yellow and wither. The powdery mildew spores are spread through the air. Use a fungicide regularly and as soon as you spot the powdery mildew.
Spotted wilt is a virus that causes circular spots on leaves that are first yellow, then brown. The virus, which can kill your plants, is spread by thrips. Use an insecticide and control nearby weeds, including grasses.
Ramularia Leaf Spot
Ramularia leaf spot is a fungus specific to sweet peas that causes large tan to yellow spots on plants, starting with the lower leaves. After infection, the leaves often will drop. The fungus survives in foliage that has been removed, so do not compost infected plants. Overly wet conditions can cause the fungus to appear. The University of California Agriculture Department recommends not planting sweet peas in the same area each year if you have problem with the fungus. Water sweet peas at ground level and use a fungicide if necessary.
Yellow leaves are often caused by a manganese deficiency, according to Cal Poly State University. The deficiency is frequently seen in soil with a very high pH. Manganese is vital for photosynthesis, and a lack of it will cause chlorosis (reduced chlorophyll). Do a garden pH test and add sulfur as needed to amend the soil.