Sweet pea (Lathyrus odorata) is a fragrant flowering annual vine or bush. Beloved by gardeners, sweet pea plants produce delicate, frilly flowers in a wide range of colours. They prefer rich, well-drained, slightly alkaline soil with a pH between 7.0 and 7.5. Sweet peas grow best in a sunny area where daytime temperatures remain between 21.1 and 26.7 degrees Celsius. A high-phosphorous fertiliser, such as 10-20-10, will encourage flower production. Poor growing conditions and pests promote diseases that can cause yellow leaves.
Pea Enation Virus
Enation mosaic virus is one of the most serious sweet pea plant diseases. The virus, which is active during hot weather, causes yellowed, stunted and mottled leaves. You may notice a decrease in the number of flowers. Aphids spread the disease from white clover and other leguminous weeds. To prevent this virus, control aphids and nearby weeds. Ask your county extension office for information on acceptable pesticides for your area.
Powdery mildew's signature symptom is a white powdery coating on leaves, but it also causes stunted growth and yellowed older leaves. The Erysiphe polygoni or Microsphaera fungus causes powdery mildew. This fungus is active during dry, moderately warm weather. To prevent infection, provide air circulation space between plants. Remove infected leaves. Fungicides are most effective in the early stages of infection. Your county extension office can provide a list of registered fungicides for powdery mildew.
Yellowed outer leaves is an advanced symptom of root rot in sweet peas. The Rhizoctonia solani fungus causes this disease, which occurs in wet, heavy soils. Other root rot symptoms include darkened, rotting crown and roots, drooping stems and leaves and plant death. Prevention is an essential control measure. Remove and destroy infected plants and avoid overwatering or watering directly onto the plant.
Some herbicides can injure nontarget plants. Amino acid inhibitor herbicides such as imazapic, primisulfuron and flumetsulam can cause chlorotic leaf margins and stunted growth in sweet peas. The plant may eventually die. To prevent herbicide injury, follow label instructions and avoid spraying during windy or wet weather. Apply herbicides carefully to avoid contact with sweet pea plants. Proper plant care and fertilisation helps to maintain plant health and avoid injury.