Small Black Spots on Sweet Peas

Honey-scented, pinafore-ruffled spring flowers clambering up garden fences and trellises can only be sweet peas (Lathyrus odoratus). Give these tendril-stemmed annuals a cool, sunny spot with organically rich, well-drained soil, and they’ll climb up to 8 feet before succumbing to summer's heat. Healthy sweet peas are a riot of white, pink, red, purple or bicolored blooms against delicate, deep-green foliage. If black spots show up to blemish their display, however, suspect fungal disease.

Ascochyta blight fungi threaten sweet peas wherever they grow. The three strains responsible for black spots on sweet peas survive in previously infected seeds or in the soil and debris around the plants. During cool, rainy weather with temperatures close to 68 degrees Fahrenheit, these fungi release spores that splash onto nearby plants. One of them, Mycosphaerella pinodes, also produces wind-blown spores capable of infecting plants long distances from their release point. These diseases threaten your sweet peas as long as cool, wet conditions persist. Infected plants develop symptoms within four days.

Mycosphaerella blight speckles your sweet peas' leaves, stems and seed pods with irregular purple-brown spots. You're most likely to find them on the plants' lower, leaf-sheltered portions, where humidity is highest. These lesions become circular and deepen to black, expanding up to 1/4-inch across. If moist conditions persist, they cover severely infected leaves. Other leaves yellow, shrivel and cling to the vines. Diseased ripe pods have dark-edged, oval lesions and may contain discolored or undersized seeds. Young ones wither before maturing.

Long, narrow purple-black spots on your sweet pea's lower stems result from an Ascochyta pinodella stem- or foot-rot infection. In the worst cases, these spots encircle the stems at and just beneath the soil line. The weak-stemmed sweet peas often lodge, or collapse to the ground. The Ascochyta lathyri fungus also attacks sweet-pea stems, rots the lower leaves and blackens the petals and flower stalks. Like Mycosphaerella, these fungi pose the greatest threats during wet weather.

Although you didn't plant sweet peas for their inedible peas, the cosmetic damage Mycosphaerella or Ascochyta infection does to these stunning ornamentals is enough to warrant treatment. While the return of dry weather halts the spreading spores and lesions in their tracks, protective measures may eliminate infections. Begin with crop rotation -- situating your sweet peas in a different part of the garden each year for three or four years and following them with Ascochyta-immune plants. By providing adequate space between the vines and watering them from beneath, you'll minimize fungus-favoring humidity. Finally, clear all debris from your garden bed each fall. Proper sanitation guards against a host of fungal and insect threats.