Spotted wilt of the sweet pea

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Introduction

In the coastal counties of California the sweet pea, Lathyrus odoratus L., is frequently afflicted with a streak disease. The cause of the disease is sometimes obscured by viruses of the mosaic type or by resemblance to the description of bacterial streak of sweet pea.⁽⁵⁾4 It has, however, been proved to be a virus identical with that responsible for the spotted wilt disease of tomato. Recently⁽⁶⁾ this virus has been reported as the cause of a streak disease of the garden pea, Pisum sativum L.

Symptoms

A characteristic symptom of the disease on sweet pea is a necrotic streaking of stems and petioles. The streaks are reddish brown to dull purple in color and in the advanced stage are conspicuous. They may run the entire length of the stem, parallel to the long axis, being found on analysis to consist of disconnected short streaks or a continuous long one. They have been seen to develop both above and below the point of virus inception.

Leaves and shoots may turn yellow and die. Early symptoms on the foliage appear as spots, usually 5 to 15 mm long, more or less circular or oval in shape, yellowish at first, with diffuse margins. Later the spots become somewhat brownish and form a pattern typical of the virus on other hosts. From these spots are developed yellowish sectors or zones, ordinarily one or only a few on each leaf. The spotting of the foliage is fairly definite in early stages, although not striking; but the general yellowing which may follow becomes a conspicuous symptom. Symptoms of the disease are shown in figure 1, A and D.

Blossoms on infected plants have been seen to develop a circular pattern in the pigment and the virus has been recovered from such material. Again, blighting of the whole shoot may occur prior to blossom formation.

Variability in the expression of symptoms has been observed. Plants infected early may yellow and die without showing other symptoms.