Transmission of celery-yellow-spot virus by the honeysuckle aphid, Rhopalosiphum conii (Dvd.)

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Celery yellow spot was first observed during the summer of 1934 in the Santa Clara Valley near Milpitas and has since been found in the celery fields near San Jose, Hollister, Salinas, and Sacramento. Symptoms have been briefly described (Severin and Freitag, 1938),4 and results of insect-transmission experiments (Freitag and Severin, 1939) presented, in previous papers. The disease causes no appreciable loss to celery growers, although 40 per cent of the plants were infected in some fields. Because the plants are only slightly stunted and because the spotted outer leaves are normally discarded in the harvesting of celery for the market, the disease is of small economic importance.

In 1935 an investigation was undertaken to study the symptoms and host range of celery-yellow-spot virus. Attempts were made to transmit the virus by means of different aphid species and by mechanical inoculations. Virus transmission by single specimens of winged and mature wingless aphids and retention by the aphid vector were the subject of experiment.

Symptoms

Usually the first symptom noticed in the greenhouse, about 14 days after inoculation of celery plants by the honeysuckle aphid, Rhopalosiphum conii (Dvd.) [R. melliferum (Hottes)], was the irregular pale green areas or spots and stripes, which rapidly became yellow (plate 1, B). This yellow spotting is the most characteristic sign of the disease. The spots and stripes are mostly along the veins (plate 1, C; plate 2, A), but are also scattered irregularly over the leaflets (plate 1, E; plate 2, E).

The yellow areas are irregular in shape and variable in size (plate 1, D; plate 2, B, C). The spots along the veins are often elongate (plate 1, C; plate 2, E) and sometimes occur at the basal portion of veinlets, where the latter join the main and lateral veins (plate 2, A). Some of the yellow spots are round and form small circular chlorotic areas (plate 2, D). The chlorotic spots may be numerous (plate 2, D) and may result in a. general yellowing of the leaflet (plate 1, E). In advanced stages of the disease the small chlorotic spots may coalesce, forming enlarged spots (plate 1, D; plate 2, B, D) and mottled areas (plate 1, F). Their yellow color gradually fades and may become white as the leaf matures.

The petioles of naturally infected celery plants develop circular white spotting (fig. 1). When the epidermis is removed from these areas, brown specks may be seen along the veins of the celery stalk (plate 2, F).