A New Virus Disease of Carrots: Its Transmission, Host Range, and Control

Abstract

A new virus disease of carrots, observed at Melbourne in 1943, is descr. It occurs throughout Victoria, and has also been recorded in New S. Wales, S. Australia, Western Australia, and Tasmania. The disease caused the virtual abandonment of early carrot production in the Melbourne vegetable area and, during the last war, was the principal limiting factor to carrot seed production. In Victoria, carrot crops are exposed to infection from Apr. to Dec, but a disease-free period occurs during the summer. Infected crops have a stunted, unthrifty appearance suggestive of mineral deficiency. Foliage symptoms consist of an irregular chlorotic mottle and marginal reddening of the lower leaves. Leaflets are distorted and reduced in size, and petioles and subpetioles are twisted longitudinally. Growth stunting is severe in susceptible vars. Stem necrosis sometimes develops on the youngest leaves of fully grown, recently infected plants, and on the older leaves when infection is of long duration, The virus is transmitted by the aphid Cavariella aegopodii. The virus has been transmitted by core-grafting, but not by mechanical inoculation methods. There is no evidence of transmission through carrot seed. Carrot is the only known natural host, but Apium ammi, A. australe, hemlock, dill, and coriander have been experimentally infected. Celery, parsley, parsnip, caraway, and fennel appear to be immune. A wide range of cultivated vars. and wild strains of Daucus carota developed infection when grown under field conditions, but several local vars. were tolerant to the disease. The vector, which is widely distributed in Victoria, occurs commonly on willow and fennel, and is a serious pest of carrot, celery, parsley, and parsnip. Histological studies of the feeding habits of the vector showed that it is a phloem feeder. The virus is of a persistent type and in one expt., the vector, after an infection feeding period of 48 hrs., remained infective during 18 days of serial transfers. There appears to be no evidence of the occurrence of a latent period. In 2 controlled expts. virus infected roots were subject to high mortality when transplanted for seed, whereas all healthy roots produced vigorous seed plants. Surviving infected seed plants lacked vigor and seed production was greatly reduced. The invert sugar content of healthy roots was significantly higher than that of infected roots. It is believed that the biological breakdown of transplanted infected roots is frequently preceded by a form of necrotic collapse directly attributable to the virus. Environmental control of the vector, and the disease, has been obtained by delaying the sowing of susceptible vars. until early summer. In a spraying trial with spring sown Chantenay carrots, weekly applns. of a proprietary DDT emulsion almost completely eliminated the vector, significantly reduced the amt. of disease, and resulted in a 6-fold increase in yield of marketable roots. Disease control has also been demonstrated by comparing the infected yields of several virus-tolerant vars., and one susceptible var. grown under exptl. conditions. A Victorian virus-tolerant selection outyielded other vars. in this trial.