Carrot Light Root Syndrome is a Possible Physiological Disorder of Carrot (Dacus carota) in California.. R. Creamer, Department of Plant Pathology, University of California, Riverside 92521. M. Wadsworth, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 78:03171. Accepted for publication 14 December 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0317D.
In recent years, reports of carrot roots harvested from early fall plantings that are unmarketable because they are light in color have increased in the Imperial and Coachella Valleys of California. It has been suggested, but not verified, that the light root syndrome is associated with lettuce infectious yellows virus (LIYV) or its insect vector, the sweetpo-talo whitefly Bemisia tabaci. Greenhouse inoculation studies of LIYV with 12 cultivars of carrots (Daucus carota L.) under a variety of environmental conditions showed that B. tabaci-A biotype could transmit LIYV to carrots, but the plants remained symptomless. Neither A nor B biotype whiteflies could transmit an agent that caused light root symptoms, nor could they induce symptoms by feeding alone, regardless of the numbers of whiteflies (5-80/plant) or lest plant age (cotyledon-bolting). The symptoms could not be induced by mechanical inoculation of the tops nor by grafting of the carrot roots. No viral dsRNA was detected in symptomatic plants, nor could fungus or bacteria be isolated from suface-sterilized symptomatic tissue. Symptomatic carrots were randomly distributed in plots in six different fields monitored over two years, although there were significant differences in the levels of light root in the different fields. Light root symptoms occurred in carrots from field plots arranged in a randomized block design in blocks pretreated with methyl bromide and/or in which carrots were protected with a nonwoven row-cover material as well as in the untreated controls. In these plots, carrot cvs. Apache and Navajo showed a lower percentage by weight of light roots than cvs. Aristopak, Avenger, Caropride, Carpak, Dominator, Fancipak, Imperator, Legend, Pacmore, or Six-Pak II. These results demonstrate that carrot light root syndrome is not associated with LIYV or whitefly feeding. The presence of symptoms despite row covers and fumigation suggests that the syndrome is not caused by a flying insect or soilborne agent and may be a physiological disorder.