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The symptoms caused by CtRLV in carrot are slight reddening and yellowing of the leaves, with moderate stunting of the plants. CtRLV commonly occurs in association with CMoV, in which case the reddening, yellowing and stunting are intensified (hence the name motley dwarf disease), and yield losses are considerable. Very similar symptoms occur in dill. In carrot seed crops in Australia, motley dwarf disease caused rotting of the roots of transplanted seedlings, and the surviving plants produced low yields of seed with poor germination (Stubbs, 1948). Parsley affected by viruses resembling CtRLV and CMoV are stunted, with varying degrees of chlorosis, the outer leaves being conspicuously yellowed, and later becoming pink or red (Frowd and Tomlinson, 1972).
In England, Cavariella aegopodii is one of the most numerous spring migrant aphids, and often accounts for up to half of all aphids caught on sticky traps in May and June (Watson, 1960). It is therefore a serious pest of carrots and causes considerable direct damage. To control the aphid, and even more for the prevention of damage caused by carrot fly (Psila rosae), commercial carrot crops in the UK and elsewhere are commonly treated routinely with insecticide. As a result the incidence of motley dwarf disease has been considerably reduced. However, the disease may remain a problem in non-commercial crops that are not treated with insecticide (Murant, 1975), and may reappear if such treatments are abandoned. In the UK and elsewhere in northern Europe, the prevalence of Anthriscus sylvestris, wild carrot and other wild perennial umbelliferous hosts ensures a continuing source of inoculum. An alternative approach is to plant virus-resistant (tolerant) cultivars.
In Australia, a hymenopterous parasite of C. aegopodii, probably Aphidius salicis, was introduced in 1962 in an attempt at biological control. There has since been a dramatic decrease in numbers of C. aegopodii and a corresponding decline in the incidence of carrot motley dwarf disease, to the extent that Stubbs et al. (1983) stated that the disease could no longer be found in Australia. However, this appears to have been an over-statement because it was subsequently reported from the Australian Capital Territory (Waterhouse, 1985).
In California, overwintering carrot crops are the main source of infection for spring-sown crops (Watson and Falk, 1994), and it is recommended to plant at a distance from overwintering crops, or if this is not practicable to grow resistant (tolerant) cultivars. However, the market cannot accept a preponderance of such cultivars, and the use of insecticides may also be necessary.
The disease was responsible for serious losses in Australia in the 1930s and 1940s, when it caused the virtual abandonment of early carrot production in the area around Melbourne, Victoria, and was the principal factor limiting carrot seed production (Stubbs, 1948). It has since declined in importance in Australia (Stubbs et al., 1983), though it can still be found (Waterhouse, 1985). The disease also caused serious crop failures in England in the 1950s and 1960s (Watson, 1960; Watson and Serjeant, 1964; Cock and Dixon, 1969). Yield losses in the Salinas Valley of California in 1969 were estimated at 50-60% (Krass and Schlegel, 1974) and the disease was still prevalent there in the 1990s (Watson and Falk, 1994).
Watson and Serjeant (1964) investigated the effect of motley dwarf disease on carrot in field experiments. Plots infected in early June and subsequently kept free of aphid infestation experienced about 50% loss of yield. Naturally infected crops suffered more because of the additional damage caused by aphid infestation, Cavariella aegopodii being a serious pest in its own right.
