Carrot motley dwarf and parsnip mottle viruses

Carrots that show symptoms of carrot motley dwarf contain two viruses, carrot mottle virus (CMV) and red‐leaf virus (RLV). CMV cannot be manually inoculated to carrot, but can be to some other members of the Umbelliferae, as well as to some species of the Solanaceae, Leguminoseae and Chenopodiaceae. The host range of RLV is limited to the Umbelliferae, and it is not manually transmissible but was transmitted by grafting. Cavariella aegopodiae Scop, transmits RLV alone but will transmit CMV only from plants infected with both viruses. Thus aphids were unable to transmit from coriander plants manually inoculated with CMV, but after these plants were infected with RLV by aphids, virus‐free aphids acquired and transmitted both viruses from them.

Aphids remain infective with both viruses for 1–2 weeks, and retain infectivity through the moult. A minimum total of about 9 hr. is needed for acquisition and transmission; vector‐efficiency increases with increasing feeding times up to severaj days.

The viruses causing motley dwarf become attenuated in the glasshouse after continued aphid‐transmission; avirulent isolates protect their hosts against infection by virulent ones.

Infectivity of saps from CMV infected plants was increased by extraction at high pH in the presence of a trace of Zn, and was associated in carrot with particles 30 mμ in diameter. Water‐phenol extracts are almost as infective as extracts in buffer, but are inactivated by 0·02 μg./l. pancreatic ribonuclease.

Parsnip mottle virus (PMV) resembles CMV in many ways, but also differs in some important respects. Unlike CMV it infects celery and parsnip, and it is transmitted by C. pastinacae as well as C. aegopodiae, the only vector of CMV (in combination with RLV). PMV is transmitted by aphids from plants infected with it alone, whether these plants were infected by aphids or by manual inoculation.

Carrot plants infected first with PMV and then with the motley dwarf virus complex developed symptoms of motley dwarf, but in coriander the reverse happened. CMV and PMV appear to interfere with each other's multiplication in the hosts, but CMV is dominant in carrot and PMV in coriander. They have some properties of distantly related strains.