CHANGES IN THE DRY-MATTER, SUGAR, PLANT FIBER AND LIGNIN CONTENTS OF SWEDE, RAPE AND KALE ROOTS IN RESPONSE TO TURNIP ROOT FLY (DELIA-FLORALIS) LARVAL DAMAGE

Plants from two genotypes of swede (Brassica napus var napobrassica), kale (B oleracea var. acephala) and rape (B napus var napus) were each inoculated at the three to four true leaf stage with 10 eggs of the t urnip root fly, Delia floralis. After 8 weeks the damage caused by the resultant larvae significantly reduced the weight of leaves, stems an d roots compared with uninoculated plants, with the greatest reduction (26-46%) being found in the roots. Dry matter content, which did not differ between either crop type or cultivar, was unaffected by larval damage. Ethanol-soluble sugar content was reduced in all cases by larv al damage, but only significantly in one rape and one swede cultivar. The effect of D floralis damage on the concentrations of individual su gars (glucose, fructose and sucrose) was crop type and cultivar depend ent. The neutral detergent fibre and lignin contents of the undamaged swede roots were significantly lower than in either the undamaged kale or rape cultivars studied. In all crop types the neutral detergent fi bre and lignin content significantly increased following D floralis da mage. The six Brassica genotypes studied fell into two groups on the b asis of host status for D floralis. The two kale cultivars and the for age rape Hobson were considered poor hosts for D floralis, with low pe rcentage pupation (38-41%) and low pupae weight (12.5-14.4 mg). The tw o swedes and the oilseed rape Ariana were considered good host for D f loralis, with both higher percentage pupation (51-59%) and pupae weigh ts (19.8-20.0 mg). The possible relationships between the root composi tion of undamaged and attacked roots, and D floralis resistance are di scussed.