Locating a resistance mechanism to the cabbage aphid in two wild Brassicas

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Abstract
Feeding behaviour of the cabbage aphid, Brevicoryne brassicae, was monitored electronically on two resistant Brassica species, B. fruticulosa and B. spinescens, and compared with a susceptible control B. oleracea var. capitata cv. Offenham Compacta. Aphids, monitored for 10 h on the under side of leaves, performed recognizable feeding behaviour on all species. Electrical Penetration Graphs (EPGs) of aphids on resistant and susceptible plants showed no difference in behaviour for aphids on resistant Brassica species compared to susceptible until stylets penetrated the phloem sieve elements when a large reduction in the duration of passive phloem uptake (E2 pattern) on B. fruticulosa was indicated. Although feeding behaviour on 6 week‐old plants of B. spinescens was similar to the susceptible controls, behaviour on 10 week‐old plants was similar to that recorded for B. fruticulosa. The mechanism of resistance is thought to be located in the sieve element as the normal sieve element salivation (E1 signal was either quickly terminated by withdrawal of the stylets from the sieve element or continued as a disrupted E2 pattern. Analysis of secondary plant compounds in the three Brassica species only identified significant differences in the glucosinolate profile. No reproducible differences were detected in the concentration of phenolics or anthocyanins. The major glucosinolate component of B. fruticulosa and B. spinescens was gluconapin rather than glucobrassicin and glucoiberin as found in the susceptible host plant. However, both pure glucosinolates and glucosinolate extracts from all three species did not reduce aphid survival on chemically‐defined artificial diets. These results suggest that the mechanism of resistance may be a mechanical blocking of the sieve element or stylets rather than a difference in the secondary plant chemistry of glucosinolates and phenolics.

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