Genetic variability for Russian wheat aphid, Diuraphis noxia resistance in South African wheat genotypes
Tolmay, Vicki Louise
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Near-isogenic genotypes 1684/Tugela and 2199/Tugela containing D. noxia resistance ex SA1684 [PI 137739 (Dn1)] and SA 2199 [PI 262660 (Dn2)] were studied in terms of antibiosis, antixenosis and tolerance, population development of D. noxia under field conditions and the probing behaviour of D. noxia. The susceptible cultivars Betta and Tugela were included for comparison. A sister line of 1684/Tugela, the commercial cultivar TugelaDn, was used to characterise the constitutive and induced components of the resistance to D. noxia. Mechanism of resistance studies showed antibiosis present in both resistant genotypes. No antixenosis was recorded in 2199/Tugela, with only low levels present in 1684/Tugela. 1684/Tugela was able to retain the same leaf area and plant dry mass as 2199/Tugela under a much higher aphid infestation level and both lines were significantly more tolerant than Betta, but did not differ from Tugela. Betta and Tugela showed higher infestation levels, both in terms of percentage tillers infested and number of D. noxia per tiller, and lower yields than the resistant genotypes. The initial rapid increase of D. noxia populations started in the beginning of October and peak aphid numbers were in early to mid- November. EPG data reflecting the probing behaviour of D. noxia revealed distinct differences between the resistant cultivars and the susceptible Betta and Tugela. The influence of resistance in 1684/Tugela and 2199/Tugela on the probing behaviour of D. noxia is different indicating that they contain two different genes, with different modes of action. While resistance in 1684/Tugela was generally reflected by disturbances in phloem activity, the resistance in 2199/Tugela was associated with a high number of short probes denoting some host recognition or epidermal factor involved in the resistance. Constitutive resistance in TugelaDn influenced the time taken by D. noxia to reach sustained phloem feeding. There was no evidence of surface or epidermal factors influencing probing behaviour in this genotype. D. noxia probing on induced resistant TugelaDn was characterised by a reduction in phloem activities on preinfested plants. D. noxia feeding on five day induced resistant plants showed more E2 fractions shorter than ten minutes and fewer E2 fractions longer than 60 minutes indicating a reduction in the ingestion of nutrients. In the case of both constitutive and induced resistance, initiation of phloem ingestion appears easier on susceptible Tugela with a higher number of single phloem salivation periods on the resistant genotype. Yield data for five susceptible and 15 resistant cultivars was analysed quantifying the percentage yield retained under severe infestation. Cultivars with D. noxia resistance from the same donor accession did not react in a similar way, neither was there any indication that they show the same measure of compensation for D. noxia damage. Compensation for aphid infestation was observed in some of the cultivars where higher yields were recorded from some infested cultivars than from cultivars kept aphid free throughout the trial. In the 2000 season, the variation in percentage yield retained between resistant cultivars equalled 63.2%, in the 2001 season 85.0 % and in the 2003 season 41.3%. This study confirms that the interaction between the aphid and each host genotype is unique. The expression of host plant resistance is not only dependent on the donor accession utilised, but also the genetic background in which it is deployed. Furthermore, environmental factors can also significantly influence the performance of host plant resistance to D. noxia.
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