The Russian wheat aphid (Diuraphis noxia Mord.): damage on Kenyan wheat (Triticum aestivum L.) varieties and possible control through resistance breeding

We studied the effect of the Russian wheat aphid (RW A) (Diuraphis noxia) infestation on seedlings and adult plants of eight Kenyan wheat (Triticum aestivum L.) varieties. The Kenyan varieties were 91B33, Fahari, Kwale, Mbuni, Chiriku, Kongoni, Nyangumi and Mbega. Two RW A resistant wheats, Halt and PI 294994, were also tested against Kenyan isolates of the aphid. All the Kenyan varieties were susceptible to RW A when compared with the resistant line PI 294994. Halt, which is a resistant variety developed in the USA, was susceptible to Kenyan isolates of RW A. This indicates that the Kenyan RW A isolates are different from the USA ones. In seedlings, the RW A damage was expressed mainly as leaf chlorosis and leaf rolling, with damage scores increasing with time. Differences among the Kenyan varieties in the extent of leaf chlorosis were observed. The most devastating effect of RW A infestation of adult plants of the Kenyan varieties was the reduction in seed set. The tight rolling of flag leaves caused by the aphid delayed ear emergence, leading to floret sterility. Infestation also reduced the quality of the seeds produced, as shown by increased rate of seed deterioration under accelerated ageing conditions, and reduced seedling vigour. The effect of infestation on seed quality was more pronounced under dry conditions. Morphological and genetic variations within PI 294994 were identified. The PI 294994 plants tested could be separated into three distinct groups, all of which had equally high resistance to Kenyan RW A. One PI 294994 derived line, designated P3, was discovered to require no vernalization and therefore to be suitable for use in a Kenyan breeding programme. Segregation in the Fz populations indicated that resistance in two PI 294994 derived lines (PI and P2) was controlled by two genes (one dominant and one recessive). For P3, the results were inconclusive since in one Fz population the segregation indicated that the resistance was controlled by one dominant gene, whereas in another population the segregation indicated that resistance was due to one dominant and one recessive gene. Work to identify molecular markers linked to RW A resistance genees) in P3 was initiated.