The linkage between the stem rust resistance gene Sr2 and pseudo-black chaff in wheat can be broken

Recessively inherited gene Sr2 has provided the basis of durable resistance to stem rust (caused by Puccinia graminis tritici) in wheat (Triticum aestivum L.) worldwide. The associated earhead and stem melanism or 'pseudo-black chaff' is generally used as a marker for this gene. Sr2 has been postulated in many wheat cultivars of India including 'Lok 1', based on associated pseudo-black chaff in adult plants, and leaf chlorosis in seedlings. However, dominant inheritance of the resistance factor operating in 'Lok 1', and a 13 : 3 (resistant : susceptible) F₂ segregation in the 'Sr2-line' ('Chinese Spring'⁶ x 'Hope' 3B) x 'Lok 1' cross confirmed that Sr2 was absent in 'Lok 1'. Susceptible plants with a pseudo-black chaff phenotype were observed in F₂ populations of 'Agra Local' (susceptible) x 'Lok 1', and the 'Sr2-line' x 'Lok 1' crosses. Most of the F₃ families derived from the susceptible F₂ segregants with pseudo-black chaff phenotypes were true breeding for the expression of pseudo-black chaff with susceptibility to stem rust. Thus, linkage of pseudo-black chaff with Sr2 in wheat can be broken, and hence, caution may be exercised in using pseudo-black chaff as a marker for selecting Sr2 in breeding programmes.