Quantification of effects of leaf spotting diseases on grain yield and market quality of durum wheat using near-isogenic lines

Leaf spotting diseases (LS) in wheat are widespread in western Canada. The most prevalent LS are tan spot and the septoria leaf blotch complex. Near-isogenic lines (NILs) for LS resistance were used to determine the impact of LS on grain yield and market quality of durum wheat. Sixteen NIL pairs, susceptible and resistant to LS caused mostly by Pyrenophora tritici-repentis (tan spot), were evaluated at the milk stage at two locations in southern Saskatchewan, from 2000 to 2002. Tan spot was the most prevalent LS on the leaves. There were differences in plant growth and disease severity among environments. However, in most cases, the resistant NILs had the lowest LS severity. Overall the resistant NILs had higher yields and lower protein concentration than the susceptible NILs, although differences for individual lines were not always significant. For the four most consistently performing pairs in the most favourable years of 2000 and 2002, a 16.2% LS reduction corresponded to increases of 17.2% for yield and 4.6% for kernel weight at Swift Current, whereas a LS reduction of 5.5% was associated with increases of 26.6% for yield and 2.8% for kernel weight at Indian Head. This indicates that in the semi-arid location of Swift Current, a greater LS reduction was required to obtain yield and quality increases similar to those at the location with the greatest yield potential, Indian Head. Protein was reduced in the resistant NILs by 4.1% at Swift Current and 7.6% at Indian Head, suggesting that improvements in LS resistance must be accompanied by breeding for higher protein concentration or recommendations for nitrogen application to maximize returns.