Associations between fungal and abiotic leaf spotting and the presence of mlo alleles in barley

The hypothesis that the increased use of the powdery mildew‐resistance gene mlo has caused the increase in spotting diseases of barley over the past 20 years was tested in field trials. Near‐isogenic lines with alleles of the Mlo gene for susceptibility or resistance to mildew in two parental backgrounds were trialled at four sites in Scotland and two in Ireland that were prone to spotting diseases, over 3 consecutive years. Mildew was controlled by sprays with quinoxyfen. Disease levels were low in the trials, the two most important diseases being scald caused by Rhynchosporium secalis and ramularia leaf spot caused by Ramularia collo‐cygni. There were high levels of abiotic spotting. Lines with mutant mlo alleles consistently developed less Rh. secalis and Ra. collo‐cygni, but more abiotic spots. This study indicates that the mlo mildew‐resistance gene has not alone been responsible for the rise in spotting diseases over the past 20 years. Possible reasons for the rise are discussed, including the interaction of the mlo gene with the environment.