Effects of plant age, leaf position, inoculum density, and wetness period on Bipolaris coicis infection in adlays of differing resistance

The effects of plant age, leaf position, inoculum density, and wetness period on the development of adlay leaf blight caused by Bipolaris coicis were evaluated using six adlay cultivars or lines of differing resistance in field plots and in the greenhouse. Levels of resistance to leaf blight among the six cultivars or lines were consistent between the 1998 and 1999 field experiments. With the aging of adlay plants, leaf blight development gradually increased in all cultivars or lines tested. Leaf blight severity was significantly greater on the lower (older) leaves than on the upper (younger) leaves. As inoculum density increased from 10(3) to 10(6) conidia per ml, the development of leaf blight increased. Disease severity also increased as the time of leaf wetness duration increased from 0 to 60 h. Wetness duration above 48 h and a high inoculum density at 10(6) conidia per ml caused severe leaf blight symptoms in adlay seedlings. The data from the greenhouse experiments consistently revealed the same trends in relative levels of susceptibility of adlay cultivars to leaf blight as those seen at late plant growth stages under field conditions.