The Assessment of Speckled Leaf Blotch in Winter Wheat in New Zealand

Disease symptoms of speckled leaf blotch (Mycosphaerella graminicola) were first observed in early August, 86 d after sowing a winter wheat field trial in New Zealand. Disease hastened senescence of all leaves, delayed the expansion of new leaves during spring and reduced the maximum size of some later-formed leaves. The absolute green leaf area was reduced by disease through to full senescence of leaves, which occurred one week earlier than in healthy plants. Disease indices calculated from the data on percentage diseased area alone indicated a decline in disease during rapid growth of the plants in spring as the temperature increased. However, this apparent decline was generated by the method of calculation and was not evident when effects on leaf area were also considered. Senescence induced by the pathogen was an important aspect of the disease syndrome.