LEAF DEVELOPMENT IN RELATION TO INFECTION BY STAGONOSPORA-NODORUM ANDSEPTORIA-TRITICI IN WHEAT

Disease and phenology were compared in 2 wheat cultivars (Aroona and S pear) differing in disease resistance and maturity, at early (April-Ma y) to late (June) sowing dates in 1991 and 1992. The cultivars had sim ilar rates of leaf development but the longer season cultivar Spear av eraged up to one leaf more on the main stem than the midseason cultiva r Aroona, depending on sowing date. The extra leaf production which de layed flag leaf emergence on Spear compared to Aroona at early sowings was associated with lower disease levels in 1991. The occurrence of l eaf rust may have diminished this advantage in 1992. Resistance to Sep toria tritici in Aroona did not affect disease progress compared to Sp ear. A comparison of disease over a segment of accumulated thermal tim e revealed that in 1991, under average seasonal conditions, the earlie st sowing experienced the most disease. In 1992, above average late sp ring rainfall occurred and time of sowing had little influence on dise ase over the same segment of thermal time. We conclude that a cultivar appropriate for early sowing on the south coast should exhibit increa sing leaf production in response to early sowing. This can result in a reduction of disease severity on the upper leaves through disease esc ape.