The effect on grain yield of adult plant resistance to mildew in oats

Lines were selected from progeny of the crosses Maldwyn × Milford and Maldwyn × Sun II which consistently exhibited small and large amounts of mildew in similar situations. These are referred to as ‘low’ and ‘high’ mildew lines respectively. In small plot field trials conducted in 1971, 1972 and 1973 the low mildew lines produced substantially more grain each year compared with the high mildew lines in plots where mildew was allowed to develop normally. In plots which were kept comparatively free from infection by spraying with tridemorph, both low and high mildew lines gave similar yield. When the unsprayed plots were compared with the sprayed plots the low mildew lines invariably showed very small losses in grain yield and the high mildew lines usually very large losses. This same trend was evident also in the parent cultivars in 1972. Maldwyn, with only 7·9% mildew cover in the unsprayed plots due to its resistance, suffered a decrease of only 9·2% in grain yield compared with losses of 17·1 and 20·3% in the more susceptible cultivars Milford and Sun II. Certain lines and cultivars sustained heavy levels of mildew without corresponding loss of grain yield, i.e. showed low sensitivity to the disease. The importance of combining low sensitivity (or tolerance) with adult plant resistance to mildew is discussed.