The preconditioning effect of day‐length and light intensity on adult plant resistance to powdery mildew in oats

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Abstract
SUMMARY: Oat genotypes with partial resistance to mildew exhibited a higher level of adult plant resistance (a.p.r.) when grown in 16 h compared with 8 h day‐length. This increased resistance was expressed in the upper leaves, and to some extent in the lower, earlier‐developed leaves.The expression of increased resistance coincided with floral initiation in Maldwyn, but preceded it in CC4761. The development of a.p.r. and floral initiation were thus not inherently linked, but conditions which accelerated the plants' ontogeny also caused earlier expression of resistance.For the most efficient testing of breeding material, plants should be grown at long day‐length and high light intensity. When plants are grown at 16 h day‐length and constant temperature of 15 oC, the leaf segments should be inoculated at the 6 to 8 leaf stage because normally susceptible genotypes showed marked resistance when inoculated later.