The problem is that septoria tolerance is at vanishingly low levels in all known genetic stocks. Do a few searches and you will find one of the best articles about screening for septoria tolerance from Scielo in Brazil. Known tolerance to septoria is polygenic and based on very small additive effects from about a dozen genes.
You must grow your plants disease free. This usually means inside a greenhouse with very careful screening for pathogens. Then you have to expose the seedlings in a controlled fashion to a specific strain of the disease organism. This must be done without exposing the primary plant to the disease ergo the use of detached leaf assay. Once tolerance has been found, then use the specific plants as parents in crosses to combine other desirable traits and/or to stabilize the disease tolerance.
DarJones
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