Some other genes of note: disease resistance

In addition to the genes for colour, some of which I've discussed in a previous post, there are a number of genes of home horticultural importance.
Foremost are probably the genes for powdery mildew resistance. There are two genes, both of which confer resistance to Erysiphe polygoni the organism responsible for powdery.
They are known as er-1 and er-2, both are recessive. The JI database notes of er-2 "Resistance broken under controlled growth conditions so not as durable as resistance conferred by er-1".
Being recessives sahould make them easy to fix, but since the only phenotypic expression is disease resistance, it is hard to distinguish whether they are present or not until E.polygoni strikes, and then how might one distinguish between them?

I have at least one line that has at least one of these genes - Delta Louisa is powdery resistant. And by the looks of my recent observations it appears that Oregon Spring, another dwarf snow pea line is also resistant. On the left of the photo is three purple podded lines - PP Select Organics, Angela's Blue, and PP Lost Seed Co, along with Swiss Giant, which all seem to be succeptible. On the right of the bed is Delta Louisa and Oregon Spring, both of which seem resistant. One line from the Jovan accessions (my research centre acquisitions) was an unknown Powdery Resistant Snow Pea, a survivor of the centre's Powdery chamber of death. It has been reluctant to germinate - of 5 seeds sown in Septenber only one survived - a subsequent resowing of 4 seeds yielded nothing. So I'm hoping for a good crop of seed from the one remaining plant. However, a bit bleary the other morning, I absent-mindedly emasculated one of its two flowers - shock horror! I've crossed it to a purple podded, so hopefully it takes and at least I will get something useful out of the error.