Due to my curiosity about this I have done a bit of research to try to answer my own question.

Aster leafhoppers must feed on an infected plant to get the mycoplasmalike organism into their system. Once that happens it takes about 3 weeks before they are able to transmit the disease to another plant.

Here in KS (and I'm sure elsewhere) there are two kinds - native and migrant. Natives are not the likely vector in most cases here.

Migrants ride southern winds to get here from Texas and Oklahoma. With their earlier spring warmups some of the overwintering southern leafhoppers have passed through the 3 week incubation period by the time they get to KS. They are the likely vector.

Garlic is green and growing long before either perennial or seedling coneflowers or rudbeckia begin growth (usually mid to late April). In my case I have begun pulling diseased established plants and seedlings by mid May. The point is that there would have been plenty of time for garlic exposure, perhaps even before the other host plants emerged.

Although I have read that aster leafhoppers may prefer plants other than garlic and that it would be low on their list it is very hard for me to believe that not one garlic plant showed anything obviously similar to the yellows over a three year period, causing me to wonder if perhaps some strains might have better resistance.

If anyone in WI or MN could shed some light on this it would be good info to pass along.

hortster