There is a lot of awareness out there of just how many diseases and pests the gardener is facing every season. I've recently become more aware of it, and didn't realize how much of a problem it is becoming. Although just reviewing in your head how many different pests and diseases make the news when they are really destructive, like the ash borer, it does start to give you that perspective. I'm not sure how much we can all do about it, but it makes sense to me that the biggest action we can take is preventative. Being aware, being careful of where we buy plants from. Asking questions of nurseries about what diseases they are dealing with. Keeping a clean garden, cleaning our tools with alcohol, Knowing the symptoms of different diseases so we recognize them quicker. Wearing disposable gloves whenever you handle any plant that you suspect has a disease. Not the way any of us want to garden. It becomes a lot of hard work. HU - that is a LOT of plants, and a worry about whether you've got it all. Since you are doing a lot of reading up on it and trying to follow recommendations, then you are doing everything you can. The link above from MOBOT, recommends replanting with plants that are less susceptible and lists them. And they suggest insects are spreading the disease, so putting in plants that those particular insects are not interested in, will deter them from continuing in your bed. I think we can all use a reminder to be vigilant. Gardening can be very rewarding but it is also a lot of time, money and work, that we invest in it and it feels awful when you have this kind of a loss and a worry. I hope that is the end of it for you.

try using an applicator like this to DRIP drops of killer on cut surfaces ... easier than a brush .. and you cant lose it.. and half filled.. even if it tips over.. it wont run out ... just wanted to share such.. no clue whats going on in your yard ... not based on these pix anyway ... is your lawn or the neighbors sprayed for weeds??? ken https://duckduckgo.com/?q=plochman+yellow+mustard&t=ffsb&iar=images&iax=images&ia=images

The OP's plant is Verbesina encelioides subsp. exauriculata, hence the smaller flowers and other differences. One interesting thing about Verbesina Ive recently discovered is that leaf arrangment is practically useless in identifying them. Several species can have either alternate or opposite foliage. This is Verbesina alternifolia, with opposite foliage. You need to ID them by the phyllaries. If going by the foliage alone, this would appear to be Verbesina occidentalis.

I don't think .so.It looks like weevil damage in the one's that are opened . The other ones just look like buds that haven't opened yet. You may also get buds that just have the cone and no petals from mites. Also with asters yellow the foliage will usually ( but not always ) have chlorosis, yours looks very healthy but I would still keep a close eye on .them..Here are some pics that should help ID the problem. Also you should snip off the flower heads that have mite damage and clean good the fall to get rid of the mites, they live inside the cone. Also look over flower heads for weevils, small black beetle looking bugs and knock them ofoff Into a bowl with water and dishdish soap ,they can swim out of just .water