Why are my asparagus plants turning brown prematurely?
Our asparagus plants are turning brown earlier than usual. Typically we cut them back near first frost time. If it is some type of fungus, should we cut earlier to protect the roots? These are older well established plants. It has been a wet humid growing season which may be the reason. The spring harvest was normal.
I suspect your hypothesis about the wet season is a likely culprit. Persistently wet soil can cause pH changes and otherwise interfere with plants’ ability to take up nutrients, and while asparagus does like moist soil, it’s not a fan of overly “wet feet” and can show some early decline. I’m inclined to advise you to remove any dead fronds (they aren’t photosynthesizing and can attract secondary problems), nurse along healthy green ones as long as you can, then cut back whenever it looks like the above ground portions are done for the season. A healthy, established asparagus bed should be able to weather the season just fine and come back next year.
This advice is based on the foliage simply yellowing/browning prematurely. If, however, you notice lesions or blisters on the fronds (see this Univ of MN Extension page on asparagus pests/pathogens for visual examples), you may have something else going on and we would treat it differently. A quick scouting of the affected foliage after looking at the photos should let you know.
In that case, or if you remain concerned after my reply, I would direct you to the UMaine Plant Disease Diagnostic Lab for confirmation before any treatment recommendations. They have an option for a visual diagnostic (less reliable, and I would advise closer pictures of the fronds in question) or lab testing of submitted samples (instructions for prepping and sending/dropping off the sample are on the page, and the results typically come back in 2-3 weeks).