First, let’s address your asparagus (Asparagus officinalis) bed ‒ asparagus benefits from regular mulching and weed control.
Mulching Asparagus for Weed Control: Keep asparagus beds well weeded; asparagus suffers from weed competition. Weeds compete with asparagus for water, nutrients, and light and reduce the yield of the spears. For healthy plants and plenty of shoots, it’s essential that you control weeds in asparagus beds. Hoeing the beds is difficult among the close-growing asparagus shoots and ferns, and even when the beds are empty, there’s a danger of damaging the delicate crowns. Fortunately, a thick layer of organic mulch provides effective weed control.
To control weeds spread layers of organic mulch over the bed. Spread the mulch in an even layer between 4 and 6 inches thick, pushing the material between the shoots. If any weeds germinate on the surface of the mulch over the growing season, simply pull them out.
I believe your asparagus are suffering from two conditions: asparagus miners and Fusarium wilt, a fungus.
Asparagus Miners: The Agromyzidae, also known as leaf-miner flies are small black fly less than 0.2 inches long. Their characteristic mining damage along the stem base is discolored reddish-brown tunnel in a serpentine pattern. The pupae are brownish red, about 0.2 inches long. Maggots are white, eel-like (early instars), or plump (later stages). They overwinter as pupae in stems and field debris.
The larvae are protected within the asparagus stem. They are a recognized vector for pathogenic Fusarium spp. responsible for Fusarium crown and root rot and early decline of their select plants. The leaf-miner fly feeds on nectar from flowers and plant fluids from asparagus beetle, if present, damage as an adult.
Fusarium Fungus: The fungus starts by working its way into the plant’s root system through the soil. Once inside, it moves in to block the vessels in the cells of the infected plant, cutting off water and nutrients as they are being transported to the plant’s extremities. To prevent further spreading of the fungus be sure to keep your garden tools and boots clean and free from soil. Contaminated tools and soles can spread fusarium pathogens to fresh soil. All equipment should be washed in a solution of bleach and water (with a ratio of 1 part bleach to 4 parts water).
Rather than using typical eradication methods that could destroy your asparagus and their bed I recommend using another action against fusarium invasions. There is a product called Actinovate, an organic product that can be used as a soil drench. Actinovate is a pesticide and fungicide that targets diseases that cause leaf blight and root rot using beneficial microbes and bacteria that help plants colonize their roots and protect foliage from invasive diseases.
The United States Environmental Protection Agency has declared Actinovate safe for both humans and the environment. The only drawback to this method is the price. In small amounts, Actinovate is quite affordable, but for soil drenching in a large garden area, using this product could wind up being very expensive.
However, before panicking please send me a few more photographs of a couple complete spears, not ones that have been cut or snapped off so I may verify the presence of leaf minors. Also, if you could cut a few stems off and look for a brown discoloration of the xylem vessels to check for fusarium. This would be an indication of infection.
Here are some additional pictures. Hope they help.
The bed has a thick layer of sawdust mulch which is probably a couple of years old. It is weedy but they are not deep and were easily removed.
The main characteristic, to me, is that the spears come up brownish, often with a bent tip. And tho the stalk may be green, the tip turns brown and withers, killing the stalk.
Jerry: I just now found this question in my junk folder, so sorry for the delay in responding. I concur with Seamus’ response but would also add that it’s not 100% clear that the asparagus has Fusarium wilt because typically the shoots are further along before becoming so dramatically symptomatic and the rot tends to start lower. However, it’s been an unusually dry and sunny late winter and spring this year. So checking for vascular discoloration inside affected spears is critical for determine if Fusarium wilt is present. Adding lime or Ca to get the soil pH above 7 is valuable for management of wilt, and can help to stall new infections.