It wasn't hard to find the reddish, circular lesions on the stems and discolored needles.
Some of the lesion were older and inside of them, dark black fungal structures were present.
My initial thought was that it had to be either Cercospora blight(Cercospora asparagi) or Purple Spot (Stemphylium vesicarium). The NCSU factsheet on Commercial Asparagus Production mentioned Cercospora blight as a problem in NC, but did not mention Purple Spot.
After looking under the scope, I didn't find any spores that looked like the ones produced by Stemphylium vesicarium, but I did see some other, very elongated spores that almost looked like leaf hairs. Sorry I didn't take a picture, but here is a link to another Cercospora species' spores.
After doing a little more research I found an APSnet Feature Article on Economically Important Asparagus Diseases in the United States. This article stated that Cercospora blight and Purple Spot are easily confused, so I decided to send it into the NCSU Plant Disease and Insect Clinic.
Luckily for me, Dr. Averre, whose pictures were included in the above article, was available to do the diagnosis! He identified that the problem was, in fact, Cercospora blight.
There are no known sources of resistance to Cercospora blight, though Jersey Gem does have tolerance to Cercospora. As a result, management must include an integrated approach of sanitation and fungicide applications. Fall removal or burial of the year's fern residue can delay the onset of disease development. This was just what the grower was going to do! Heavily infected ferns should be removed.
Cultural management techniques that reduces leaf wetness are very important for the management of Cercospora blight. These techniques include:
- Using drip or furrow irrigation to decrease leaf moisture. If overhead irrigation must be used, it should be scheduled so that the foliage has time to dry before nightfall.
- Increasing row spacing. This will improve air circulation within the planting.
- Plant asaparagus in areas with good air movement to encourage leaf drying.
Also, Oklahoma State University has a nice factsheet on Cercospora blight of asparagus.