Avoid asparagus rust, which produces reddish brown spots on the stems and leaves, by planting resistant cultivars. Minimize damage from Fusarium wilt, which causes spears, leaves, and stems to be small with large lesions at or below the soil line, by purchasing disease-free roots and using good garden sanitation. Crown rot causes spears to turn brown near the soil line. Prevent crown rot by planting in raised beds, maintaining good drainage, and keeping soil pH above 6.0.
If your asparagus bed does become infected by disease organisms, your best option is to start a new bed in a distant part of the garden, using newly purchased or grown plants.
If young spears turn brown and become soft or withered, they may have been injured by frost. Cover spears with mulch or newspaper when freezing nights are predicted.
Don’t harvest any spears during the first 2 years that plants are in the permanent bed. They need to put all their energy into establishing deep roots. During the third season, pick the spears over a 4-week period, and by the fourth year, extend your harvest to 8 weeks. In early spring, harvest spears every third day or so; as the weather warms, you might have to pick twice a day to keep up with production. Cut asparagus spears with a sharp knife or snap off the spears at, or right below, ground level with your fingers.
This article is courtesy of Rodale's Ultimate Encyclopedia of Organic Gardening.