Pests

The most serious disease of asparagus is Fusarium root rot, caused by the fungus Fusarium oxysporum f. asparagi. This organism is present at very low populations in most soils, where it grows slowly on organic matter in the soil.

Planting disease-free seeds or crowns of the host asparagus causes a slow build-up of the Fusarium population in the soil. However, planting infested seed or infected crowns leads to a rapid build-up that may adversely affect performance of the first asparagus planting, and permanently contaminate the garden in that location for asparagus in the future. The Fusarium infects the root system and kills the feeder roots. As a result, plant vigor declines, spear size decreases, and the weaker plants may die.

Fusarium can be identified by reddish-brown color of the feeder roots, reddish-brown spots and streaks on the storage roots, and large lesions on the base of the spears and stalks at or below the soil line.

The symptoms are much more severe when the plants are under stress due to excessively long harvests, poor drainage, competition from weeds, and damage from insects and diseases.

The best way to avoid Fusarium root rot in the garden is to plant surfaced-sterilized seed in disease-free soil. Although surface-sterilized seed is not generally available, you can treat the seed yourself.

Soak the seed for 2 minutes in a solution of 1 part laundry bleach (composed of approximately 5 percent sodium hypochlorite) plus 4 parts of water. Rinse the seeds for one minute with cool running water and plant in soil that has never grown asparagus previously, or start seedling transplants in a disease-free artificial soil mix.

Although research to control Fusarium by the use of fungicides is under way, no practical treatment is yet perfected. Progress is being made on development of Fusarium-resistant strains of asparagus at the New Jersey and California Agricultural Experiment Stations.

Rust (Puccinia asparagi) is another common fungus disease of asparagus in the East and Midwest, and certain valleys in California. Dew or other free water on the plant for 10 hours is enough for spore germination.

Ten days after germination, the fungus appears on the surface of stems and branches as small rust-colored spots, containing spores that cause spread of the disease. In the fall the rust colored areas produce black, over-wintering spores that can infect the plant the following year. Severe rust destroys much of the foliage, reducing reserves for the next year's crop.

Rust can be partially controlled by fungicides, and several asparagus strains selected in the East and Midwest have some rust resistance.

Several species of thrips are often a serious problem, primarily on small plants in a nursery or on transplanted seedlings in the garden. Thrips are small, white, flying insects which are very difficult to see. They suck plant juices and cause the green needles and stems on the young plants to turn dull gray-green, then brown. Insecticides can be used to control thrips.

The asparagus beetle (Crioceris species) is a serious pest every year, although the insect populations vary from season to season. Adult beetles, which look like slender lady-bird beetles, lay small black elongated eggs on end in rows on asparagus brush. The larvae (small, dark green worms) do the actual damage by eating the green epidermis from the fern.

Beetles can kill very young seedlings, and seriously damage fern growth of mature plants. They can be controlled by insecticides.

European asparagus aphids invaded the eastern United States in recent years. They are blue-green, with a metallic sheen (like aluminum) when in clusters on plants. The aphids feed on young growing shoots, and inhibit the elongation of internodes, producing a kind of rosette (bushy stunted growth). Heavy infestations of the insects can seriously decrease foliage area needed for photosynthesis.

Fortunately the European asparagus aphid has many natural enemies, and seems to be subsiding as a pest.

Harvesting

An important culture requirement of asparagus is that the crop must be grown for two full growing seasons before harvest begins. This is necessary to allow the plants to develop an adequate storage root system to produce spears during the first harvest season and beyond. Any harvesting or damage to the brush during the first two growing seasons dwarfs the plants and can reduce yield for the life of the bed.

When the first spears emerge in the spring, merely snap off spears 7 to 10 inches long, with tight heads, leaving the tough stub on the plant. The upper portion which snaps off should be "all green" and "all tender". Harvest all spears that come up during the harvest season.

A good general rule for length of harvest season for all areas except the cool central valleys of California is the 2-4-8 week sequence. Harvest for 2 weeks the third year the plants are in the garden; 4 weeks the fourth year, and 8 weeks the fifth and following years. In the cool central valleys of California, a 4-8-12 week sequence is best.

When the harvest season is approximately half completed, 5 to 6 inches of soil may be carefully ridged over the row. This lowers the temperature around the crown and increases spear size. The ridge should be raked level right after the last harvest.

Asparagus spears harvested by snapping.

White asparagus, which has a distinctive flavor, can be produced by ridging 10 to 12 inches of soil over the row in the spring when the first spears emerge. When the tip of the spear breaks through the ridge of soil, carefully remove some soil from around the spear, and use a long knife or asparagus knife to cut the spear about 8 inches below the tip.

If the harvest from one day is not enough for a meal or if the asparagus is to be consumed later, wash the spears, place the cut ends in a shallow pan of water and immediately Put them in the refrigerator. Good quality can be maintained for several days if the spears are kept at 35 to 40 F. A 40-foot long row of asparagus will yield approximately 10 to 25 pounds of spears during the average season.