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Asparagus production guide: pest management

Insects

Asparagus grown in Minnesota is relatively free of insect pests compared to many other vegetable crops. The asparagus beetle (black with white markings) is the most common insect that attacks asparagus. Adult beetles, which over-winter under debris along field edges, begin to move to asparagus as the plants first emerge in spring. Beetles feed on the spears and glue rows of black eggs, usually on the tips. These eggs hatch in approximately one week and the fleshy, dark gray larvae then move to the foliage on which they feed. The larval stage lasts two to three weeks, after which the larvae drop to the ground, burrow into the soil, and pupate (cocoon stage). Two or three generations are produced during the growing season in Minnesota.

The twelve-spotted asparagus beetle (orange with black spots) also may be present. Its life cycle is similar to that of the asparagus beetle, differing primarily in that the larvae feed on the developing berries. Consequently, spotted asparagus beetles cause relatively minor damage to the spears or fern.

When asparagus beetles are present at harvest, they can cause extensive damage. Early-season feeding by adult beetles causes a distorted "shepherd's crook" growth of the spear. The presence of eggs also renders the spears unacceptable for market.

On nursery seedlings, defoliation of the plants by asparagus beetle larvae should be monitored carefully; insecticides should be sprayed if a field has 5-10 adults/100 crowns or 2% of the spears have eggs. After harvest, limited feeding by larvae on established plantings may be tolerated; treatment threshold for adults increases to 5-10 adults/10 crowns. Several insecticides are labeled for controlling asparagus beetles.

Cultural controls for both species of beetles include following good cultural practices that promote plant vigor and thorough harvesting of spears to reduce the number of beetles that hatch in the spring. Chemical control of adult beetles may have to be repeated, since the beetles emerge from overwintering sites over an extended period.

One of the most devastating insect pest that attacks asparagus is the asparagus aphid (Brachycorynella asparagi). The aphid was first reported in southern and central Minnesota in 1982. The asparagus aphid is a minute, blue-green sucking insect that usually feeds on asparagus fern. In the process of feeding, it injects a toxin into the asparagus plant that is translocated down the stem into the dormant buds. The toxin causes the buds to elongate into new shoots prematurely, producing a "witch's broom," or a dwarfed, very bushy, short plant with silver, blue- green color. Under severe insect pressure, all the buds on the crown may "break," causing the plant to have none left over for the following season, essentially terminating the plant's life. Although the relationship between aphid infestations and economic damage is unknown, younger plants, including those 1-2 years after transplanting, are most susceptible to damage. Several naturally occurring biological controls usually prevent this aphid from reaching the damaging levels typically observed in the western production areas of California and Washington state. The adults and larvae of several lady beetle species, other insect predators, a parasitic wasp (specific to aphids) and fungi (triggered by warm, humid conditions) all help to reduce aphid infestations. When populations are increasing rapidly and biological control does not appear to be effective, malathion (Cythion® 5E) should be used at 2 pints per acre. Sevin® is not recommended for aphid control.

The asparagus aphid lays its eggs in late summer or early fall. The eggs over-winter on the fern and fall to the ground by spring. Unharvested asparagus, which ferns out in early spring, is highly susceptible to early aphid infestations because egg masses are allowed to hatch and the aphid's life cycle begins. Asparagus that is harvested into early summer is not at risk until the fern is allowed to develop. The aphid feeds only on the fern, not on asparagus spears. Removing asparagus fern in late fall after it has dried down greatly reduces potential aphid infestations the next year, but this is seldom possible before the first snowfall. Cutworms can cut off asparagus spears below ground and even cause damage by feeding on the tips of spears above ground. Shoots damaged by cutworm feeding develop into crooked spears and must be picked and culled. Chemical control is warranted if one or more worms/10 crowns are found. Pyrethroid insecticides, particularly under cool, spring conditions, will usually provide good cutworm control. Consult the Midwest Vegetable Production Guide for the most up to date information which is revised each year.

Weeds

A number of herbicides are available for use on asparagus (Midwest Vegetable Production Guide). Depending on the type of asparagus plantation (direct-seeded nursery, transplants, crowns, or established asparagus beds), choosing the most effective herbicide and rate represents a challenge to the grower. The desired herbicide should produce long-term weed control, be safe to use on the asparagus fern, and be legal to use. As outlined, each type of asparagus plantation presents a different set of problems.

Direct-seeded asparagus

Producing asparagus crowns in a nursery for eventual digging and establishment of new production fields is still a popular practice. The slow germination and emergence rate of asparagus seedlings and their slow growth rate present outstanding weed control problems. Therefore, the major weed control objective in nursery production of crowns should be to use an herbicide that has long weed-killing activity yet remains safe on the delicate asparagus seedlings. The herbicide should remain active at least until the seedlings are large enough to be mechanically cultivated safely. This may take as long as two to three months after seeding.

Transplants

The use of seedling transplants to establish new asparagus acreage is sometime used in Minnesota. Since furrow opening and planting is a one-step operation, herbicides to control weeds must be applied "over-the-top" of the transplant fern or directed to the ground after planting to avoid the asparagus fern. Primary concerns include: selecting a herbicide and a rate that is toxic enough to control weed species but not seriously set back the growth of the asparagus plant, and determining the application method.

Crowns

Traditionally, most new asparagus production fields are established by planting one-year-old nursery grown asparagus crowns into deep furrows. Since the first new shoots may take many weeks to emerge and grow to a size that can be cultivated, weeds in the furrow may become large and not controllable by cultivation. Therefore, an easily applied pre-emergence herbicide of long, dependable activity is necessary to reduce the number of mechanical cultivations needed to keep fields weed-free.

Established asparagus

Many herbicides are now labeled for use on established asparagus and can be applied before the harvest begins (preemergence), after harvest (delayed application), before and after harvest (split application), or throughout the harvest season (multiple application). The problems facing the grower are choosing an application method that complements the operation and choosing a chemical and rate that will control weeds after harvest for the duration of the growing season.

Diseases

There are several important diseases that can cause significant asparagus losses: Fusarium root and crown rot, rust, and Stemphylium purple spot. Refer to the (Midwest Vegetable Production Guide) for the most up to date recommendations on the use registered fungicides available to manage these diseases.

Fusarium wilt root, and crown rots

A common, soil-borne fungus, Fusarium moniliforme, is the cause of asparagus crown rot. The fungus is found in soil where asparagus is grown. Generic terms used to describe asparagus crown rot are seedling blight, decline disease, and replant problems. A second Fusarium disease, caused by Fusarium oxysporum f. sp. asparagi, causes root rot, wilt, and seedling blight. This pathogen causes the water conducting vessels to plug, producing wilting of spears and ferns.

Infection commonly occurs when Fusarium moniliforme enters the roots and spreads throughout the plant. Symptoms of asparagus crown rot include wilting of mature plants during hot summer weather, stunting, yellowing, seedling blight, and death. Infected areas of the crown turn brownish in color as cells that transport water and nutrients become clogged due to the infection. Cutting open affected plants reveals dark, reddish-brown colored decay of lower stems, crowns, and roots. Later, portions of the crown begin to dry up until the entire plant dies. Scattered wilting throughout the fernstalks is more indicative of Fusarium oxysporum f. sp. asparagi; Symptoms caused by this disease usually occur midsummer when 1 or 2 fernstalks per plant turn yellow. This leads to large gaps in the asparagus field and significantly lowers crop production. Fusarium-infected plants increase in number until the crop is too sparse to harvest economically. The positive diagnosis of Fusarium moniliforme is based on laboratory detection of the fungus associated with typical symptoms on the plant.

Fusarium moniliforme survives in crown and stem lesions of diseased, old asparagus plantings. Fungal spores are spread by air currents and on the surface of contaminated asparagus seeds.

Fusarium diseases are extremely difficult to manage once the fungus is established in an asparagus field. Primary controls are choosing healthy, Fusarium-tolerant varieties of plants obtained from a reputable source, and planting in fields not previously used for asparagus. Fusarium-resistant varieties for Minnesota growers include Jersey Giant, Jersey, Knight, Jersey Prince, and Viking KB3. Disease intensity can be increased by nutrient stress, drought, and insect damage. To help minimize losses due to these diseases and to establish a vigorous asparagus field, only disease free plants should be planted on well-drained soil. Soil pH should be maintained between 6.5 and 7.5 with moderate levels of fertility. To minimize disease, control pests, diseases, weeds, and avoiding excessive harvesting that stresses plants and predispose them to disease. Once Fusarium becomes established in the field, there are no simple controls. Since the pathogen is soil-borne, new beds should never be planted in fields previously in asparagus.

Asparagus rust

Asparagus rust, caused by Puccinia asparagi, occurs in varying amounts wherever the plant is grown, and attacks asparagus ferns during and after the cutting season. In addition to asparagus, Allium species, such as cooking onions and chives, are also susceptible. There are no alternate hosts such as is common with other Puccinia rusts.

Asparagus spears are usually harvested before extensive rust symptoms appear. Symptoms are first noticeable on the growing shoots in early summer as light green, oval lesions, followed by tan blister spots and black, protruding blisters later in the season. The lesions are symptoms of Puccinia asparagi during early spring, mid-summer and later summer to fall. Severe rust infections stunt or kill young asparagus shoots, causing foliage to fall prematurely, and reduce the ability of the plant to store food reserves in the crown.

The orange spores are the key sign for this disease. Run your hand over an asparagus spear and examine your palm for orange-colored spores. Laboratory techniques may also be used for diagnosis of asparagus rust.

Spores overwinter on host plant residue, germinate in early spring, and produce new infections on growing asparagus spears. The light green, oval lesions are surrounded by a concentric ring pattern. In young plantings, before stalks are harvested, lesions develop yellow spore-bearing structures in concentric rings. Wind and splashing rain can spread spores to branches and fern needles, where they germinate in the presence of water drops.

Plants affected by rust are more susceptible to Fusarium crown and root rot.

Plant rust-resistant varieties of asparagus, such as Jersey Giant or Viking KB3 which are reported to grow well in Minnesota. Remove volunteer asparagus within 300 meters of commercial plants, and locate new plants away from established fields. Plant well-spaced rows oriented in the direction of prevailing winds to maximize air movement and facilitate drying after rain. It is important to note that plants affected by rust are more susceptible to Fusarium crown and root rot. Several growers are using the culture practice of increasing the row width from four to five feet to allow more air movement around the plants therefore allowing the plants to dry out earlier from rains or heavy dews.

Labeled Fungicides: Timely fungicide applications will provide reasonably good control of rust (Midwest Vegetable Production Guide). During periods of high humidity and extended rainfall, applications should be made every seven to ten days.

Stemphylium purple spot

Stemphylium purple spot, caused by Stemphylium vesicarium, was first found in Minnesota asparagus fields in 1988.

Symptoms on the spears appear as elliptical sunken, purplish spots, which may cause rejection of product. The disease produces brown to tan lesions with dark purple margins on the ferns. In spring, spore are produced from last year's infected plants and spread by wind and water to newly developing plants. Infection occurs through natural openings and wounds on current season asparagus tissue with favorable temperatures and moisture from rainfall or irrigation.

Good sanitation is the key to good disease management, and incorporation of asparagus debris from the previous season into the soil in the fall results in less disease severity than when debris is left on the soil surface. Volunteer asparagus seedlings can become infected during the harvest season and may serve as a source for disease increase as well as reservoir to carry the disease from the harvest period when the spears are removed to when the ferns are allowed to grow. Research from Michigan State has determined that the Tom-Cast disease forecaster alerts growers as to when environmental conditions are favorable for purple spot disease development (extended dew, and rainy periods coupled with high humidity and warm temperatures) and to when fungicides are necessary. In these studies, incorporation of the Tom-Cast disease forecaster enabled growers to reduce fungicides while still successfully managing purple spot disease of asparagus.

Postharvest, handling, and storage

Pack asparagus upright with damp absorbent pads under the spears. Asparagus spears will bend toward light, so horizontally packed spears will noticeably bend upwards. Do not keep spears in pans of water for very long, or microbial infection can occur. Cool spears immediately after harvest to prevent bacterial soft rot infection. Store asparagus for up to 3 weeks at 36° F. Asparagus will turn flaccid and dull gray-green if kept for more than 10 days at 32° F. Avoid exposure to ethylene gas, as ethylene can cause toughening of spears.


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