Asparagus pest management

Cultivation

Cultivation is most often used between the rows, to uproot small weeds. However, it can also be used cautiously before or after the harvest season during very tight time intervals. Some asparagus producers cultivate the rows in the early spring, after snow-melt but before spear emergence.

If the final spear harvest is cut below the soil surface, in-row cultivation may also be possible immediately following the last harvest. At both timings (early spring and after last harvest), the cultivation must be very shallow, less than three inches, and should be done before new spears start emerging. This is challenging and is not always possible.

If the spears have started to emerge, cultivation may hit the spears and damage them, reducing yield and introducing disease. After harvest, cultivation can damage emerging spears if they are not all cut below the surface.

Spring cultivation

• The main role of early spring cultivation is to incorporate fertilizer and to eliminate winter annual weeds.
• This soil disturbance also stimulates growth of weed seeds by warming the soil and moving small buried weed seeds to the surface where they can germinate.
• Growers should consider whether it is really necessary to cultivate each spring. 

Post-harvest cultivation

• Post-harvest cultivation should be done immediately following the final harvest, and only if spears were harvested below the soil surface.
• Post-harvest cultivation can uproot small emerged weeds to keep the soil relatively weed-free until the ferns become established enough to outcompete weeds.
• Shallow cultivation will not reliably control thistles or other weeds with taproots and rhizomes or large established annual weeds.

Cover crops

Rather than cultivating between the rows, many asparagus growers avoid cultivation and grow no-till asparagus. This can improve soil health by greatly reducing soil disturbance. No-till asparagus growers can either plant cover crops in the aisles between the rows, or allow the natural vegetation to grow.

Beyond weed control, cover crops can provide additional benefits including soil health improvements, reducing compaction and keeping walkways clean. They can also make harvest easier during rainy conditions that would otherwise make the soil muddy and difficult to walk on. 

Perennial cover crops

• Perennial cover crop mixes usually contain fescues, perennial ryegrass and clover.
• Perennial cover crops and natural ground covers should be mowed periodically to reduce weed seed production. 
• Perennial cover crops can spread into the rows, potentially competing with the asparagus plants and crop and reducing yield.
• Cover crop roots compete with the asparagus crowns for soil resources including water, nutrients and space, and thus may decrease spear yield especially when resources are limited.

A cultivator with sweeps may be used to cut into the edges between the aisles and rows, to prevent cover crops from spreading into rows. Certain herbicides can be used in the rows before and after harvest to keep them clear.

Annual cover crops

Annual cover crops may also be used between rows. This may be a helpful option for newly planted fields since they are quicker to establish, but there is limited research available on the use of annual cover crops in asparagus.

• Organic growers may choose to use annual cover crops to prevent spread into the asparagus rows.
• As the stand matures, the rows of asparagus will start to widen into the aisles. Growers may use annual between-row cover crops for the first couple of years to keep weed competition low.
• As spears start to fill in the spaces between rows, they may transition away from cover crops.

Flaming (propane weeding)

Flame weeders, also called propane weeders, emit heat from propane-powered torches to kill weeds. Despite the name “flame weeder,” this tool kills weeds by heating them, not by burning them directly with the flame.

• Flame weeders come in many different shapes and sizes, from individual torches on a backpack model to pull-behind implements for tractors.
• Flaming can be used carefully within the rows, during stand establishment, or in the aisles.
• Like cultivation, flaming is most effective on small, annual broadleaf weeds.
• Grasses tend to have concealed growth points that are protected from heat, and, while flaming will kill the above-ground tissues of perennials, it does not reach the below-ground storage tissues. 

Hand-removal

At times, hand-weeding is the best option for large annual and perennial weeds and for removing weeds in the rows during the harvest season.

• In the absence of herbicides, hand removal is the most effective option for managing Canada thistle and quack grass in asparagus stands.
• Mowing and cultivation are not sufficient for eliminating thistles in the rows, as it takes repeated passes throughout the season to control thistles with these methods.
• Flaming has been found ineffective on thistles and quack grass; it does not impact the belowground rhizomes (spreading roots) that these plants use for reproduction. 

Mulching

While not a commonly used practice, the aisles between the rows can be mulched using wood chips, straw, or landscape fabric in order to smother weeds between rows. 

A number of herbicides are available for use on asparagus. The most effective product and rate will vary depending on the type of asparagus beds (newly planted crowns vs. established beds) and the time in the season.

The herbicide program should produce long-term weed control, be safe to use on the asparagus fern and follow state and federal pesticide regulations. As outlined below, new and established asparagus present different sets of requirements.

Herbicide types and application timings

There are several reasons why growers choose to use herbicides (either synthetic or organic) in asparagus. Many Minnesota asparagus growers are also growing strawberries, and therefore are too busy in late June to spend time on weed management after asparagus harvest.

One to two well-timed applications containing both pre- and post-emergent herbicides can reduce the amount of time spent on weed management. It also reduces soil disturbance by reducing the need for cultivation. 

Herbicides can be applied in the following timeframes: 

• Before planting crowns (new stands).
• Early spring before spears emerge.
• During harvest after cutting all emerged spears.
• After final harvest but before ferns grow.
• To ferns after the harvest season.

When to use pre-emergent herbicides

Pre-emergent herbicides in the spring reduce weed emergence for several weeks during the busy harvest period. A second pre-emergent application may be done after harvest, to prevent weeds until the ferns are large enough to outcompete them. 

When to use post-emergent herbicides

Post-emergent herbicides can be used in the early spring to kill winter annual weeds, and after harvest to kill weeds that came up during harvest. In general, weeds are more susceptible to herbicide control when they are less than six inches tall. Larger weeds will still require hand removal, as described above.

Preventing herbicide injury

Herbicide injury, either from drift from neighboring fields or mismanaged in-field application can damage spears and stunt asparagus production. Common causes of herbicide injury include exceeding labeled rates, using the wrong nozzle type or sprayer pressure, failing to calibrate the sprayer, and spraying during weather conditions or crop stage not permitted by the label. 

For example, while halosulfuron (i.e. Sandea) can be applied to ferns after harvest according to the label, it should not be broadcast over the rows. It must only be applied using a directed or shielded sprayer to prevent direct damage to ferns.  Additionally, if broadleaf herbicides such as dicamba or 2,4-D are applied too late in the spring once spears start to emerge, they can cause spear damage.

Another common scenario for herbicide injury is when a grower applies a postharvest herbicide once some ferns have started to emerge. Spraying the whole field during this stage could injure the ferns. In this case, the producer may mow the entire field a few inches above the soil surface and apply a labeled herbicide before ferns begin to grow back.

Herbicide damage in one year can decrease yield the following year by injuring the ferns and crowns. If herbicide damage is severe (i.e. if it killed a significant portion of the ferns), the grower should consider reducing the harvest period the following year in order to allow the stand to recover. Monitor the rate of spear emergence the following spring to determine if this is necessary.

Good weed management is critical for establishing high yielding, healthy new asparagus beds. The newly planted crowns have very small root systems, so just a few weeds around each plant can impact fern growth and subsequent yields. We strongly encourage growers to keep their new stands as weed-free as reasonably possible.

As described previously, most new asparagus production fields are established by planting one-year-old nursery-grown asparagus crowns into deep furrows. Since the first new shoots take several weeks to emerge and grow to a size that can be safely cultivated, weeds in the furrow may become large and not controllable by cultivation.

A pre-emergence herbicide with long, dependable activity will reduce early season weed populations and decrease the number of mechanical cultivations needed to keep fields weed-free.

The asparagus beetle (black head, red prothorax, black and red wing covers usually with six whitish spots) is the most common insect that attacks asparagus. Adult beetles overwinter on the previous year’s ferns, and under debris along field edges. 

Adult asparagus beetles emerge in the spring around the time of spear emergence, feed on the spears, ferns and flower buds, and lay eggs on the plants.

The eggs are rather distinctive and easy to observe as they are black, usually sticking out at a 90-degree angle from the spear and ferns.

• Eggs hatch in about one week and the fleshy, dark gray larvae then move to the foliage to 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.

Larval feeding on the spears causes the spears to grow in a “shepherd’s crook,” making them unmarketable. This happens because the side of the spear that is damaged by the beetles’ feeding grows slower than the undamaged side.

The appearance of eggs on the spears is also considered unmarketable.  Larvae feeding on newly planted ferns can cause extensive defoliation and should be closely monitored. 

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

Managing asparagus beetles

Monitoring

Common asparagus beetles are much more active and easy to detect during the afternoon hours on sunny days. To accurately estimate the percentage of plants infested, it is critical to do beetle sampling during the afternoon (anytime between 12 and 6 p.m.). Sampling for beetles early in the morning, on most days, will typically underestimate the actual infestation level. 

Cultural control and prevention

Since asparagus beetles overwinter in the dead ferns from the previous season, removing the ferns in late fall rather than waiting until spring will help minimize asparagus beetle populations in the field.

The short growing season in Minnesota often means that the ferns are not yet fully dormant when our region receives the first snowfall. It is important to wait to remove the ferns until they are fully dormant (color change from green to brown), but it is difficult or impossible to remove the ferns with snow on the ground. 

Depending on the situation, growers may be forced to wait until the spring to remove the ferns.  When this is the case, growers should completely remove them from the field and burn them as soon as possible in the spring, rather than leaving them in the field.

Some growers will burn the ferns in-field instead of mowing and removing them. This decision will depend on the practicality of removing them, the individual’s comfort with field burning and city or county ordinances around burning. No matter what method you use to destroy the dead ferns, this step is important for managing asparagus beetles.

Asparagus beetles can also overwinter in residues like straw, leaves, cover crops and other dead plant matter. While cover crops play helpful roles in asparagus production,  cover crop detritus will also serve as an overwintering site for asparagus beetles. If populations become severe, try scaling back on cover crop production to evaluate whether asparagus beetles are reduced as a result.

Promoting plant vigor and harvesting spears regularly and thoroughly will help reduce the number of beetles that hatch in the spring.

Chemical management

Organic and synthetic insecticides are available for control of asparagus beetles. They should be applied if at least one of the following criteria (action thresholds) is reached:

• One out of 10 plants (10%) have adult beetles.
• 50-75% of the plants have asparagus beetle larvae.
• 2% of spears have dark, oval-shaped eggs.

Organic insecticide options for asparagus beetles include Entrust (spinosad) and neem oil. However, Entrust cannot be used on spears intended for harvest, because it has a 60-day pre-harvest interval. 

Synthetic insecticide options for application to spears include Assail 30SG, Lannate LV, Lorsban 4E, malathion, Perm-Up, and Sevin XLR Plus. These are the products that have a short pre-harvest interval, allowing for spear harvest, and are also effective on this insect pest.

Synthetic insecticides for application to ferns after harvest include Dimethoate 400, Dimethoate 4E, Radiant SC, and Sevin XLR PLUS. Note that all of these products except for Sevin have long pre-harvest intervals, excluding them from being used on spears.

European asparagus aphid (Brachycorynella asparagi) populations are only high in some years in the upper Midwest. However, in the rare years when large populations do appear, it can cause severe economic loss.

The aphid was first reported in southern and central Minnesota in 1982. The asparagus aphid is a minute (about 0.06 inch long), blue-green sucking insect that usually feeds on asparagus fern.

The aphid feeds only on the fern, but can still be quite destructive. While feeding on the ferns, the asparagus aphid injects a toxin that damages the health of the plant. The toxin gets transported down the plant, from the ferns into the dormant buds underground.

The toxin causes the buds to elongate into new shoots prematurely, producing a "witch's broom," or a dwarfed, very bushy, short plant with a 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.

This damage results in decreased yield and sometimes plant death the following spring. Because of the damaging nature of this toxin, the action threshold for this insect is low; only a few aphids per plant can cause substantial damage. Younger plants, including those one to two years after transplanting, are most susceptible to damage.

Managing asparagus aphid

Like asparagus beetles, asparagus aphids also overwinter on the ferns. However, it is the eggs that overwinter rather than the adults. Because they rely on these ferns as their sole host, fern removal is important for managing this pest.

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. 

Insecticide options

When populations are increasing rapidly and biological control does not appear to be effective, chemical control may be used. Researchers on the west coast, where asparagus aphid is a common pest, have spent decades developing best management practices for controlling it.

Dimethoate, Cythion 5E, Di-Syston, and Fulfill are effective as foliar applications. In a controlled study by Michigan State University, Fulfill was found to be the most effective of these options. Sevin® is not recommended for aphid control.

Organic insecticide options include insecticidal oils and PyGanic. However, the efficacy of these products is less thoroughly documented, and they may not be as effective as the synthetic options listed above. 

Biological control

Lady beetles and parasitic wasps (Diaeretiella rapae) have been found to decrease populations in certain cases. However, very high aphid population levels are difficult to manage with biocontrol alone.

Several species of cutworms successfully overwinter each year in Minnesota. Cutworms, which feed on a number of vegetable crops, cut off asparagus spears and ferns near the base of the plant.

They are active starting in April and continue feeding all season, but are most destructive in the early season when plants are smaller. Therefore, they do impact actively growing spears, and must be monitored closely early in the spring season. In addition, some species feed primarily at dusk or during evening hours; therefore, sampling near dusk can be beneficial. 

If a cutworm feeds on the base of a spear but does not completely sever it, the spear will grow crooked as the undamaged side grows faster than the damaged side. This damage can be mistaken with asparagus beetle damage. 

Managing cutworms on asparagus

Chemical control is warranted if one or more cutworms per 10 crowns are found. Pesticides can be used as a foliar spray. Pyrethroid insecticides, particularly under cool, spring conditions, will usually provide good cutworm control.

Effective synthetic insecticides include carbaryl (Sevin), chlorpyrifos, cyfluthrin, and permethrin. Permethrin is more active in cool spring temperatures, and trials have found it to be more effective than chlorpyrifos. They can be band-applied over the rows, where the cutworms are most active, and tank-mixed with pre-emergent herbicides in the early spring as spears are emerging. 

Spinosad baits may be used for cutworm control on ferns, but are not labeled for use on spears.

Asparagus Rust is caused by the fungus Puccinia asparagi. Lesions on the ferns that can become large, reducing fern growth and subsequent stand productivity.  

Symptoms

Small, uneven, cream-colored “scab”-like lesions on the stems of ferns, orange-colored stripes on the stems with more advanced infections, and early senescence (dieback) of the ferns. Asparagus rust does not cause symptoms on the spears.

Conditions

It is more common in newly planted fields that are not harvested. This is because the act of harvesting spears removes the fungal spores that form on them in the early spring (basidiospores and aeciospores). These early spores are necessary for subsequent fern infections to occur. When they are removed via spear harvest, it is unlikely that the orange “rusty” fern infection will form. 

Management

In Minnesota, asparagus rust can often be controlled organically without the use of fungicides. Regular harvest of all spears will greatly reduce the risk of asparagus rust forming on the ferns, and this is often sufficient for mature stands.

In non-harvested stands, or in cases where new and mature stands will regularly be in close proximity, the use of newer varieties with better disease resistance can help reduce, but not eliminate, risk of infection. If rust is found on the ferns, removing infected ferns before the spring helps slow the spread of the disease the next season by interrupting the disease cycle.

In severe cases, specific fungicides active on asparagus rust can be applied.

Purple Spot is caused by the fungus Stemphylium vesicarium. Severe infection can cause the ferns to senesce (shut down for winter) early and eventually decrease yield and productivity of the infected plants. While it can infect spears, this is not considered the main impact of purple spot.

Symptoms

Small purple, ovalur spots on the spears and ferns, early senescence of ferns.

Conditions

Previous studies of no-till asparagus have suggested that purple spot may be more common in no-till stands because the fungi remain in detritus left in the field. Cool, wet weather and blowing sand contribute to purple spot infection on spears and ferns. Blowing sand is abrasive to the spears, creating disease entry points.

Management

Because the fungal spores survive on dead ferns over the winter, remove the previous year’s dead ferns from the field or burn them in the field to decrease infection of the next year’s spears. In fields with sandier soil, consider using cover crops and wind breaks to reduce wind-blown sand.

Chlorothalonil fungicides have been found effective on purple spot.

Cercospora leaf spot is caused by the fungus Cercospora asparagi. Like other diseases, fern damage from Cercospora causes yield loss the following year by reducing fern growth and speeding up senescence. Repeated infection in multiple years can reduce the lifespan of an asparagus stand.

Symptoms

The symptoms start on the lower parts of the ferns and spread upward. Symptoms include browning of the ferns, oval lesions with tan or gray center and reddish brown border, and yellowing over entire ferns in advanced cases. 

Conditions

Cercospora spreads and infects ferns from June to August, and is stimulated by warm, humid weather. Therefore, Cercospora risk increases as the fern canopies grow large and create humid environments. Risk also increases with frequent rainfall during hot periods, and when excessive irrigation is used.

Management

Minimize the amount of time the fern canopy is wet:

• If irrigation is necessary, irrigate in the mid-morning so that the ferns can dry quickly.
• Space rows further apart to promote air movement within the stand. As described in the planting section above, plant rows at a minimum of four feet. Research in Oklahoma showed that spacing rows at six feet instead of five feet increased air flow and slowed canopy closure. 

Additionally, burning the dead ferns in late fall or early spring destroys fungal disease spores, delaying or reducing infection later on. If cultural controls prove insufficient for controlling a significant Cercospora infection, select foliar fungicides can be applied throughout the growing season with a focus on disease prevention during wet, hot, humid periods.

Phytophthora root rot is caused by oomycetes (or water molds) in the Phytophthora genus. It is not a common pathogen in Minnesota, but growers should still practice prevention. Phytophthora asparagi is the most common species that causes damage to asparagus in the Midwest.

As the name suggests, Phytophthora root rot starts in the roots, but it can spread to the crowns over time if conditions are right. Phytophthora can go unnoticed for years in asparagus because it begins in the roots and spreads slowly.

Symptoms

At first, you may simply notice reduced yields and fern development. If you were to dig up a plant, you would notice reduced root vigor and discoloration.

The first aboveground symptoms are water-soaked lesions or a general water-soaked appearance on the spears at the soil line. As the disease progresses, spears can become crooked or hook-shaped, and the water-soaked lesions eventually become brown and shriveled.

In severe or advanced cases,  the spears become yellowish-brown inside. In a field, growers will typically see distinct affected patches, particularly in low lying areas.

Conditions

Phytophthora is soilborne and typically causes damage in wet conditions. It is particularly problematic in areas with poor drainage, and during seasons with frequent, heavy rainfall. The severity of symptoms can vary from year to year, but once a plant is infected, there is no cure.

Management

Prevention is key. Buy healthy crowns, and inspect them for disease at planting. Phytophthora symptoms in crowns manifest as grayish colored, water-soaked roots.

Select your planting site carefully, and make sure there is ample drainage. If possible, avoid planting new asparagus crowns in an area that was previously infected. The variety Jersey Giant is less susceptible to P. asparagi than Jersey Supreme or Millenium.

Fusarium crown and root rot is caused by a variety of soil fungi in the Fusarium genus and is common in all places where asparagus is grown. It typically affects mature stands of asparagus, but it can also affect seedlings. If the crowns are infected at the time of planting, Fusarium can spread quickly throughout a field. 

Symptoms

In older plantings, Fusarium is typically first noticed due to chlorosis, or yellowing of the ferns. This process can be gradual, with a general decline in productivity and growth.

Eventually, the chlorosis, or yellowing of tissues, progresses down the stem and infected tissues become necrotic. In younger plantings, Fusarium can simply manifest as wilt, which will often progress to kill infected plants.  

Conditions

Most soils naturally contain strains of Fusarium that are capable of infecting asparagus. Soils with a low pH and low organic matter are more likely to have problems. 

Management

Start with healthy crowns; inspect crowns to ensure that the roots are white, not discolored and not shriveled. Avoid planting asparagus in areas where it has been grown within the past five years, and if possible select a site with good drainage, high organic matter and a moderate to high pH.

Fusarium has been associated with deep plantings; in fields with known Fusarium problems, consider planting at a more shallow depth. Focus on organic matter building strategies in your soil, and test your soil regularly to prevent Magnesium and Manganese deficiencies. Drought, weed competition, insect pressure and overharvesting have all been correlated with increased severity of Fusarium infection. 

Most of the common varieties in Minnesota are tolerant to Fusarium including Jersey Giant, Jersey Knight, Jersey Supreme, Purple Passion and Millennium. 

In fields with known Fusarium problems, fumigating, or using anaerobic disinfestation, and incorporating manure or other organic matter can help to reduce pathogen populations in the soil.