Asparagus plants are prone to a wide range of diseases. In most cases, the visible symptoms will help to determine the cause of the infection, and treatments are generally available.
But diagnosing the source of the infection can be challenging.
This guide will help you to distinguish between the fungi, water mold, and viruses that could be afflicting your plants.
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Some of these diseases are more readily treatable than others, and we will provide advice on how to deal with the pathogens that can be controlled.
Common Asparagus Diseases
Asparagus plants are vulnerable to a wide array of fungi, some of which prefer cool, moist weather, while others attack in warm, dry weather.
This fungus that causes anthracnose, Colletotrichum gloeosporioides, thrives in hot conditions (77°F) and is primarily a problem on the East Coast.
To determine whether your asparagus is under attack, look at the base of the main stem to check where the fern first branches out. If there is an anthracnose infection, you will see light pink lesions that will turn brown later. Eventually, the spores will form concentric rings that are also visible.
Anthracnose spreads in a variety of ways:
- By wind and water
- Through contact of healthy plant parts with infected ones
- Via seed saved from infected plants
Asparagus rust, caused by Puccinia asparagi, is an extremely complex fungal disease that threatens crops throughout the world.
While plant breeders work hard to create varieties that are resistant to asparagus rust, the fungus eventually overcomes this resistance, and the plants are no longer protected.
This type of rust has four life cycles that are active at various points throughout the year, and the spores undergo radical physical changes from one phase to the next.
The most damaging phases occur in the summer and produce rust-colored spores, which is where the disease gets its name. These spores are called aeciospores, and they rapidly spread the infection with the help of wind and/or rain.
New infections are spread by the next kind of spores – urediniospores – which damage the foliage the most seriously. They can infect the plants in the presence of rain, dew, or overhead irrigation.
An infection with this fungus can kill young plants, reduce the quality of the spears, and make the plant more susceptible to crown rot.
You can help reduce the number of spores that overwinter and start infections in the spring by burning all the crop residue in the fall – or at least bagging it up and disposing of it off your property.
Destroy wild asparagus or any volunteers near your plantings. Plant the rows of asparagus in the direction of the prevailing winds to promote good airflow.
You can also use the microbial fungicide CEASE, available via Arbico Organics, or various synthetic fungicides to control this disease.
Cercospora Needle Blight
Cercospora asparagi is a fungus that causes lesions to appear on the leaves and stems. It can be a horrific problem in warm humid environments like those in North Carolina and eastern Oklahoma.
Favorable conditions include 95 percent or higher humidity and average temperatures of 77 to 86°F. However, it does not cause much damage in cooler or drier climates.
Small oval grayish-tan lesions with purple borders first appear on lower portions of the ferns. The infected ferns turn yellow to brown and then die prematurely. Yield losses can be high the following spring.
Spacing rows six feet apart can increase the movement of air, so that the foliage will dry out more easily, and planting rows in a north-south direction will enable the prevailing southerly winds to dry the foliage.
Research on the use of fungicides to control this disease has been extremely promising. In Oklahoma, researchers obtained yield increases of up to 2,000 pounds per acre following a year in which they treated the crop with three different fungicides.
In cultivar trials via the North Carolina Cooperative Extension Service, researchers found that chlorothalonil, available as Bonide Fung-onil Concentrate was an effective fungicide against this disease.
Crown and Root Rot
Crown and root rot, caused by Fusarium oxysporum f. sp. asparagi and Fusarium proliferatum, can be so serious that it drove a lot of commercial asparagus operations on the East Coast out of business in the 20th century, according to an article in Phytopathology by the plant pathologist Wade Elmer.
The fungi are ever present, even after the soil is no longer being used to grow asparagus. Such soils can still contain propagules from 30 years ago! Therefore, chemicals are not feasible to control them.
You can tell if your plants have crown and root rot because the fleshy roots will have lesions and may have almost entirely rotted away if the infection is severe. You may also be able to see reddish-brown lesions on the lower stems and/or crowns. In addition, the ferns will turn yellow.
You should assume that the soil in which you are growing your asparagus plants is contaminated. Therefore, it is critical to avoid stressing your plants, helping to increase their chances of survival in the presence of these pathogens. Steps to take include irrigating regularly, so they do not get drought stressed, and controlling weeds and insects.
Keeping the pH at 6.0 or higher can also help to keep your plants safe. Research suggests that treating the soil with salt (sodium chloride) can help restrain infections.
Also be careful not to do any tilling that could damage the crowns of your plants, since that can spread the infection. Do not rotate to corn, since it is susceptible to infection by Fusarium and can serve as a reservoir of the pathogen.
Fusarium culmorum is a dreadful pathogen that lives in the soil and affects a wide array of plants.
Regrettably, the fungus can survive for long periods in the soil in the form of resting spores called chlamydospores – as long as 30 years!
The first sign that your asparagus plants are under attack will be a single dead plant stem. If you look more closely, you will see an elongated, desiccated, pale red or orange lesion just below or near the surface of the soil.
Water transports the fungus, and infections are most commonly observed after long periods or rain or in wet conditions. The fungus enters the plant through wounds and spreads from there.
Sources of wounds include:
- Harvest damage
- Phytophthora infection
- Growth cracks
- Damage caused by pests such as asparagus miner flies or spotted snake millipedes
Fusarium wilt occurs more frequently in sandy soil rather than heavier types of soil.
Unfortunately, there is no effective method to control F. culmorum. Preventing infection by minimizing wounds to the plant is your best bet.
In addition to attacking asparagus plants, this widespread fungus (Botrytis cinerea) can cause serious disease on a number of high profile crops, including strawberries, tomatoes, and grapes.
It causes similar symptoms in all of these plants, starting with dead flowers that will be covered with a gray film of fungal threads. From there, the infection spreads to other parts of the plant.
The wind disperses this fungus, which can spread rapidly. It is most problematic in plots where the asparagus plants dry slowly, and under hot and humid conditions.
When conditions are wet, the spores will enter the plant through petals, wounds, or dead plant parts. From there, the fungus will rot the tissue.
This fungus can persist for a long time, even during drought or in cool temperatures, by forming resting spores called sclerotia.
One way to prevent this disease is by planting the rows in the direction of the prevailing winds, so the plants will dry rapidly. Limiting the use of overhead irrigation will also help keep the plants dry and limit infection.
Given how widespread and serious this disease is, fungicides are often used to control gray mold. However, this type of fungus is exceptionally quick at developing resistance to fungicides.
Another problem is that the resistance varies between locations. A fungicide will work in some locations and not others.
Therefore, the advice on which chemicals to use is constantly changing. Your best bet is to check with your local county extension office to find out what fungicides are recommended in your area.
Biofungicides are another option to consider.
Purple spot, caused by Stemphylium vesicarium and Pleospora herbarum, can be a serious problem for newly emerging asparagus spears, or the presence of the fungi that cause it may be of no consequence, depending on the weather.
New spears can be affected when the weather is cool and wet, resulting in extreme crop losses for home gardeners as well as commercial growers. However, the disease subsides when the weather warms up.
Symptoms include small reddish-purple lesions on the lower halves of the new spears. Sometimes, the disease is much worse where the wind blows against the spears. This happens because the sand that blows in the wind creates wounds that the fungi exploit to enter the plant tissue.
Ascospores produced on asparagus fern debris left on the ground from the previous season may serve as a source of infection. Bagging and removing or burning this debris will greatly help to reduce the potential for infection.
You will need to check the recommendations in your area to determine if you can treat this disease with fungicides. If so, chlorothalonil is effective.
You can find chlorothalonil as Bonide Fung-onil Concentrate, available via Amazon.
This serious fungal disease, caused by Helicobasidium purpureum (Rhizoctonia violacea), afflicts asparagus, sugar beets, carrots, and chicory.
The aboveground parts of the plants will not show symptoms, but a telltale sign is the mysterious loss of plants in specific parts of a plot. These bare spots in your garden will increase in size every year as the fungus spreads from one plant to another.
Unfortunately, this disease can survive in the soil for long periods of time, via resting structures called sclerotia.
Certain types of weeds can be good host plants, including creeping thistle, field bindweed, and couch grass.
Your only option is prevention. Don’t grow asparagus in plots that have been affected by red rot in the past. Remove any infected plants by hand. If your plot is affected, don’t subject it to any mechanical treatments, because they may spread the fungus.
The optimal temperature for disease development is approximately 68°F, and the fungi seem to prefer soils that have a high content of humus and a low pH.
Asparagus is among the hundreds of different kinds of plants that can be infected by Sclerotinia sclerotiorum. Infections typically occur late in the growth stage of the ferns. White mold is primarily a threat when conditions are hot and moist.
The most distinctive symptom is a white discoloration of the main stem or lateral stems. As the disease progresses, the fungus produces resting structures called sclerotia in the white mycelia. These structures look like tiny rat droppings. At this point, the affected stem parts will die.
Since the sclerotia produced in the stems enable the fungus to survive for up to 10 years, it is imperative that you remove and destroy infected plant tissue.
However, this fungus can become so entrenched in your crop that you may be better off starting over in a different part of your garden.
Water Molds (Oomycetes)
Pathogens in this class were originally classified as fungi before they were determined to belong to a totally different group of organisms known as oomycetes.
The pathogens are still commonly called water molds due to their ability to spread in moist conditions.
Phytophthora Crown, Root, and Spear Rot
As if it weren’t bad enough that Fusarium can cause crown rot on asparagus, Phytophthora asparagi can also cause this severe affliction.
This disease first manifests as soft, water-soaked lesions that occur slightly above or below the soil line. As the infected spears grow, a bending structure that looks like a shepherd’s crook is visible. The ferns of infected plants will turn yellow, and the crowns will die.
The pathogen produces oospores that live in the soil for an extremely long time until they sense the presence of a potential host. Then, they germinate and infect the roots and crowns of the plants.
At this point, another type of spore forms on the infected tissues. When moisture is present, these produce mobile zoospores that can swim in water. They are easily splashed by rain onto the spears, where they cause infection.
One way to avoid this disease is to refrain from planting in low-lying fields or areas that drain poorly.
There are fungicides that can be used against this pathogen as well.
As if all the asparagus diseases caused by fungi and the water mold are not enough, this crop is also vulnerable to viral diseases.
Asparagus Virus I and II
Asparagus plants can be infected with one or both of these diseases, but you may not realize they are infected due to a lack of visually apparent symptoms. They do result in reduced vigor and an increase in susceptibility to other pathogens.
If a plant is under siege by just one of these viruses, it will remain fairly vigorous. However, when both viruses are present, the infected plant will have lower vigor and less chance of survival, particularly if it is also young.
Aphids can transmit both types of viruses, although controlling them does not seem to help to control these pathogens. Virus II can be transmitted by pollen and is more common in older cultivars.
Consult your local county extension agent to learn the best techniques to control these viruses.
So Many Pathogens
Asparagus plants can be faced with a number of maladies, some of which are more easily controlled than others.
Our hope is that this guide will help you to diagnose damage to your plants and hopefully learn appropriate control measures.
Even the dreaded Fusarium crown and root rot can be partially brought under control with cultural treatments.
If your plants do have a pernicious disease, there may well be available techniques to help prevent infection in the future.
Have your asparagus plants suffered from an infection? If so, let us know in the comments, and feel free to share any questions you may have as well!
And for more information on growing asparagus in your garden, check out these guides next: