Strawberry Plant Diseases: Problems Caused by Bacteria, Fungi, Molds, and Viruses

Strawberry diseases can affect fruit, flowers, leaves, roots, and crowns of strawberry plants, and sometimes cause the collapse of the whole plant. While many of the problems caused by bacteria, fungi, molds, and viruses can be treated when symptoms are observed, some of the most serious strawberry plant diseases cannot be treated after the plants are in the ground. But home growers and organic growers will find many organic options that reduce strawberry diseases, even if they don’t eliminate them, in the A to Z listing below.

Angular Leaf Spot

Angular Leaf Spot

Angular leaf spot first appears as water-soaked lesions on the undersides of leaves and calyxes around fruit. As the bacterial ooze dries, the next thing you will see is a white powder, followed by purple or red discoloration of the leaf at an angle from the stem to which it is attached. There will be yellowing (chlorosis) around the damaged area of the leaf.

Sometimes this infection with Xanthomonas fragariae bacteria can cause the entire plant to die suddenly, but this is rare. This condition usually doesn’t have a major impact on fruit production by itself, but it can be one among many factors that keep you from getting a good crop. Commercial growers who ship their strawberries across state lines will find their products under quarantine if inspectors find angular leaf spot, so they have to keep this problem under control.

Plant sprays containing copper control angular leaf spot in young plants. An antibiotic for plants called validamycin can control the problem in mature plants. But if you are raising strawberries for home consumption, the most practical approach is soil management.

The bacterium that causes angular leaf spot can only survive on plant matter. If cannot grow on vermiculite or rock used in hydroponic growing, and it won’t survive the winter if strawberry plants and their roots are removed from the ground or their growing containers.

Anthracnose

Anthracnose on strawberries

Anthracnose is a fungal infection that can attack the crown, stems, leaves, and fruit of strawberry plants. You can recognize anthracnose by the orange ooze of spores it forms on the parts of the plant it kills. These spores are spread by overhead splashing water. If you don’t have any rain and you don’t do any overhead watering after fruit has set, you can at least save your crop, although plants may be damaged.

This fungus can survive in the ground for up to nine months after the strawberry plant on which was growing dies. It’s important not to leave infected plants in the ground or to turn them under after they die. It’s OK to compost anthracnose-infected strawberry plants if your compost pile heats up enough to kill pathogens.

Preventing the spread of anthracnose begins with your selection of the strawberry plants you put out. Don’t buy plants that were grown in the ground or irrigated with overhead sprinklers. You can kill any anthracnose that transplants may have acquired in the nursery by immersing the plants in hot water as soon as you get them home, but this method only works if you have a thermometer and you keep track of time.

First, it is important to soak your transplants in a warm water bath, 85 to 86 degrees Fahrenheit (about 30 degrees Celsius) for 30 minutes so they won’t be shocked when you put them in hot water. Then your strawberry transplants need to be immersed in water heated to 113 to 115 degrees Fahrenheit (45-46 degrees Celsius), no hotter and no colder, for 10 minutes, no less and no longer. Leaving the plants in their hot water bath too long may damage them, and not heating the water hot enough will not kill the fungus. Finally, your plants need to be allowed to cool off at room temperature for half an hour before you transplant them.

Alternatively, you could dip transplants into a mixture of the fungicides Abound (azoxystrobin) or Switch (cyprodinil + fludioxonil).

It’s possible to pretreat the soil into which you are planting strawberries with the fungicide InLine (1,3-dichloropropene + chloropicrin) to reduce anthracnose problems. If you do this, you should scatter the product over the entire patch rather than concentrating it near plants. Commercial strawberry growers used to fumigate the soil with methyl bromide plus InLine to stop anthracnose, although this chemical combination is decidedly toxic to more than just the fungus.

Another approach to preparing the soil is to “solarize” it by covering it with black plastic the summer before planting. This approach works in hot, sunny summer climates. If you grow strawberries during the summer, you won’t get good results from solarizing during the winter.

Once you get your strawberry plants in the ground, it’s important to avoid overhead watering that causes splashing. Drip irrigation or micro-sprinkler systems are preferable.

There are no “organic” fungicides that kill anthracnose, but you may be able to reduce the problem by choosing your location for planting strawberries carefully. One of the ways anthracnose is spread to strawberries from other plants that harbor the disease but are not damaged by it. Tomatoes, zinnias, peppers, celery, delphiniums, and vetch planted near your strawberry plant can keep it infected with anthracnose. Close proximity to almond trees and pines can also be a problem.

Leather Rot Causes Brown Spots on Strawberries

brown spots strawberriesLeather rot is a relatively common pathogenic fungus (Phytophthora cactorum) that can affect strawberries in any stage of development. It can infect green strawberries. It can infect ripe strawberries. And, it typically only causes minimal damage to commercial farmers. However, home gardeners can loose many strawberries to the fungus if care isn’t taken to avoid infection.

Infection of healthy fruit and subsequent brown spots on strawberries from leather rot typically happens during periods of wet weather. Rainy April, May, and June weather sets the stage for infection. The spores of the fungus stay in the soil and can infect strawberries when there are periods of extended wetness. If water remains in contact with fruit for an hour when the temperature is between 62 and 77 degrees Fahrenheit, infection is likely if spores are present. Brown spots on strawberries will come soon after infection as the pathogenic fungus sets up shop in your fruit.

Brown spots on strawberries are most easily seen on immature fruit. If the fruit is still green or white, the spots will be most visible. Ripe fruit can get leather rot also. However, the brown spots will not usually be as noticeable. Sometimes, the discoloration will even be a purple color or just a darkening of the strawberry in the infected area. Usually, along with discoloration, there will be an odor and unpleasant taste in infected berries.

How to Prevent Brown Spots on Strawberries

Preventing leather rot infections is the key to harvesting healthy strawberries.

  1. The first and critical step is to pick a location for your strawberry bed that is well-drained. Good soil drainage prevents standing water. The absence of standing water makes difficult for the fungus to remain in contact with strawberries long enough to infect them.
  2. Use a thick barrier mulch. A good layer of clean straw can prevent the spread of leather rot. Clean straw help reduce or eliminate splash from rain. It can assist in drainage. And, it keeps the strawberries themselves from resting on the soil where the pathogenic fungus lurks.
  3. Avoid shade. Plant your strawberries in full sun to minimize the conditions favorable to fungal infection.
  4. Plant the rows parallel to the direction of the prevailing winds. Most locations receive there weather, more or less, from the same general direction. If you plant your strawberry rows with the prevailing wind, the fruit and leaves of your strawberry plants will dry more quickly.
  5. Avoid excessively dense plantings. Be sure to maintain adequate space between your strawberry plants to facilitate drying. Additionally, inappropriate application of nitrogen fertilizer can cause dense vegetative growth that will shade the berries (see 2. above!) and create an environment conducive to infection.
  6. Pick fruit early. As soon as the plants dry each day, go picking! Removing the strawberries as soon as possible during the day can reduce infections.
  7. Watch for and remove fruits with any brown spots on strawberries. It is vital for the health of remaining plants that strawberries with brown spots be removed as quickly as possible from the strawberry patch. If strawberries with leather rot are left in the field, the causal organism will multiply and spread.
  8. As a last approach, fungicides can be used.

Pay attention to your strawberries! If they get brown spots, cull them as soon as possible. Leather rot can be a pain in the neck. If you set up your strawberry bed in a way that helps the fungus instead of impairing it, remedy as many factors as you can. Good luck!

Bortrytis Fruit Rot

Bortrytis Fruit Rot on strawberries

Bortrytis fruit rot is a problem for strawberry growers all over the world. This ubiquitous fungus can attack fruit before or after harvest, transforming a beautifully red and juicy strawberry into an unappetizing and infectious mass of gray mold in just hours. Bortrytis fruit rot also attacks strawberry flowers. In conditions of relatively low temperatures, between 58 to 72° F (15 to 22° C), and high humidity, up to 80 percent of your crop can be lost to bortrytis fruit rot.

Bortrytis spores land on mature leaves and wait for them to die. As the leaf matures and begins to decay, the fungus produces spores that land in flowers. They may kill the flower outright, or land deep inside stamens where they lie dormant waiting for the fruit to form and start producing sugars. As the strawberry becomes sweeter, the fungi grow. The characteristic gray mold may become noticeable before or after the fruit is harvested.

One way to stop the growth of bortrytis on ripe strawberries is to chill them immediately after harvest. Waiting even an hour can give the fungus a chance to destroy the newly picked fruit. Strawberries need to be chilled to between 32 to 37° F (0 to 3° C) and held at those temperatures until just before they are eaten. If you see a moldy berry in a clamshell or a flat, remove it immediately to prevent contamination of all the other strawberries in the container.

If gray mold is a repeat problem in garden strawberries, try spacing them out so more air can circulate around the leaves and flowers, creating an environment in which mold cannot flourish. Or grow your strawberries in hanging baskets. If you are growing strawberries in a raised bed, plant them along the edges so their runners hang over the edge and get more air and sunshine. Remove “mummy berries” that can spread the fungus to other plants.

If you are growing your strawberries in a greenhouse, turn up the heat two to four times a week so your plants spend the night at about 77° F *(25° C). This will discourage the growth of the fungus. Make sure your fans are on a timer so there is adequate air flow over your crop.

Sometimes it is possible to fight fungi with beneficial bacteria. Sprays of Bacillus subtilis QST 713 (sold under the brand names Cease and Serenade). Streptomyces griseoviridis (sold under the brand name Mycostop), and Streptomyces lydicus WYEC 108 (sold under the brand name Actinovate AG) applied before any signs of Bortrytis infestation will help. So, will sprays of potassium bicarbonate to keep the surface of leaves and flowers too alkaline for growth of the fungus. The bacterial sprays and potassium bicarbonate are non-toxic and may be acceptable for organic certification. (Ask your agricultural extension agent for the exact details of regulations in your state or province.)

Commercial strawberry growers often attempt to control bortrytis gray mold with toxic chemical agents. The problem with trying poison gray mold is that it reproduces so often that mutations occur that it the ability to resist fungicides. Among the fungicides that are unlikely to be helpful are fenhexamid (sold under the brand name Elevate), pyraclostrobin + boscalid (sold under the brand name Pristine), cyprodinil + fludioxonil (sold under the brand name Switch), and thiophanate-methyl (sold under the brand name Topsin). Your extension agent can help you find fungicides that are still effective against bortrytis gray mold in your area.

Charcoal Rot (also known as Macrophomina)

Charcoal rot is a disease of strawberries also known as Macrophomina (capitalized because the name is taken from the scientific name of the fungus). It is caused by the fungus Macrophomina phaesolina. This fungal infection has the same symptoms as another fungal disease of strawberries, Fusarium wilt. Charcoal rot comes from the soil into the roots and infects the crown of the plant. At first, growth will be stunted but the plant will be green. Then all the leaves except those closest to the crown die, and finally the crown itself dries up into orange and brown plant matter.

Charcoal rot tends to occur when strawberries get so much water that their roots don’t get enough oxygen, and when plants are under extreme stress from heat and drought. It can also take over when strawberry plants are invaded by mites, which are also more common during heat and drought.

Plowing infected strawberry plants back into the soil perpetuates the disease, but it is not just strawberry plants that can be affected. Charcoal rot also occurs in about 500 vegetables and grains, including alfalfa, corn, chickpea, cabbage, peanut, pepper, sorghum, sweet potato, and wheat. Soil in which any of these plants has been grown may be contaminated.

Getting rid of charcoal rot is challenging with organic methods. Solarization, covering the ground with black plastic to let summer sun heat and sterilize it, may help. Rotating strawberries and other susceptible plants with broccoli or mustard plants reduces the number of charcoal rot spores in the soil, as does working mustard plant meal (the vegetable, not the condiment) into the soil.

If you can find them, plant Chandler or Seascape strawberry plants. They have some resistance to charcoal rot. And always be careful to avoid bringing contaminated soil or compost into your strawberry patch.

Fusarium Wilt

Fusarium Wilt

Fusarium wilt is a fungal disease that can wipe out your entire strawberry crop. Caused by the fungus Fusarium oxysporum, this common strawberry disease shows up in hot and dry weather. It can cause slow growth, stunting, wilting of older leaves, and discoloration of crowns. It may just reduce your yields, or, if your plants are under stress from heat and drought, kill your entire strawberry bed or strawberry field.

Fusarium wilt is a soilborne condition. It’s the kind of pathogen you can bring into your strawberry patch with contaminated soil or compost. Rototiller blades, shovels, hoes, and plows on tractors can carry the fungus from one field to another. Leaving dead leaves, roots, or fruit from infected plants on the soil or turning it under at the end of the growing season can perpetuate the problem for the next growing season. A minor problem one year can result in the complete destruction of your crop the next.

So, what can you do about this persistent, potentially devastating strawberry disease?

  • Solarize your soil by placing black plastic over your strawberry bed the summer before you plant. The heat will kill pathogenic bacteria and fungi in the soil.
  • Plant varieties of strawberries that have some resistance to Fusarium wilt, such as San Andreas and Ventana.
  • Always buy high-quality transplants from nurseries that can tell you how they protect their stock from Fusarium wilt.
  • Alternate your strawberry patch with a broccoli patch. Residues from broccoli suppress the Fusarium fungus.
  • If you feed your strawberry plants compost, make sure any plant matter in it is completely decomposed. The Fusarium fungus feeds on the process of decay and won’t grow once decomposition is complete.
  • Don’t use tools that have been exposed to plants that had wilt around plants that have not been exposed to wilt unless you disinfect them with bleach first.
  • Be kind to your plants. Make sure they are adequately irrigated. Treat spider mites. Shelter them from extremes of heat and drought.

Everything about Fusarium oxysporum isn’t bad. The fungus has the ability to dissolve gold from soil and coat itself in gold leaf. If you happen to be gardening over a literal gold mine, Fusarium wilt will let you know about your mineral riches.

Leaf Blotch

Leaf blotch is a fungal disease that is transmitted by rain drops that hit the ground and splash up to the leaves of the plant. It causes gray to tan blotches on older leaves, and it can sometimes damage the calyx, the green leaves around a strawberry fruit. The strawberry will still be edible, but it will not be as attractive. A diagnostic test for leaf blotch is the presence of tiny black or brown fruiting bodies in affected leaves.

Leaf blotch is not something you treat. It’s something you prevent by planting your strawberries through a sheet of plastic mulch. Perforated red plastic mulch prevents soil-borne diseases from reaching the above-ground parts of the plant and results in larger, sweeter fruit.

Mucor Fruit Rot

Mucor Fruit Rot on strawberry

Tiny injuries in the surface of a ripe strawberry let the fungus that causes Mucor fruit rot inside to mutiply and destroy the berry. (The term “Mucor” is capitalized because it refers to part of the scientific name of the fungus.) Within a few hours, the fungus will liquify the strawberry. Then what is left of the fruit will be covered with white furry spikes that will eventually bear black spores.

There aren’t any fungicides that treat Mucor fruit rot. You can greatly reduce the disease by making sure that you handle ripe fruit gently, so it is not bruised or cut. You also need to be sure to remove the entire berry from the strawberry plant. Leaving the crown of the berry attached to the plant leaves many entry points for Mucor fruit rot spores that may be floating in the air. Unfortunately, once the berry is damaged, there is no way to protect it from Mucor fruit rot, even in cold storage. It’s important to treat ripe strawberries gently.

Rhizopus Fruit Rot

Rhizopus fruit rot is a strawberry disease remarkably similar to Mucor fruit rot, except Rhizopus spores will appear dry and Mucor spores will appear to be incased in some kind of slime when you inspect diseased berries with a magnifying glass. Unlike Mucor fruit rot, Rhizopus fruit rot is stopped when strawberries are put in cold storage.

Pallidosis-Related Decline

Aphids transmit viral diseases in strawberries
Aphids transmit viral diseases in strawberries

Pallidosis is a viral disease of strawberries transmitted by aphids and whiteflies. Viruses in this family can cause strawberry mottle, strawberry crinkle, strawberry vein banding, and strawberries with yellow edges. Your plants may produce fewer runners but there is no damage to fruit and little reduction in fruit production. Sometimes the virus increases growth in the top of the plant, making it top heavy. If you can control aphids and whiteflies, you can control this viral disease.

Phytophthera Rot

Strawberry roots infected with Phytophthora fragariae
By Liberato, JR – http://dev.padil.gov.au/pests-and-diseases/Pest/Image/4738, CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=37701124

Phytophthera rot is a relatively rare but devastating disease in your strawberry plant. Early in the year, plants affected by phytophthera show stunted growth. At some point when the weather has warmed and the plant has grown, the whole plant will suddenly collapse.

Phytophthera is a “water mold.” It grows readily on free water and in water-soaked strawberry beds and strawberry fields. Water-soaked soil enables this pathogen to produce sporangia that quickly release billions of “spores,” more accurately described as zoospores, because they can swim through standing water.

When this happens, the mold that had affected just one or two plants can quickly spread to every other plant in your strawberry planting. The mold destroys the roots of the plant, and when the roots are no longer functional, the plant dies.

Once this mold has taken over your patch or field, its spores can lie dormant in the soil for years until they find a new host. Phytophthera infects not just strawberries, but over 200 other plants, including potatoes, peppers, plants in the Cabbage Family, and asparagus.

Chemical treatments for phytophthera such as mefenoxam (Ridomil Gold) and phosphonates (Aliette, Fosphite, and others) are systemic. They have to be absorbed by the entire plant. But you may be able to avoid outbreaks of phytophthera simply by preventing standing water: Stop sprinkler leaks, don’t let equipment create ruts in your strawberry bed, and don’t irrigate plants by flooding. Just avoiding overwatering can make a huge difference in your strawberry plants’ vulnerability to this devastating disease.

Powdery Mildew

Powdery Mildew

The first signs of powdery mildew are white, fluffy patches on leaves. Young leaves will curl upward. As the colony of mildew grows and ages, leaves will turn first purple and then red on their lower surface and then show purple, red, and brown blotches on their upper surface. Powdery mildew seldom affects mature leaves because it prefers young, growing tissue.

When the mildew colony has taken over a leaf, it produces spores. These spores land on flowers. The deformed flowers are also covered with white, fluffy patches and fail to develop properly so they do not bear fruit. When powdery mildew spores land on immature fruit, they also stop developing. Fruit at a later stage of development, about 10 or 15 days after flowering, is immune to powdery mildew, but the seeds can be infected. Mature strawberries affected by mildew have a “seedy” appearance and a moldy flavor.

Powdery mildew is usually something you bring into your garden or onto your farm through infected transplants. It grows faster in warm (60 to 80 degrees Fahrenheit, or about 16 to 26 degrees Celsius) humid conditions, but droplets of moisture on leaves from rain or overhead irrigation inhibit it. Direct sunlight also reduces the growth of mildew.

What can you do to stop powdery mildew on your strawberries?

  • Be selective about where you get your plants. It’s better to buy plants that have been out in the sun instead of plants you find in a damp, poorly ventilated greenhouse.
  • Buy cultivars that are more resistant to powdery mildew. There are no strawberries that are immune to the disease, but Albion, Chandler, Florida Radiance, San Andreas, Seascape, and Sweet Ann strawberries are more resistant to mildew than Camarosa, Monterey, and Ventana strawberries.
  • Protect your plants from powdery mildew with preventive applications of micronized sulfur or some of the same soaps used for protection from insects. Both of these relatively non-toxic products have to be applied before powdery mildew appears. Using these products more often than once ever 14 days reduces fruit production. Baking soda in water (a tablespoon, or about 15 grams, in a gallon, or about 4 liters, of water) also helps.
  • There are a number of fungicides that kill powdery mildew, including Abound, Pristine, Merivon Quintec, Rally 40 W, and Torino. The cost of these products could be greater than the value of your strawberries if you are a home gardener, and you should consult your agriculture extension agent about produce selection if you are a commercial strawberry farmer. Organic methods, of course, also work in commercial production.

Verticillium Wilt

Verticillium Wilt on strawberry roots
Healthy strawberry plant (left) and strawberry plant infected with verticillium wilt (right), photo by Howard F. Schwartz, Colorado State University, Bugwood.org

Verticillium is a fungus that can lie dormant in the soil for years until it detects the presence of a healthy strawberry plant less than 2 millimeters (about 1/10 of an inch) away. When this occurs, the fungus will grow long hyphae that start to feed on the plant.

The strawberry plant will be severely stunted, but it is possible that neighboring plants will grow in around it if you aren’t planting your strawberries too far apart. If verticillium wilt is a continuing problem in your garden or on your farm, try to locate strawberries of the cultivars Albion or Camino Real which are more resistant to the disease. There are no fungicides that can control verticillium wilt once it is in the soil.

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