Strawberries With Leaf Scorch – Treating Strawberry Leaf Scorch Symptoms
By: Tonya Barnett, (Author of FRESHCUTKY)
It is easy to see why strawberries are one of the most popular fruit crops grown in today’s home gardens. These simple to grow berries are not only versatile in the kitchen, but insanely delicious when compared to their supermarket counterparts. Their small, compact size also lends their growth to those tending container gardens, as well as larger plantings. By recognizing the signs and symptoms of common strawberry issues, like scorched strawberry leaves, growers are able to harvest succulent berries for year to come.
What is Leaf Scorch on Strawberry?
Scorched strawberry leaves are caused by a fungal infection which affects the foliage of strawberry plantings. The fungus responsible is called Diplocarpon earliana. Strawberries with leaf scorch may first show signs of issue with the development of small purplish blemishes that occur on the topside of leaves.
Over time, the spots will continue to grow larger and darken. In severe cases, dark spots may even cover entire portions of strawberry plant leaves and cause them to completely dry and fall from the plant. Though the foliage of the infected plants is not aesthetically pleasing, it is seldom that the presence of this fungus impacts the quality of the strawberry crop itself.
Treating Strawberry Leaf Scorch
While leaf scorch on strawberry plants can be frustrating, there are some strategies which home gardeners may employ to help prevent its spread in the garden. The primary means of strawberry leaf scorch control should always be prevention.
Since this fungal pathogen overwinters on the fallen leaves of infected plants, proper garden sanitation is key. This includes the removal of infected garden debris from the strawberry patch, as well as the frequent establishment of new strawberry transplants. The creation of new plantings and strawberry patches is key to maintaining a consistent strawberry harvest, as older plants are more likely to show signs of severe infection.
When making new plantings always ensure that good planting practices are implemented. These practices include the use of proper plant spacing to provide adequate air circulation and the use of drip irrigation. The avoidance of waterlogged soil and frequent garden cleanup will help to reduce the likelihood of spread of this fungus.
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John Hammel, UW-Madison Plant Pathology
What is common leaf spot? Common leaf spot of strawberry (also known as Mycosphaerella leaf spot, Ramularia leaf spot, strawberry leaf spot, bird’s-eye spot, gray spotness, and white spot) is a common fungal leaf disease that affects both wild and cultivated strawberries throughout the world. Common leaf spot was once the most economically important strawberry disease, but the use of resistant strawberry varieties/cultivars and improvements in methods for growing strawberries have been effective in managing the disease and reducing its impact. Today, the disease is often a cosmetic problem and typically has little impact on yield or fruit quality.
What does common leaf spot look like? Symptoms of common leaf spot can occur on leaves, fruits, berry caps, petioles, and runners. The most noticeable symptoms of the disease are small, round, necrotic (i.e., dead) spots on strawberry leaves. Initially, these spots develop on the upper leaf surface and are deep purple to red in color. The spots eventually develop tan, gray or almost white centers with distinct reddish-purple to brown borders. During warm, humid weather, uniformly rusty-brown spots without purple margins or light colored centers may develop instead. Spots can occur on the undersides of the leaves as well, but these spots tend to be less vibrant in color. As the disease progresses, spots enlarge to ⅛ to ¼ inch in diameter and may merge together, in extreme cases leading to leaf death. Spots on berry caps, petioles, and runners resemble those produced on upper leaf surfaces. Shallow, black spots (¼ inch in diameter) may develop on infected fruits, and are often surrounded by brown or black, leathery tissue.
Where does common leaf spot come from? Common leaf spot is caused by the fungus Mycospharella fragariae, which can enter a garden on infected strawberry plants or via windblown spores from nearby strawberries. Once introduced into a garden, the fungus is spread predominantly by splashing water from rain or sprinklers used for watering. M. fragariae is most active when temperatures range from 65°F to 75°F, with periods of high rainfall and humidity. M. fragariae survives the winter on dead strawberry leaves and other plant parts, and is moved to new foliage in the spring by early season rains.
How do I save strawberry plants with common leaf spot? Once common leaf spot develops on strawberry plants, the plants cannot be cured. If the disease is detected early, its development may be slowed using fungicides. Keep in mind however, that common leaf spot is often merely a cosmetic issue and the use of fungicides may not be warranted. If you decide that fungicide treatments are needed, select a product that is labeled for use on strawberries and that contains captan, myclobutanil or copper as the active ingredient. Use copper-containing fungicides only prior to flowering. If you decide to use a myclobutanil-containing product, alternate applications of this product with applications of a second fungicide containing another active ingredient. This will help prevent selection of myclobutanil-resistant variants of the common leaf spot pathogen. Be sure to read and follow all instructions on the label(s) of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.
How can I prevent common leaf spot in the future? When establishing your strawberry patch, consider planting resistant strawberry varieties and use certified, disease-free nursery stock. Examples of resistant varieties include ‘Crimson King’, ‘Earliglow’, ‘Glooscap’, ‘Ogallala’, and ‘Ozark Beauty’. Plant strawberries in full sunlight, in well-drained soils, and with proper spacing to optimize air circulation and create a drier environment that is less favorable for the common leaf spot pathogen. See University of Wisconsin bulletin A1597 (“Growing Strawberries in Wisconsin”) available at http://learningstore.extension.wisc.edu for additional details on proper planting.
Once plants are in the ground, avoid overhead watering (i.e., DO NOT use a sprinkler) as this will splash the common leaf spot pathogen from plant to plant, and provide a wet environment that is more favorable for the fungus to infect. Instead, use a drip or soaker hose for watering. For similar reasons, DO NOT work in your strawberry patch (e.g., weeding, thinning plants or harvesting fruit) when it is wet wait until the patch is dry.
For June-bearing strawberries, bed renovation techniques (in particular mowing) can be useful in managing common leaf spot. See University of Wisconsin bulletin A1597 (mentioned above) for details on proper renovation techniques. At the end of the growing season, remove strawberry plant debris to minimize sites where the fungus can survive the winter. Deep bury, burn (where allowed by local ordinance) or hot compost this material.
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In annual production systems, the disease can come on transplants or tips and build up in the plug production phase in the early fall in the field. However, the pathogen does not cause damage the following spring. In the matted row system, the population can build up to damaging levels. Spores are produced in the spring and midsummer on lower leaf surfaces of dead leaves infected in the previous year, and are spread by wind and splashing rain. Disease increase is favored by leaf wetness during warm weather (68-86°F), and is likely to become more significant on older plantings of susceptible varieties. One ascospore generation (the starting sproes) and several overlapping generations of conidia (spores) are produced every year. Apothecia, a small mushroom-like structure, generally form on infected leaves in the fall and forcibly discharge ascospores, which are wind dispersed, in the springtime. Acervuli are found throughout the season in lesions or on residue of the foliage. Under dry conditions the acervuli can go into dormancy, but once moist conditions return, they again become active and exude conidia in a sticky mass. The conidia are disseminated to new infection sites where they directly penetrate and grow intercellularly.
Annemiek Schilder, Michigan State University Extension, Department of Plant Pathology - May 9, 2006
Editor’s note (2006): This article was originally published on May 5, 2004, in the Fruit CAT Alert. It is being republished at this time for your convenience.
There are more choices for disease control in strawberries than ever before. This can be a rather bewildering experience, as growers have to consider the disease control spectrum, efficacy ratings and cost per acre for each product. This article aims to help strawberry growers in the decision-making process by outlining unique aspects of several strawberry diseases, characteristics of the newer fungicides, and by suggesting several possible fungicide programs. A few notes on specific diseases:
1) Control of leaf diseases, such as common leaf spot, scorch, Phomopsis leaf blight and angular leaf spot may only be needed on susceptible cultivars. Some leaf diseases can spread to the berries (e.g., Phomopsis can also cause a fruit rot), or berry caps (angular leaf spot and scorch). If these have been a problem in the past, start fungicide sprays before bloom.
2) Leather rot (Phytopthora cactorum) is best controlled by growing strawberries in well-drained soil and by applying straw mulch between the rows to prevent the berries from touching the soil (where the fungus resides) and prevent soil from splashing up onto the berries. If there still is a problem, use Ridomil Gold or Aliette for control. Some phosphorous acid products such as Agri-Phos (similar to Aliette) may also work, but have not been evaluated on strawberries in Michigan. Spray during bloom and fruit development.
3) Angular leaf spot is a bacterial disease characterized by translucent leaf spots and blackening of the berry caps. It is favored by cool, wet weather and nights with temperatures close to freezing. The bacteria are spread by rain splash or by irrigation water. Copper (e.g., Kocide, Cuprofix, Bordeaux, etc.) is the only chemical that works against this disease. Some labels suggest adding lime as a safener to reduce the risk of crop injury. In susceptible varieties, start spray applications before bloom to prevent multiplication of the bacteria on the leaves before they jump to the berry caps.
4) Botrytis gray mold, the predominant fruit rot in most areas where strawberries are grown, primarily enters the berries through the blossoms, which means that chemical control should be focused on the bloom period. The Botrytis fungus can produce numerous spores on dead leaves and other plant matter and spreads easily by wind. Make sure to protect the king blooms especially, since these provide the largest berries. The other period for control is pre-harvest, since Botrytis can spread rapidly from infected berries to ripe and overripe berries. Pre-harvest sprays reduce post-harvest rots and increase shelf life of the berries.
5) Most other fruit rots, including anthracnose, tend infect the berries somewhat later in the season, i.e., during the green fruit or ripening stage. Anthracnose fruit rot is favored by warm, humid conditions and can spread rapidly during rains or frequent irrigation. In cool seasons, it tends to appear closer harvest or may not show up at all. Anthracnose fruit rot can be identified by black sunken lesions with wet, orange (and sometimes gray) spore masses in them. The anthracnose fungus is able to multiply on the leaves without visible symptoms, which may explain its sometimes widespread and sudden appearance in fields.
New fungicide characteristics (prices are estimates for comparative purposes only and may vary depending on the supplier and quantity purchased). Please follow label directions carefully before use.
Pristine (pyraclostrobin and boscalid) contains a strobilurin and an analid active ingredient. This fungicide is a very broad-spectrum material and has excellent activity against leaf spots, powdery mildew, and fruit rots, including Botrytis gray mold. It is surface-systemic (i.e., it is somewhat mobile within the wax layer on the plant surface) and has limited back action. The fungicide gets rainfast quickly. The label rate is 18-23.5 oz/acre (approximate cost: $32-$42/acre). The number of applications is restricted for fungicide resistance management. PHI=0 days.
Cabrio (pyraclostrobin) is a strobilurin-type fungicide with excellent broad-spectrum activity against leaf spots, powdery mildew and fruit rots . However, it does not provide much control of Botrytis gray mold. It is surface-systemic and has limited back action. The fungicide gets rainfast quickly. The label rate is 12-14 oz/acre (approximate cost: $16-$18/acre). The number of applications is restricted for fungicide resistance management. PHI=0 days.
Abound (azoxystrobin) is also a strobilurin-type fungicide with good to excellent broad-spectrum activity against leaf spots, powdery mildew and fruit rots. It does not have much activity against Botrytis gray mold. It is surface-systemic and has limited back action. The fungicide gets rainfast quickly. The label rate is 6.2-15.4 fl oz/acre (approximate cost: $12-$30/acre $24 at the 12-oz rate). The number of applications is restricted for fungicide resistance management. PHI=0 days.
Elevate (fenhexamid) is a fungicide with a new chemistry that has excellent activity against Botrytis gray mold. While fenhexamid has some systemic activity, it should be used as a preventative fungicide. The fungicide gets rainfast quickly. Can be used to alternate with fungicides in other chemical classes. The label rate is 1.5 lb/acre (approximate cost: $41/acre). PHI=0 days.
Captevate (fenhexamid and captan) is a pre-mix of Elevate and Captan. It has excellent activity against Botrytis as well as moderate to good activity against anthracnose and other leaf spot and fruit rot diseases. This formulation appears to perform a bit better than a tank mix of Elevate and Captan. The label rate is 3.5-5.25 lb/acre (approximate cost: $42-$63/acre) . At the high rate, the dose is equivalent to 1.5 lb Elevate and 5 lb Captan. The PHI=0 days.
Switch (cyprodinil and fludioxonil) is a mixture of a systemic and protectant active ingredient (both are new chemistries). Switch has excellent activity against Botrytis gray mold and moderate to good activity against anthracnose and scorch. The label rate is 11-14 oz/acre (approximate cost: $39-$50/acre). PHI=0 days.
Older fungicides such as Topsin M, Captan, Thiram, Sulfur, etc. remain effective disease control tools. The approximate prices per acre: Topsin M $16/acre Captan $13/acre Thiram $8/acre Sulfur $1-2/acre and copper formulations: $3 to $4/acre.
This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).
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Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - July 15, 2015
Keep leaves of perennial strawberry beds healthy by removing old leaves at renovation and applying fungicides as needed to control foliar diseases into fall.
Common leaf spot on strawberry leaves. All photos by Annemiek Schilder, MSU
The 2015 growing season has been rainy with generally moderate temperatures, conditions that are particularly conducive to fungal and bacterial diseases. In many strawberry fields, leaf spots are common though may not necessarily be severe. A few leaf spots won’t do much harm, but lots of them will reduce photosynthesis of strawberry leaves and therefore the plant’s ability to feed itself and may cause strawberry leaves to die prematurely. A loss of leaf area can weaken strawberry plants, reduce growth and make them more prone to winter injury. Furthermore, the various leaf spot pathogens can overwinter in the field and increase disease pressure next year. Most of these pathogens also infect runners, petioles, berries and berry caps.
In general, cultivar susceptibility and weather conditions determine which diseases occur in a planting in any particular year, since most foliar pathogens are already present on the farm or are introduced with the planting material. Even if they start out at very low levels, they can build up over time. Some cultivars that tend to be more susceptible to foliar diseases are Redchief, Raritan, Annapolis, Eros, Idea, Kent, Glooscap, Midway, Mira, San Andreas and Selkirk. All leaf spots are favored by extended leaf wetness and moderate temperatures of 65-75 degrees Fahrenheit, with angular leaf spot being especially promoted by nighttime frosts. In addition to leaf spot diseases, powdery mildew can occur on susceptible cultivars, but this disease tends to flourish under warm, dry conditions.
There are three main fungal pathogens and one bacterial pathogen that cause leaf spots in Michigan strawberries. They can be distinguished by the appearance of the symptoms. Common leaf spot, caused by the fungus Mycosphaerella fragariae, is characterized by round, light gray lesions with purple rims. Leaf scorch, caused by the fungus Diplocarpon earliana, is characterized by solid purple to brown spots on the upper leaf surface. Leaf areas between spots may turn yellow to bright red. Severely infected leaves turn brown, curl up and appear scorched.
Powdery mildew on strawberry leaves.
Leaf scorch on strawberry leaves.
Phomopsis leaf blight, caused by the fungus Phomopsis obscurans, has purplish-brown, lens-shaped lesions with a light brown center. Lesions are often found along leaf veins or as V-shaped necrotic areas along the leaf’s edge. Older leaves become blighted and die prematurely. Angular leaf spot, caused by the bacterium Xanthomonas fragariae, has spots that appear angular and reddish-brown on the upper leaf surface and water-soaked on the lower leaf surface. Spots look like tiny translucent windows when the leaf is held against the light.
Phomopsis leaf blight on strawberry leaves.
Angular leaf spot on strawberry leaves.
Nonchemical control options include selecting a well-aerated site, planting resistant or tolerant cultivars, using disease-free transplants, reducing humidity and leaf wetness by avoiding dense canopies and tight row-spacing, not over-fertilizing plants, applying overhead irrigation in the early morning to speed up drying of leaves and removing old leaf material and fruit mummies from the field at renovation. With respect to cultivar choices, it is hard to find a cultivar that is resistant to all foliar diseases, especially angular leaf spot. Or, they may be resistant to leaf spot and scorch, but not to Phomopsis leaf blight. However, the following strawberry cultivars are fairly resistant to fungal leaf spots: Allstar, Cavendish, Brunswick, Evangeline, Clancy, Guardian, Jewel, Lester, Mesabi, Ovation, Primetime, Seneca, St. Williams and Surecrop. For a list of cultivars and their disease susceptibilities, see page 213 of the “2015 Michigan Fruit Management Guide,” Michigan State University Extension Bulletin E0154.
As far as chemical control is concerned, while there are some differences in the timing and biology of the leaf spot fungi, they mostly respond similarly to broad-spectrum fungicides and can be managed collectively. The exception is angular leaf spot. Since this disease is caused by a bacterial pathogen, it does not respond to most fungicides except copper. Below is a table of efficacy ratings of fungicides for leaf spot control in strawberries. Since infections often occur on the leaf underside, good coverage of both leaf surfaces is needed, especially of young leaves, which are most susceptible.