Protect strawberries from foliar diseases after renovation

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

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.

Symptoms

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.

strawberry powdery mildew
Powdery mildew on strawberry leaves.

strawberry leaf scorch
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.

strawberry phomopsis leaf blight
Phomopsis leaf blight on strawberry leaves.

strawberry angular leaf spot 
Angular leaf spot on strawberry leaves.

Management

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.

Effectiveness of fungicides for strawberry foliar disease control

Fungicide

Active ingredient

Common leaf spot

Phomopsis leaf blight

Leaf scorch

Angular leaf spot

Powdery mildew

Abound

azoxystrobin

Good

Moderate

Good

Not effective

Good

Armicarb

potassium bicarbonate

Good

Poor

Poor

Poor

Moderate

Cabrio

pyraclostrobin

Excellent

Good

Good

Not effective

Good

Captan

captan

Moderate

Moderate

Moderate

Not effective

Not effective

Copper**

copper

Moderate

Moderate

Moderate

Good

Poor

Flint

trifloxystrobin

Good

Good

Good

Not effective

Excellent

Fontelis

penthiopyrad

Not known

Not known

Not known

Not known

Excellent

Inspire Super

difenoconazole + cyprodinil

Excellent

Good

Not known

Not effective

Excellent

JMS Stylet Oil**

paraffinic oil

Not known

Not known

Not known

Not known

Good

Kaligreen

potassium bicarbonate

Moderate

Moderate

Not known

Not known

Moderate

Mettle

tetraconazole

Good

Good

Not known

Not effective

Good

Oxidate

hydrogen peroxide

Moderate

Poor

Poor

Not effective

Moderate

Phostrol**

phosphorous acid

Not known

Not known

Moderate

Not known

Not known

Pristine

pyraclostrobin + boscalid

Excellent

Good

Excellent

Not effective

Excellent

Procure

triflumizole

Not known

Not known

Not known

Not effective

Good

Quadris Top

azoxystrobin + difenoconazole

Excellent

Good

Good

Not effective

Excellent

Quilt Xcel

azoxystrobin + propiconazole

Excellent

Good

Good

Not effective

Excellent

Quintec

quinoxyfen

Not known

Not known

Not known

Not effective

Good

Rally

myclobutanil

Good

Good

Not known

Not effective

Good

Serenade

Bacillus subtilis

Good

Moderate

Not known

Not known

Moderate

Sulfur**

sulfur

Not effective

Not effective

Not known

Not effective

Good

Thiram

thiram

Poor

Moderate

Moderate

Not effective

Not effective

Tilt

propiconazole

Good

Not known

Not known

Not effective

Good

Topsin-M

thiophanate methyl

Good

Good

Good

Not effective

Moderate

Topsin-M + Captan

thiophanate methyl + captan

Good/ Excellent

Good/ Excellent

Good/ Excellent

Not effective

Good

Torino

cyflufenamid

Not known

Not known

Not known

Not known

Excellent

**Phytotoxicity may occur under slow, drying conditions (copper, Phostrol) or at high temperatures (sulfur, oil, Phostrol).

Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.

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