The fungus (Verticillium albo-atrum) that causes Verticillium wilt can be very damaging to strawberry plantings. Several common garden crops (tomatoes, potatoes, peppers, eggplants) and some woody ornamentals are also susceptible to this disease. The fungus can survive in the soil for many years. Wounds are not necessary for infection: it occurs by direct penetration of roots in infested soil. In new strawberry plantings, disease symptoms appear about the time runner production begins (mid-July). In established plantings, the symptoms appear as the fruit begins to ripen.

Symptoms
1. Older outer leaves wilt, turn brown around the edge and between the veins (Fig. 1).
2. Leaf Petioles may turn red.
3. The crown and roots may show a brown discoloration indicating decay (Fig.2).

Control:
The fungus can be introduced into "clean" soil by infected planting stock or by using implements contaminated with Verticillium-infested soil.
The disease is most easily controlled by planting resistant varieties of strawberries (see Table 1 for a list of varieties and their disease resistance). To reduce the chances of Verticillium wilt, do not plant strawberries following tomatoes, potatoes, peppers, or eggplants.
Preplant soil fumigation should be done in the autumn prior to spring planting, to help control Verticillium wilt. Proper soil preparation for several months prior to fumigation is essential. Consult the MSU Fruit Spraying Calendar (Extension Bulletin E-154) for soil fumigant recommendations.

Control:
Select a well drained planting site with loamy soil. Purchase resistant plants from reputable growers. Resistance to red stele disease is a relative term, however. There are at least five distinct biological races of the causal fungus and all varieties of strawberries are not resistant to all races of the fungus. Resistant varieties are an effective means of control. Also, consult the MSU Fruit Spraying Calendar (Extension Bulletin E-154) for chemical control recommendations.

The exact cause of this root rot is not known. Any condition that causes root injury or generally weakens plants will produce symptoms of black root rot. One or more of the following factors are associated with its appearance: soilborne fungi, nematodes, drought, winter injury, excessive fertilizer application, or excessive soil moisture. Plants may wilt, appear stunted, or die outright. The leaves may turn purplish with red petioles.
By the second growing season, the main, or perennial roots, of a healthy strawberry plant become woody. They have a dark brown or black outer covering and the inside is usually lighter colored. There should be masses of fine white feeder roots branching from the main roots. A plant affected by black root rot will have brown or black (inactive) feeder roots or none at all. The outer root covering will die and pull off easily. The white core of the main roots may turn black in patches or along the entire length of the root (Fig. 2 & Fig. 4).

Control:
If the disease is present in an established bed, abandon that bed and choose a new site. The alternative is to plow the plants under, work the soil for several months, fumigate the soil in the autumn, and plant healthy strawberry plants in the spring. Consult the MSU Fruit Spraying Calendar (Extension Bulletin E-154) for fumigant recommendations. Proper cultural practices can help minimize the occurrence of black root rot. Plant certified disease-free stock into a fertile, well-drained sandy loam. Incorporating organic matter at planting time stimulates beneficial biological activity in the soil. Adequate summer mulch prevents drought stress, and a layer of mulch over the plants for winter prevents winter injury to the crown.

Grey Mold

This disease is caused by the fungus, Botrytis cinerea, which overwinters on plant debris on the ground. Rainy or humid periods favor disease development. At the beginning of bloom, the fungus attacks the blossoms and causes a blossom blight leading to considerable crop loss. Fungus spores form on the blighted blossoms and infection spreads to both green developing fruit and ripening fruit. The disease first appears on fruit as small, water-soaked areas which are soon covered with gray, fuzzy-spore masses (Fig. 5).

Control:
Fungicide sprays beginning at first bloom and continuing at 7 to 10 day intervals through harvest are effective in controlling gray mold. See Extension Bulletin E-154 for fungicide recommendations.

Stem End Rot

Control:
Apply the first fungicide spray when leaves have expanded in the spring and flower buds are just emerging from the crown. Subsequent sprays primarily aimed at control of gray mold will control stem end rot for the duration of the season. Consult Extension Bulletin E-154, Fruit Spraying Calendar, for fungicide recommendations

Leather Rot

This disease is caused by the fungus Phytophthora cactorum. This disease can become very important in some years, given favorable environmental conditions. Ripening fruit clusters that are touching the ground in standing water after prolonged warm rains suddenly (in a day or two) turn gray-brown and become mushy. The fruit stems often become rotted. The disease progresses rapidly and a rotten smell pervades the field. After a few days, infected berries start to dry out and become leathery. If infected berries are cut longitudinally, the vascular system of the fruit is darkened and shows up as dark streaks radiating from the fruit core outward (Fig. 7).

Control:
Straw mulch on the ground to keep berries off the soil is a very useful cultural control. See Extension Bulletin E-154 for chemical control recommendations.

Leaf Diseases

Leaf Scorch

The fungus (Diplocarpon earliana) causing leaf scorch can live through the winter in old, infected strawberry leaves. Spores are easily spread by splashing rain or by mechanical means, such as by machinery or people passing through an infected planting. The first symptoms are irregular purple spots 1/4" in diameter scattered profusely over the upper leaf surface (Fig. 8). Spots may appear at any time during the growing season. As the spots age, black pimple-like fruiting bodies of the fungus are produced in the center of each spot. Spots may rapidly become so numerous that the entire leaf dries up and looks scorched" as though by fire. Spots may also occur on petioles, stolons and fruit stalks.

Control:
Plant resistant varieties. Remove and destroy leaves after fruitbearing. The gray mold/stem end rot fungicide program will usually control scorch. Consult Extension Bulletin E-154 for appropriate fungicides.

Leaf Blight

Control:
See control for stem end rot.

This disease is caused by the fungus Mycosphaerella fragariae. The fungus overwinters on old, infected, strawberry leaves. In the spring, the pathogen attacks young leaves, leaf stalks, calyces (caps) and stolons. Infection is favored by cool, wet weather. Leaf spots are circular and 1/8 to 1/4 inch in diameter (Fig. 10). The center of the lesion or spot is tan and has a purple border. This same fungus can also cause a fruit disorder called "black seed," but it is not usually important in Michigan.

Control:
Certain varieties are more resistant than others. The normal fungicide sprays applied to control fruit rots will usually control leaf spot. However, leaf spot disease can build up during the first year after planting if growers do not apply many fungicide sprays to non-bearing plants. Consult Extension Bulletin E-154 for fungicide recommendations.

Control:
Use resistant varieties.

There are several virus diseases that can infect strawberries. Visible symptoms may not be evident in the plants. Plant vigor is generally affected, how-ever, causing the plants to die when weather conditions become unfavorable. Visible symptoms can include leaf crinkling, variegation, cupping, and stunting. Plants showing these symptoms should be removed and destroyed. The predominant virus-like disease of strawberries in Michigan is "June Yellows" (Fig. 11). The main symptoms are stunting and yellowing of leaf margins.

Control:
Purchase only certified virus-free strawberry plants. Many strawberry viruses are transmitted by aphids. The only way to assure that the plants remain reasonably virus-free is to carry on an aphid spray control program throughout the growing season. However, if the strawberry bed is renewed every 2 to 3 years, virus build-up should not become a problem and no control measures are necessary.

Fig. 3. Discolored root core (stele) resulting from red stele root rot.

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References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned. This information becomes public property upon publication and may be printed verbatim with credit to MSU Extension. Reprinting cannot be used to endorse or advertise a commercial product or company.

MSU is an Affirmative-Action/Equal-Opportunity Institution. Extension programs and materials are available to all without regard to race, color, national origin, sex, disability, age or religion.
Issued in furtherance of Extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Gail L. Imig, extension director, Michigan State University, E. Lansing, MI 48824.
Produced by Outreach Communications and printed on recycled paper using vegetable-based inks.
Major revision, destroy previous editions-10:94-3M-TCM-SP. Price 75 cents, FILE 28.361