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Bloom is a time for strawberry disease control

This article was originally scheduled for release on Monday, June 1, 2020, but was postponed in light of events in the Twin Cities and throughout the country protesting the killing of George Floyd. 

Author: Annie Klodd, Extension Educator-Fruit and Vegetable Production

Strawberry bloom and fruit set in Annapolis berries, on June 1, 2020 in White Bear Lake, MN.

Bloom is an exciting time in the strawberry field, but it is also an important time for managing strawberry diseases.

While we can't see the disease symptoms at this time, disease spores are actively spreading and infecting the plants and blossoms. The infections that occur now can cause the symptoms that we see later.

Therefore, it is important to control diseases at this point in the season to prevent damage to fruit and leaves. Keeping diseases at bay during this time will lead to lower fruit and leaf infestations later in the season and allow better fruit quality, healthier plants, and higher marketable yields.

During bloom, effective organic or synthetic fungicides should be applied from 5-10% bloom until flowers have finished blooming, according to the application intervals on the product labels.

Diseases active during strawberry bloom in Minnesota:
  • Anthracnose
  • Botrytis gray mold
  • Powdery mildew
  • Septoria leaf spot
  • Leaf blight
  • Leather rot

How our 2020 weather impacts fruit disease management

Most fruit diseases thrive in warm, humid, rainy conditions. The fungal species responsible for each disease have a range of temperatures and rainfall requirements that promote their infection and spread. While the specific conditions depend on the specific disease, these conditions often include warm, but not hot, temperatures, rain, and prolonged leaf wetness. 

Powdery mildew is somewhat of an exception to this; while infection is stimulated by wet weather, repeated subsequent rainfall tends to reduce further spread of the infection on the leaves. 

Bloom began in much of Minnesota between May 23-27. Bloom was at 5-10% in White Bear Lake on Tuesday, May 26 and full bloom on Monday, June 1 with some fruit set. During this time, a weekend of heavy rainfall was followed by warm daytime temperatures and mild nighttime temperatures. 

These warm temperatures and rain at the onset of bloom stimulate blossom infection of botrytis gray mold and anthracnose. Leaf infection by powdery mildew, leaf spot and other leaf diseases was already occurring prior to bloom but continues during bloom as temperatures become increasingly suitable to infection.

Anthracnose

Fungicide applications during bloom should include active products that are effective for control of anthracnose, particularly if this disease has been an issue in the field previously. 

Anthracnose can infect the blossoms, petioles, and stems in the early season. Blossom infection leads to small, hard, misshapen fruit, and can cause increased fruit rot.

Anthracnose has become more prevalent in the northern US in recent years. Yet, I have noticed that some growers in Minnesota only start applying fungicides for anthracnose during the harvest season, in order to retroactively reduce infections on mature fruit. 

Delaying applications is not a best practice for anthracnose control, especially if weather is warm and humid during bloom. Ideal conditions for anthracnose development are 77-86 degrees F, in conjunction with a rain event. Delaying these applications until the harvest season allows anthracnose to infect the blossoms, causing small, hard, misshapen fruit. Delaying control also increases the risk of infection on mature fruit.

Organic fungicide options for anthracnose include the Serenade products, with Serenade OPTI reported to have the strongest efficacy. However, applications are most effective at the beginning of bloom or shortly before bloom. Other options include hydrogen peroxide, Actinovate, and Double Nickel. Regalia, serifel, neem oil, and potassium bicarbonate may be used, but their efficacy is not well documented. Refer to the Organic Production and IPM Guide for Strawberries and the manufacturer labels for more information.

Synthetic fungicide options: There are a number of effective fungicides for anthracnose control, listed in the Midwest Fruit Pest Management Guide. Growers must be careful to rotate fungicide modes of action to prevent pathogens from developing resistance to the most effective chemistries. Product options include Abound, Topsin, Captan, CaptEvate, Flint Extra, Luna Sensation and Pristine.

When selecting fungicides, in addition to considering resistance, growers should also consider which other diseases the products are effective on and plan accordingly. Additionally, observe pre-harvest intervals. According to the Midwest Fruit Pest Management Guide, "under heavy anthracnose disease pressure, all fungicides should be combined with Captan" for optimal control. However, applying Captan close to harvest should be avoided, as it can cause visible residues. 

Botrytis gray mold

Botrytis gray mold infections are most active between 65-75 degrees F with rainfall. If there has been at least one rain event since your last bloom-time fungicide application, the next application should continue to include a product with strong efficacy on botrytis. Applications are recommended at pre-bloom, early bloom, and after fruit set. 

The timing of the application should influence the product being applied. For example, Regalia is an organic biological fungicide that acts by inducing the plant's natural defenses against pathogens. Therefore, it is most effective if applied before the onset of infection; before bloom. Bloom and post-bloom applications should include different products that protect the plants from active infection.

Organic fungicide options for bloom include Optiva and Serenade (Bacillus subtilis), potassium bicarbonate, and hydrogen peroxide. Again, refer to the Organic Production and IPM Guide for Strawberries and the manufacturer labels for more information.

Synthetic fungicide options: There are many effective synthetic fungicides effective on botrytis. However, growers must rotate fungicides with different modes of action to prevent resistance from developing. Learn the mode of action for each product you apply, by looking at the FRAC code. Do not rely solely on, or make repeated applications of the same fungicide mode of action. 

Botrytis populations with resistance to Topsin and Scala have been identified and should be used sparingly. Pristine and Rovral are at relatively high risk of resistance. Other products include Inspire Super, Fontelis, Captevate (Captan + fenhexamid), and Switch. 

Refer to the Midwest Fruit Pest Management Guide for further guidance on rotating these products and on preventing resistance. Also refer to the manufacturer labels.

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