Reasons for Small Fruit Size in Strawberries

Small fruit size in strawberries is a common occurrence in Minnesota berry farms. Troubleshooting the cause of the problem and addressing it will help ensure higher marketable yields and happy U-Pick customers.

There are several potential causes of small or deformed strawberry fruit. These include:

• Poor pollination
• Tarnished plant bug
• Weed pressure and over-crowding
• Old strawberry plantings
• Anthracnose infection on blossoms
• Poor nutrient management

Here, we will discuss a few of these issues. I do not describe nutrient management or disease management in this article, but the links directly above provide helpful guidance on those topics.

Poor Pollination

A pollinator box in an orchard. Photo: Annie Klodd

Better pollination leads to larger fruit. Strawberries are called aggregate fruits because they have multiple ovules per fruit that can be pollinated during bloom. These are observed as the many seeds (achenes) on the outside of the fruit. The more ovules that are pollinated per fruit, the larger the fruit will be, because the fruit tissue develops around the achene of each ovule that is pollinated. Therefore, if pollination is poor, fruit are likely to be smaller. 

While strawberries do self-pollinate, self-pollination alone is not likely to pollinate all ovules on the blossom. Pollinators such as honeybees and bumblebees increase pollination and fruit size.

Cool, rainy weather or broad-spectrum insecticide application during bloom can decrease pollination by reducing pollinator activity.

Tarnished plant bug or frost damage

Tarnished plant bug on a strawberry. Photo: Jeff Hahn, UMN Extension

Damage from tarnish plant bug (TPB) nymph feeding causes the fruit to be deformed and often smaller in size as a result. The damage is a hard, seedy area at the tip of the berry. Next to spotted wing drosophila, Minnesota growers I speak to often cite tarnished plant bug as the other top strawberry pest in our region. 

Start scouting for TPB prior to bloom, when the flower buds are green or white, and continue scouting during bloom and fruit set. Manage weeds, which can harbor TPB, and do not plant clover or alfalfa nearby. 

Since TPB attack strawberry flowers, growers should scout and apply insecticides at appropriate intervals throughout bloom. Select organic or synthetic insecticides according to recommendations in the Midwest Fruit Pest Management Guide and apply according to the label instructions. Numerous Extension articles are available online for growers to optimize their TPB management programs.

Tarnished plant bug injury on strawberries. Photo: David Handley, University of Maine Cooperative Extension.

Frost damage during bloom can also cause the fruit to be deformed and undersized. Frost damage is easily mistaken with TPB damage, but has a less predictable injury pattern. I provided more details about this in this week's Fruit Update article.

An unripe strawberry with damage that is likely due to frost damage, but could also be due to tarnished plant bug feeding. Photo: Annie Klodd

Weed Management

Weed competition in strawberry fields steals away resources like water and nutrients that strawberry plants need to produce large, healthy fruit. Weed management in strawberries is so important that it is standard practice for growers to send crews out to hand-weed weekly. Even small U-Pick farms in Minnesota, with small labor forces, will send their families or employees out to weed fields by hand during the season.

In additional to cultural weed management practices such as straw between rows, growers should use effective herbicides or hand-removal in the rows to minimize weed competition. Herbicide drift and product usage can be reduced by spot-spraying individual weeds such as dandelions and thistles rather than covering the whole field.
 

A weed that has been spot-treated in a strawberry field. Photo: Annie Klodd

Dual Magnum recently received an indemnified label for use in Minnesota strawberries, among other specialty crops. Growers wishing to use Dual Magnum must download and carry the indemnified label. Click here to watch a webinar about using Dual Magnum by the Minnesota Fruit and Vegetable Growers Association, and get your indemnified labels here. 

Old Stands

Reduced fruit size is a normal symptom of an aging stand. Replacing the stand regularly, rather than trying to extend its lifespan beyond its productive years, is a best practice to maintain high yields of high quality berries. 

Commercial strawberry beds should live for 3-5 years before replacement. The peak of productivity is in years 2 and 3 after planting, and plants typically have 2-3 productive years before they lose vigor and begin senescence. As stands age, they can also become overcrowded, nutrient-depleted, and accumulate pathogens if not properly managed and renovated each year.