Deformed strawberries are a common frustration. This phenomenon, frequently described as “cat-facing” or “button berries,” is not a disease but rather incomplete development or external damage that occurred during the flower or early fruit stage. Diagnosing the exact cause requires a close look at the type of deformity and the growing conditions present during the critical spring period.
Incomplete Pollination and Temperature Stress
The strawberry is an aggregate accessory fruit; the fleshy part develops from the receptacle tissue of the flower, not the ovary. For the fruit to swell properly and symmetrically, the hundreds of tiny ovules—the eventual achenes on the berry’s surface—must be successfully fertilized. Each fertilized ovule releases a hormone that signals the surrounding tissue to expand, resulting in a large, symmetrical berry.
When pollination is incomplete, the unfertilized sections fail to expand, creating the characteristic lumpy, misshapen fruit known as cat-facing. While strawberries are self-fertile, wind alone is often not enough to transfer sufficient pollen to all the ovules within a single flower. Insects, particularly bees and flies, are necessary to ensure the 15 to 25 visits needed per flower for full fertilization and optimal fruit size.
Extreme temperatures during the flowering period are a common cause of poor pollination. Temperatures falling below 55°F or rising above 85°F can significantly inhibit pollen viability. Cold temperatures also reduce the activity of insect pollinators, limiting the transfer of pollen between the flower parts.
Frost damage can kill the central, most developed pistils (female parts) of the flower. Since the first flower to open (the “king berry”) is the most susceptible, this damage often results in a berry deformed at the tip or center. This injury leaves a concentration of small, hard, unfertilized achenes in the damaged area, and temperatures below 30°F are sufficient to cause it.
Specific Insect Feeding Damage
Certain insect pests cause deformation by feeding directly on the developing flower or very young fruit. The damage is highly localized, leading to a distinct type of misshapen berry different from simple pollination failure. The most common culprit is the tarnished plant bug (Lygus lineolaris).
The nymph stage of the tarnished plant bug uses piercing-sucking mouthparts to feed on the achenes of the newly formed fruit. When the bug sucks the juices from a seed, it destroys that seed, and the surrounding tissue loses its ability to produce hormones. The rest of the berry continues to expand, resulting in a localized, hard, seedy, and stunted patch, often concentrated at the berry’s tip.
The cyclamen mite (Steneotarsonemus pallidus) can cause severe stunting and deformation. These mites feed deep within the crown on the newest tissues, including the developing flowers and leaves. Fruit on severely infested plants will be dwarfed, bronzed, and shrunken, with seeds that stand out prominently on the surface.
Chemical Exposure and Mineral Imbalances
Chemical contamination or a lack of specific nutrients can also lead to fruit malformation. Herbicide exposure, even from minute amounts of drift originating from a neighboring lawn or field, can cause significant distortion. Plant growth regulator herbicides, such as 2,4-D, are particularly damaging.
Exposure to these chemicals often results in a distinct, elongated, flattened, or fan-like appearance in the fruit, sometimes referred to as fasciation. The chemical interferes with the plant’s growth hormones, causing twisting and abnormal cell division, which is often accompanied by misshapen or cupped leaves.
Mineral deficiencies can also present as deformities. Boron, a micronutrient, is essential for proper pollen tube growth and fruit set. A deficiency in boron can lead to small, hard, unevenly developed berries with a bumpy texture and sometimes a constricted, “belt-squeezed” appearance.
Calcium is crucial for cell wall structure and is poorly mobile within the plant. A lack of calcium during the rapid development of the fruit can cause the cells at the berry’s tip to fail, meaning the tip does not develop fully. This often leaves the fruit small and hard, with a dense grouping of seeds at the underdeveloped end.