In Japan, strawberries are produced in the off-season (June to November) in cool regions; however, the high temperatures and strong sunlight limit fruit production. Dark septate endophytic fungi (DSEs) support growth and flower bud formation of plants grown in environments unsuitable for plant growth. In this study, we investigated the effects of DSE on dry matter production and flower bud formation in strawberry plants grown in the summer and autumn. The seeds were sown in soil mixed with DSE on 5 February 2024. The DSEs used were Cladophialophora chaetospira SK51 (S) and Cc. MNB12 (M), and Veronaeopsis simplex Y34 (Y). Plants were planted in a plastic house on April 18. The total dry weight was significantly increased by DSEs. This is because S and Y-cultured plants did not show a significant decrease in leaf emergence under high temperatures, unlike those grown with M; however, its leaf area was larger than that of the control. This resulted in a larger leaf area for receiving light and higher cumulative light reception and light-use efficiency. Although the DSEs increased cumulative fruit yield, the harvest period was limited to July because of the extreme summer heat. In addition, there was no difference in the budding date or flowering date between the treatments. These results suggest that DSEs improve light use efficiency, thereby increasing total dry matter weight and contributing to increased fruit yield in summer-autumn cultivation.