It's midsummer and the tomatoes are ripening fast. However, many gardeners may be noticing yellow shoulders (yellowing or greening on the top of the tomato fruit) and other color disorders on their fruit. Unlike other tomato problems, yellow shoulders isn't caused by insects or diseases, but by environmental conditions and genetics. Even though yellow shoulders disorder doesn't harm the fruit, it does reduce the nutritional quality and affect the flavor.
Researchers at Ohio State University have found yellow shoulders can be prevented by changing the soil pH and organic matter content. Tomatoes grown on soils with 2.4 percent organic matter had more incidences of yellow shoulders than those grown on soils with 3.4 percent organic matter. When the pH was lowered from 6.7 to 6.4, the amount of yellow shoulders decreased. Also, tomatoes grown on soils with high potassium levels had less of a problem with yellow shoulders.
Although this research won't help you change the amount of yellow shoulders on tomatoes this year, it suggests some steps you can take next year. Grow varieties less susceptible to this condition; increase the organic matter by adding compost; and do a soil test to check the pH and potassium levels, adding any necessary amendments to maintain a pH between 6.2 and 6.5 and to insure high levels of potassium in your soil.
For more on tomato color disorder research go to Ohio State University (www.oardc.ohio-state.edu/tomato/ysd/clrsoil.pdf.