- Physiological Disorders of Tomatoes Affect Color of Fruit
- By David Francis
Ohio State University
Quality in tomatoes means different things to different people. Dr. Charlie Rick, the late University of California (UC)-Davis geneticist and leading expert on the tomato, said that if you wanted a great tasting tomato you should grow it yourself and pick it fresh.
In production agriculture, quality often extends beyond flavor to appearance and nutritional quality. For example, the most important limitation to cost efficiency in production and processing of tomatoes intended for whole-peel and diced products are physiological disorders that affect the color of the fruit. Because the pigments that make tomato fruit red are pro-vitamin A and lycopene, poor color translates into less nutritious fruit. Color disorders affect as much as 65% of the processing tomato crop.
Discolored regions under the skin of ripe tomato fruit are typical of yellow shoulder disorder, the most common color disorder. The severity of symptoms ranges from internal white tissue to distinct yellow or green sectors. The diversity of symptoms has lead to a number of names including yellow eye, green shoulder, yellow tag, internal white tissue, and yellow shoulder, all of which refer to one disorder. Yellow shoulder is a disorder that involves modified development, not delayed ripening of fruit.
Developmental abnormalities include a reduction in the size of cells within the fruit. These cells also have a more random arrangement of in the sectored tissue relative to normal tissue. The green chloroplasts in tissue affected by yellow shoulder fail to develop red pigment. These alterations occur very early in fruit development and are not reversed by delaying harvest. In fact, delaying harvest to let colored sectors catch up is ineffective and will reduce the quality of unaffected fruit.
Most color disorders are difficult to “solve” because they involve an interaction between many factors. Tomato variety, soil, weather and the interaction of weather with the plant and the soil all play a role in yellow shoulder disorder. Although the environmental conditions that lead to color disorders are difficult to control, some of the factors that contribute to the incidence and severity can be managed.
Color quality begins with the soil. Research in California and the Great Lakes region has demonstrated that available potassium, magnesium, calcium, are important factors in the soil. Soil pH, organic matter, and potassium fixation capacity are also important factors. Uniform color requires more available potassium than is necessary for yield alone. The most complete study to date by Tim Hartz and co-workers at the UC-Davis, demonstrated that the incidence of yellow shoulder varied among fields from 0%-68% of fruit affected. In their studies the incidence of yellow shoulder was lower in fields with high potassium status of both soil and plant. Soil application of either potassium or gypsum, to increase the ratio of available potassium to magnesium, reduced color disorders.
The results from California indicate that managing potassium and the ratio of potassium to magnesium may help reduce color disorders. Foliar applications of potassium were not effective. However the California studies also showed that when soil potassium fixation is high, adding potassium and gypsum will not provide an economic solution to the problem.
Ongoing research sponsored by the USDA’s Initiative for Future Agriculture and Food Systems is yielding results that will help manage risk related to color disorders. Tomato production in Indiana, Michigan, Ohio, Pennsylvania, and New Jersey occurs on soils with more diversity than is found in the central valley of California. Soils in the Great Lakes region vary in texture, mineralogy, cation exchange capacity, and the availability of nutrients. Despite this diversity, the variables identified by Hartz and colleagues in California have predictive value for the Great Lakes growing region. The profiles for soils at risk for yellow shoulder disorder can be found using the links available under “Managing Color Disorders” at www.oardc.ohio-state.edu/tomato/.
In addition to potassium, calcium and magnesium, work in the Great Lakes region over the last two years has suggested an important role for soil organic matter and pH. Tomatoes grown on soils containing greater than 3.4% organic matter produce fruit with a low incidence of yellow shoulder disorder while tomatoes grown on soils with organic matter below 2.4% produce fruit with a high incidence of yellow shoulder disorder. Tomatoes produced on soil at a pH of 6.4 have a low incidence of yellow shoulder while tomatoes grown on soil in excess of 6.7 have a high incidence.
Choice of variety is also important. Growers and consultants are encouraged to work with processors and other end users to choose varieties with more uniform color. As we learn more about the genetic basis for color uniformity, it is becoming clear that the deep red color associated with high lycopene “crimson” tomatoes does not always yield uniform color. Varieties with improved color and improved color uniformity are available.
As tomato growers and crop consultants develop strategies to manage the crop for color and nutrient quality, it may be useful to keep in mind the following established facts. Because soil conditions in the Great Lakes region are far more diverse than those found in California, there are not yet general recommendations in our region for amending high-risk fields. The management goal is to increase available potassium and/or decrease available magnesium. Available magnesium may be reduced by adding calcium or by reducing soil pH to below 6.5. Tomatoes will grow well in soils with a pH from 6.2 to 6.5. Adjusting the pH of soils to higher than 6.5 with lime will be counterproductive to management of yellow shoulder. Increasing the soil organic matter will increase the available potassium. Adding potassium, potash, or gypsum may be cost prohibitive and ineffective on soils with high magnesium, high potassium fixation capacity, or a pH greater than 6.5.
Tomato fruit quality management principles.
1) Management strategies must be aimed at prevention of color disorders. The alterations in fruit are triggered very early in fruit development.
2) Know your soils. Pay close attention to exchangeable K+, Mg+, and Ca++, K+ fixation capacity, pH, and organic matter.
3) Use varieties that are less susceptible to yellow shoulder disorder.
4) Uniform color requires more available K+ than is necessary for maximum yield.
5) Manage soil conditions to increase available K+ and/or decrease available Mg+. Maintaining a pH between 6.2 and 6.5 and high organic matter may be more effective than amendments.
