The journey of a tomato from an unopened flower to a mature, harvestable fruit is a tenuous one. Many things can happen along the way that can render the fruit of lesser value or, in some cases, totally worthless. A number of tomato ripening disorders have been witnessed this year and in recent past years. They include blotchy ripening, gray wall, white core and yellow shoulder. Collectively the literature refers to these as tissue ripening disorders.
First, it is important to remember that these are disorders—not diseases. Disorders normally are caused by environmental conditions whereas diseases are cause by plant pathogens. Disorders cannot be spread from an affect plant to a healthy one; diseases can. Symptoms of blotchy ripening, gray wall, white core and yellow shoulder often occur when the plant is under stress or when the environment changes rapidly. Periods of hot weather following unusually cool, raining example are a good example of the latter.
There are several keys to controlling tissue ripening disorders; the first being variety selection. For example, tomatoes with the "green shoulder" trait are more prone to develop yellow shoulder than those that have the uniform-ripening gene. Additionally, some varieties are more likely to develop white core than are others. Review University field trail data for ripening disorder ratings before selecting a tomato variety; new varieties should be grown on a limited scale the first year.
Second, proper management of the plants’ leaves (crop canopy) is very important. Here we have a bit of a dilemma since yellow shoulder is more prone to develop in open canopies. The heat stress associated with fruit exposed to the sun has been indicated as a cause for yellow shoulder. In contrast, blotchy ripening often is associated with dense crop canopies. Fruit hidden from the sun in dense foliage are much more likely to develop blotchy ripening then those less shaded. A good crop canopy allows for air circulation which is important for preventing foliar diseases. It will be heavy enough to allow for shading of the fruit but without excessive vegetation.
Third, and (arguably) most important is potassium nutrition. Nearly all of the literature involving the aforementioned ripening disorders links them to potassium in some way. Tomatoes are very heavy potassium feeders and a shortage of the element during fruit development and ripening often leads to fruit ripening problems. Depending on soil test results, tomatoes should be supplied with about 200 pounds of potassium (expressed as K20) per acre to grow a productive crop. One-half of this should be applied as “plow down”, before the plastic is laid.
Additional potassium during the ripening period should be applied through the drip irrigation system. Beginning when the oldest tomato fruit are the size of a dime, begin applying potassium nitrate at the rate of between 10-12 lbs./acre/week. Double this rate when the oldest fruit are about 2 ½ inches in diameter until time of peak harvest. Finally, the rate can be stepped back to 10-12 lbs./acre/week just after peak harvest until the end of the crop.
The above recommendations are for potassium expressed as K20. Make certain you know the percent potassium in the fertilizer used and adjust injection rates accordingly.
In summary, fruit ripening disorders must be prevented; they cannot be cured. Strategic variety selection, good management of the crop's canopy and proper potassium fertilization can go a long way to preventing these disorders.
REVISED: November 23, 2015