By Brad Bergefurd, Extension Educator Horticulture/Agriculture and Natural Resources
Not only was the 2015 growing season one of the wettest on record, but the season had some of the lowest average temperatures for an extended period of time in recent years. The season-long extreme environmental conditions caused many problems for all vegetable growers, but fresh market tomato growers were particularly affected due to high percentages of fruit physiological disorders with some farms experiencing up to 50 percent of fruit affected. Thanks to greatly appreciated grant funding from the Ohio Vegetable and Small Fruit Research and Development Program (OVSFRDP), researchers have shed some light on reducing the economic impact of these tomato fruit disorders through cultural management and fertility research.
Yellow shoulder disorder and other tomato fruit disorders are a wide-spread problem annually, especially with extended hot and dry growing conditions at blossoming and fruit development. Yellow shoulder disorder seems to impact all-sized fruit and is characterized by areas at the top of the fruit and shoulders of fruit that stay green or yellow and as the fruit ripens, tend to turn a more intense yellow color. These areas never will ripen properly, even if left to hang on the plant for an extended time. The area beneath the yellow shoulder is firm and poor tasting which makes the fruit unmarketable and not desired by consumers. Unfortunately, the cause of this problem is complex and researchers have been investigating cures for almost thirty years with limited success. The complexity of the disorder is increased since environmental conditions as well as tomato plant physiology lead to the disorder and there is no real solution. Factors that can increase the severity of these disorders include cloudy weather, wet and cool conditions, high nitrogen, low potassium, and compacted soils. Some of the cultural and crop management practices that fresh market tomato growers can do to ease the symptoms and possibly reduce crop losses will be covered here.
One of the main causes of this disorder, that we have limited control over, is intense heat. High temperatures prevent lycopene production, the red pigment in the tomato fruit, most often in the shoulders of tomato, as this part is more commonly exposed to the direct rays of the sun. Researchers have measured fruit temperatures of between 86 degrees and 105 degrees Fahrenheit morning through evening hours in July 2012, one of the hottest months on record for southern Ohio. When temperatures are greater than 85 degrees, lycopene production begins to cease, whereas at temperatures below 85 degrees, lycopene consistently produces.
Inside the plant we see a reduction in potassium (K) just before yellow shoulders are seen. This year in our tissue testing we saw drops in K of 3-4% in a matter of weeks going from 4-6 percent, which is in the good range, to 2-3 percent which is in the poor range. Usually within a week or two of this drop, yellow shoulder will be expressed. Therefore, early detection and management are critical for control. Drops in calcium (Ca), nitrogen, and at times magnesium (Mg) have also been observed as we move into mid-July and early August, the hottest months of the year. We also have observed this disorder in high tunnel tomatoes; however, it is usually a month or so earlier, when temperatures in the tunnels begin to climb around Memorial Day. High tunnel tomato growers will apply a 10 to 15 percent shade cloth to tunnels around this time in an attempt to reduce the heat stress in the tunnels. This disorder is expressed in plants that are under some stressful growing conditions when the plant is under a heavy fruit load. These stresses can be too little water, too much heat or high amounts of plant disease or insect infections.
For now, recommendations from our research conducted at the OSU South Centers over three years is to closely monitor plant nutrient levels, especially nitrogen and potassium levels, on a regular basis throughout plant development beginning around the time of first flower cluster formation. Timeliness is so important to take the necessary corrective actions to avoid or reduce this disorder. A major limitation is getting the plant nutrient analysis results back in a timely manner. A considerable amount of time is required to collect leaf petiole samples, dry samples, send them to a commercial lab for analysis and then receive the results, which could take several days, and more typically a week.
Plant sap extraction and analysis can be completed in the field using a quick-test method to speed up the collection of tissue testing results and to help make more timely fertilizer program adjustments. Instruments are commercially available that can be used to directly measure nutrient concentration and that do not require a laboratory setting for accurate calibration and use. These pocket-sized meters currently cost about $500 and are simple to use. We have taught and demonstrated the use of these instruments at workshops and field days over the years. A sample size normally collected for petiole testing with conventional methods will yield more than sufficient sap to obtain a reading with this type of meter. Using the plant tissue test results as a guide, necessary and timely adjustments should be made to nitrogen, potassium and calcium fertilizing programs to keep fertility levels within sufficiency ranges. Plants respond well to fertigation of fertilizer directly through drip irrigation with higher amounts better applied through this method. Foliar fertilizing may help, but it is difficult to raise the potassium levels 2-4 percentage points as would be needed in most cases through foliar applications alone and plant injury could result. More information on plant petiole sap testing for vegetable crops along with plant nutrient charts which show nutrient sufficiency ranges can be found on the University of Florida fact sheet http://edis.ifas.ufl.edu/cv004. This fact sheet explains the testing process in more detail.
Yellow shoulder disorder is also a varietal problem, as some varieties have been observed which express the symptoms more than others. Bergefurd and other members of the South Centers horticulture research team partnering with Dr. Matt Kleinhenz of the OSU Horticulture & Crop Science Department have conducted important tomato variety evaluations, especially on new variety releases and the use of grafted plants, to observe resistance or tolerance to yellow shoulder and other tomato fruit disorders. Many tomato evaluations, including research performed at the OSU South Centers, are available for comparison annually in the Midwest Vegetable Variety Trial report published by Purdue University. This publication is a compilation of vegetable variety research performed throughout the Midwest United States and makes for easy comparison between trials. The Midwest Vegetable Variety Trial report is available at the Purdue University vegetable crops web site https://ag.purdue.edu/hla/fruitveg/Pages/MVVTRB.aspx
From our extensive tomato physiological fruit disorder research conducted at the South Centers over three years, we recommend that for most growers the best practices to prevent yellow shoulder will be to intensively tissue test tomatoes from first flower cluster for Calcium, Potassium and Magnesium levels. From this information, growers could apply Potassium Nitrate, Sulfate of Potash, Potassium Carbonate, Calcium Nitrate, Calcium Chelate, and Magnesium Oxide to reduce the potential for this disorder. Our experience has proven that nutrients applied through fertigation are necessary to prevent yellow shoulder disorder of tomato.
Full research reports of this and past years’ fruit and vegetable experiments are available on the OSU South Centers website at http://southcenters.osu.edu/horticulture/. For more information on the tomato research or other horticulture field trails, contact Brad Bergefurd, Extension Educator Horticulture/Agriculture & Natural Resources at firstname.lastname@example.org or at 740-289-2071 ext. 136.