By Jinsheng Huang, Sieglinde Snapp, Darryl Warncke, Mary Hausbeck, Brian Cortright, Ron Goldy,
Michigan State University

Tomato shoulder check is a physiological defect involving multiple micro-cracks and senescent cells on the fruit epidermis (skin). This can be a serious problem associated with variable weather: hot, dry alternating with moist conditions.

Losses for specific harvests can be high, reducing marketable fruit by as much as 80 percent. Findings are preliminary at this stage, but cultural practices may help reduce tomato shoulder check. Tomato quality research conducted in 2001 appeared to indicate high tunnel covers and spraying fruit with boron may provide some help in controlling this problem. These results are only preliminary. The high tunnels reduced the yield (fresh fruit weight) of shoulder check tomatoes by 53 percent and decreased the number of fruit with the defect from 55 to 28 percent in a sample of 100 fruit . Spraying fruit with a combination of boron and calcium reduced yield of fruit with the defect by 22 percent, where 41 fruit had the defect compared to 55 in the control unsprayed fruit. By contast, Surround WP (kaolin clay) spray made the problem worse: it increased yield (fresh fruit weight) of fruit with shoulder check by 24 percent. Similar resutls were found at every harvest.

A greenhouse study and applications on a grower’s field revealed similar results. Weekly boron sprays reduced the number of fruits with check by as high as 80 percent in the greenhouse, while calcium spray alone did not significantly reduce shoulder check. This suggests boron is the helpful nutrient. Results are preliminary and it is important not to add excess boron or toxicity problems may develop. Do not exceed 0.5 lb/acre of elemental boron applied foliar, nor exceed 3 lb/acre boron applied to soil.

Over the past two seasons we have found tomato shoulder check was most likely to occur with rapid movement of water and solutes into dry tomato fruit. Around 45 percent of the fruit were found to have check defect by August 20 in Benton Harbor, Michigan, just three days after one inch of rain. Three more days of an additional 2-1/2 inches of rainfall was associated with yield losses of up to 70 percent from shoulder check. A similar pattern occurred the summer of 2000.

High tunnel covers formed a small greenhouse, preventing rain from contacting plants. The measured leaf wetness under the plastic tunnels was lower than non-covered plants. These temporary tunnels improved fruit quality substantially, with as high as 80 percent yield without any shoulder check defect.

Spraying fruit provides a less expensive solution than high tunnels to improving fruit quality. Boron plays an important role in plant cell wall formation and the transport of potassium to guard cells for the proper control of internal water balance. Boron also assists in binding calcium to the cell walls. Boron deficiency is common with high soil pH and highly alkaline water. Boron also leaches easily, so coarse, sandy soils and high rainfall may cause temporary soil shortages. Further, boron may be taken up early in the season but then become immobilized in the plant and not available for rapidly growing plant parts, such as fruit. Our results are very preliminary and will be tested again next year.

The recommended foliar spray rate of boron element is 0.2 to 0.3 lb/acre when this nutrient is deficient. The most popular soluble sources of boron are Borax (Na2B4O2 . 10H2 O, 11 percent B) and Solubor (Na2B10O16 . 0H2O, 20 percent B). These boron fertilizers are soluble and immediately available to plants. These products include natural, mined and purified boron and are priced at around $60 per 50 pounds. Broadcast and incorporation of boron is recommended for deficient soils at a rate of 1/4 lb/acre up to 3 lb/acre of elemental boron. Boron should be used only in very small amounts, 2 lb/acre applied to soil is the maximum for sandy sites because it can become toxic to plants if over-applied. Never apply more than 1/2 lb of actual boron as a foliar spray.

Research conducted by Dr. Mary Hausbeck and Brian Cortright over the last two seasons indicate that the fruit defect occurred regardless of variety grown. Use of recommended fungicides also did not significantly influence shoulder check incidence. Thus, no change is recommended regarding which varieties should be grown and which disease protection strategies should be followed.

In our trials, spraying water weekly on dry fruit - water alone with any nutrients added - reduced the number of fruit with shoulder check by about 10 percent. We are continuing our investigations of how water and boron reduce fruit defects. The water treatment may help by keeping moisture at the fruit surface constant. Other means to improve consistency in plant water status may help as well, such as careful scheduling of irrigation.