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A research report from Dr. David Sugar, Oregon State University, Medford

Bartlett pear growers practice a variety of techniques to increase the size of the fruit on their trees, in order to take advantage of the greater value returned for large fruit. Hard pruning, detailed hand thinning, and precise irrigation and nutrient management are critical elements of a large-fruit strategy. Growers may also apply the hormones NAA (naphthaleneacetic acid) or NAD (napthaleneacetamide) as fruit thinners, although the results have been inconsistent and highly sensitive to temperatures following application.

At the Southern Oregon Research and Extension Center near Medford, we have been investigating fruit size enhancement programs by focusing on two distinct approaches:

• Applying urea at full bloom (when approximately 80% of the flowers are open)

• Applying MaxCel, which contains the hormone 6-benzyladenine, at approximately 10 to 12-mm (just under 0.5 inch) average fruit diameter, which typically corresponds to 10 to 15 days after petal fall.

Urea

Urea is a common orchard fertilizer, and contains 46% nitrogen by weight. In our studies, we used 5 and 7.5% urea solutions. Solution percentages are based on weight; since 100 gallons of water weighs 833.3 pounds, a 5% urea solution contains 42 pounds of urea per 100 gallons water, and a 7.5% urea solution contains 62.5 pounds urea per 100 gallons water. Depending on the size of the urea granules, varying amounts of time are necessary for fertilizer-grade urea to dissolve in water. Dissolving the urea through a screen or adding it to the spray tank slowly with continuous agitation is important to assure that the material is completely dissolved. Since spray application of urea solutions adds a significant amount of nitrogen to the orchard, this should be taken into consideration when designing the total fertilizer program for the orchard.

Over the five seasons in which we have conducted trials in a cooperating grower’s orchard, urea treatments have increased the average fruit weight, the percentage of fruit size 90 and larger, and the yield of fruit size 90 and larger as compared to untreated, unthinned controls (Table 1).

At the same time, the treatments tended to reduce fruit set and the total yield of fruit. The fruit size benefit was not observed in every year of the study. In fact, most years the slight increase in fruit size from the 5% urea treatment was not statistically significant, but the significance became apparent over the five-year period. The 7.5% concentration of urea, on the other hand, had a stronger effect and increased fruit size significantly most years (see Figure 1).

Researchers at the Washington Tree Fruit Research Commission have reported that they have not found Bartlett fruit size enhancement from 5% urea sprays at 80% bloom in most of their trials in various locations in Washington. The reasons for the differing results in different locations are not clear, but this points to the need to evaluate new techniques in individual situations to assess their value.

Urea may act in two ways to promote fruit size. The caustic (chemical burning) effect appears to damage some blossoms, resulting in reduced fruit set (thinning) and reduced total yield, as shown in Table 1. In addition, the nitrogen applied directly to the blossoms in the urea sprays may provide a nitrogen boost during the rapid fruit growth that occurs after blossom pollination and fertilization. Researchers in Argentina (Mariela Curetti and Enrique Sanchez) found that blossoms and fruitlets of Bartlett pear had increased nitrogen content in the days following 5% urea application, which dissipated within seven days of treatment.

Urea sprays at 5 or 7.5% concentrations will burn leaves. Fortunately, very little leaf surface is typically exposed at 80% bloom on Bartlett pear trees. The burn that occurs on emerging leaf tips does not appear to cause any subsequent problems. We have observed no negative effects on the fruit finish of Bartlett pears, but urea sprays may increase russet on d’Anjou and Comice pears, so contact with these varieties should be avoided.

MaxCel

MaxCel became available for use on pears in the United States in 2007. The label indicates use rates between 75 and 200 ppm of 6-benzyladenine and provides a dilution chart for calculating the concentration. The label also advises that applications will be most effective when the maximum temperature is above 65°F on the day of application and the following two to three days. Washington State University’s “Crop Protection Guide for Tree Fruits” indicates application timings of 5 to 15-mm fruit diameter. In southern Oregon, an average fruit diameter of 10 to 12 mm typically occurs about 10 to 15 days after petal fall. Other 6-benzyladenine products exist; MaxCel is the brand that we used in our studies.

We have evaluated MaxCel in Bartlett pears during the past two years, using a rate of 125 ppm. In these trials, the MaxCel treatment increased average fruit weight, the percentage of fruit size 90 and larger, and the yield of fruit size 90 and larger as compared to untreated, unthinned controls (see Table 2). The distribution of fruit sizes in response to treatments is shown in Figure 2.

MaxCel treatments may also promote fruit size in two ways. In our study, treatment with MaxCel at 125 ppm of 6-benzyladenine reduced fruit set and overall yield. Researchers in Argentina (Gustavo Gimenez and Claudia Dussi) reported reduced fruit set in Bartlett pear using the higher rate of 150 ppm 6-benzyladenine, applied from petal fall to 16 days after petal fall, with fruit diameters up to 19 mm. The hormone 6-benzyladenine has also been identified as a stimulant of fruit cell division. Both of these mechanisms (reducing fruit set and stimulating cell division) may enhance fruit size. However, excessive application rates could lead to overthinning of fruit. We did not observe any negative effects on the quality of Bartlett pears at the rate used in this study.


Conclusion

Both urea and MaxCel treatments, at the rates and timings used in these experiments, reduced the crop of Bartlett pears while increasing the proportion of large-sized fruit. Neither of these treatments can be expected to eliminate the need for hand thinning to meet current fruit size requirements for economic success in Bartlett pear growing, but could promote fruit size increase independently and make hand thinning more efficient. Treatment programs in which urea is followed by or mixed with MaxCel are being evaluated, along with programs for other pear varieties.

This research was funded in part by the Fresh and Processed Pear Committees.