The Effects of Dormex on Replacing Lack of Chilling in Kiwifruit

Arlie A. Powell

Horticulturist-Fruits, Auburn University

January, 1997


Kiwifruit were introduced into the southeastern U.S. in the mid 1980's for commercial production. Prior interest in growing this fruit crop was limited to occasional home plantings. As a result, little cultural knowledge was available upon which to develop a production management system. A large commercial nursery from New Zealand, which operated for a period of several years in South Carolina, was primarily responsible for initiating interest in establishing a commercial industry in this region.

Experimental as well as commercial plantings were first established in early 1987 at several locations in southern and central Alabama. Commercial plantings were mainly established in the milder southern counties of the state situated closer to the Gulf of Mexico. Winter chilling (<7.2C) in these areas usually average 700 to 1000 hours, but has been as low as 500 to 800 hours about 50% of the time over the past 10 years.

As new plantings began fruiting (3rd to 4th leaf), it became apparent that although vegetative growth was quite good throughout the state, flowering and fruit production were extremely limited with the 'Hayward' variety in the milder southern counties. Because fruiting has been quite satisfactory in central counties where chilling usually averages 1000 to 1300 hours, it appeared that lack of chilling was largely responsible for the poor flowering and fruiting in southern counties.

Much of the general literature from other countries, including New Zealand, indicated about 400 to 600 hours <7.2C were needed to satisfy the chilling requirements of kiwifruit. However, there was no sound scientific basis for this information.

Workers in California (Grant et al., 1994) report that 'Hayward', the leading female commercial cultivar, has a chilling requirement of 600 to 850 hours <7.2C. Monty and Kramer are two additional female cultivars with similar chilling requirements. They also list Bruno, Abbott and Allison as low chilling (50 to 250 hours) female cultivars.

However, recent studies in South Carolina (Caldwell, 1989) have shown that 950 to 1100 hours <7.2C are required for optimum vegetative growth and flowering of Hayward and Matua, while 750 to 950 hours are needed for Bruno and Tomuri. Matua and Tomuri are the most common male cultivars used as pollinators.

Observations over the past seven years in southern areas of Alabama clearly indicate that 'Hayward' can break bud (although somewhat slowly) and make robust vegetative growth with only 500 to 600 hours of chilling <7.2C. However, development of flower buds (10 to 30 per plant) and cropping (0 to 25 fruit per plant) are extremely limiting. Similar observations in central Alabama where chilling averages 1000 to 1300 hours <7.2C show 'Hayward' performs optimally, vegetatively and in flowering and fruiting (100 to 200 fruit per plant). Matua and Tomuri produce very adequate flowering for pollination with only 500 to 600 hours of chilling although greater chilling appears to enhance flowering.

Thus, the lack of flowering and fruiting of 'Hayward' (Bruno crops quite well in this area but has poor fruit quality) associated with the mild winters in the Mobile Bay- Gulf Coast area have prevented this crop from being produced successfully in this region on a commercial basis. Because plants in this area usually suffer less winter freeze damage it would be ideal for kiwifruit production if cropping could be improved. For this reason a number of studies were conducted in the Mobile Bay area beginning in 1991 to evaluate the use of hydrogen cyanamide (Dormex) to replace lack of chilling and enhance flowering and fruiting of the 'Hayward' cultivar. This seemed a reasonable approach given the fact that Dormex was already being used in milder areas of a number of other countries to replace lack of chilling and enhance cropping of kiwifruit and other fruit crops (Kristof, 1991).

Materials and Methods


Annual studies to evaluate the effect of Dormex on replacing lack of chilling in kiwifruit were conducted in a commercial planting in Baldwin county, AL, situated about five miles from Mobile bay. Studies were conducted from 1991 through 1995 (except for 1992). Results from 1991, 1993 and 1994 are reported in this paper. In all studies, treatments were imposed by making a single spray application of hydrogen cyanamid (Dormex; SKW Trostberg AG, Trostberg, Federal Republic of Germany) on the entire canopy of 'Hayward' plants to near point of drip using a small power sprayer operating at 50 psi. Approximately 0.35 to 0.45 liters were required for adequate coverage on plants which were spaced 2.4 m x 4.8 m. Plants were trickle irrigated and standard recommended pest management, fertility and other cultural practices were followed.

During the 4-year study treatments were imposed between 20 Feb. and 1 Mar. after annual chilling for the area involved was complete and while buds were still visibly dormant. Chilling hours (<7.2C) were measured from 1 Oct. until 28 Feb. each year. Weather conditions at time of application ranged from sunny to partly cloudy with air temperatures of 10 to 21 C. Plants were in their 5th leaf at the beginning of the study in 1991 and in their ninth leaf by 1995.

Dormex 4.0 SL, as a 49% a.i. specially stabilized aqueous solution, was applied at rates of 0, 2, 3 and 4% by volume in combination with 0.25% x-77 Spreader surfactant. Single plant plots were used as experimental units and studies were designed using randomized complete blocks with four to five replicates per treatment. Data was taken using branch counts to measure budbreak, vegetative growth, flower production and fruiting. Total yield at harvest was determined from fruit counts per plant. Fruit size, shape, soluble solids and firmness were measured using 10-to 15-fruit samples per plant at 1 to 3 harvest dates. A handheld pressure tester (Effegi penetrometer rated 0-12 kg) and a handheld refractometer (Atago temperature compensating, 0-32% Brix) were used to measure fruit firmness and % soluble solids respectively. Flesh firmness and soluble solids were also determined after refrigerated storage at 4C for 5 and 8 weeks (depending upon harvest year) plus a week of room temperature (24C) in 1993. The SAS general linear model procedure was used to analyze all studies. Means were separated by Duncan's multiple range test (SAS Institute, 1989).

Literature Cited

Grant, J.A., V.S. Polito and K. Ryugo. 1994. Flower and fruit development. p. 14-17. In: J.K. Hasey, R.S. Johnson, J.A. Grant and W.O. Reil (eds.). Kiwifruit growing and handling. University of California, pub. 3344. F.A. Bliss. 1994. The genus Actinidia. p. 9. In: J.K. Hasey, R.S. Johnson, J.A. Grant and W.O. Reil (eds.). Kiwifruit growing and handling. University of California, pub. 3344.

J. Caldwell. 1989. Kiwifruit performance in South Carolina and effect of winter chilling. Proc. Ala. Fruit and Vegetable Growers Assoc. 10:127-129.

SAS Institute. 1989 SAS/STAT user's guide. 4th ed. Version 6. SAS Inst. Cary, N.C.

G. Reeder. 1991. Personal communication. Research and Development, SKW Trostberg AG, Trostberg, Federal Republic of Germany.


Budbreak and growth of new shoots was measured in 1991. The highest rate of Dormex (4%) resulted in significantly more budbreak on 1-year-old wood than the control (Table 1). However, at the highest Dormex rate, only 38% of the buds forced. These findings agree with Caldwell (1989) who reported that a max budbreak of only 50% was attained by 'Hayward' when plants were exposed to 1100 h of chilling at <7.2C and no budbreak occurred if chilling was less than 300 h. It appears that forcing of remaining buds may be inhibited to do so by first buds to break. Workers in California (Grant et. al, 1994) reported that the number, vigor, fruitfulness and uniformity of buds that force into growth in spring, are thought to depend on the level of winter chilling received by vines. Although no data is shown, observations over a five-year-period indicated that Dormex applications at all rates resulted in advanced budbreak of 2 to 6 days. This was especially noticeable in springs (1991 and 1993) following mild winters. Accumulation of chilling hours (<7.2C) during the winters of 90/91, 92/93 and 93/94 were 505, 495 and 909 respectively. When natural chilling was high in 1994, no differences were evident in time of budbreak among treatments. Budbreak usually occurred between 10 Mar. and 20 Mar. some 2 to 4 weeks after application of Dormex.

Growth measurements made in 1991 indicted Dormex resulted in 70 to 90% more shoot elongation than the control although differences were not significant. Similar measurements were not made in subsequent years, but careful observations indicted Dormex treated vines continued to make more early shoot growth than controls. The very promotive effect of Dormex on leaf as well as fruit buds of a number of fruit crops has been reported by other workers (Erez, 1987; Dozier et. at, 1990). Dormex was not required to replace lack of chilling in the leaf buds although grater budbreak and stronger early growth consistently resulted from its application. No phytotoxicity was apparent from any of the Dormex rates used during this 4-year study.

Dormex at all rates resulted in significantly greater flower bud formation and number of fruits produced per tagged shoot in all years but especially so during 1991 and 1993 following very mild winters.

After mild winters the 4% Dormex rate increased the flowers and fruits produced from usually less than 1 to generally 4 per tagged shoot. Fruit set was increased by all Dormex rates over the control following mild winters (1991 and 1993), but no differences were apparent in 1994 following a high chilling winter. Studies in 1995 (not reported) in a second location in southern Alabama resulted in essentially the same level of flowering and fruiting per shoot for 1991 and 1993.

Workers in California (Grant, 1994) indicated that almost all dormant buds on a cane contain flower primordia (formed in midsummer) and therefore have the capacity to flower. During midsummer, the primordia are undifferentiated protrusions without form and remain in this form until just before budbreak the subsequent spring. Flowers may form on up to 6 of the first basal nodes of current shoots. They indicate that the number of flowers produced per inflorescence varies at each node for 'Hayward'. Inflorescences on heavily cropped plants may have up to 3 flowers on many nodes.

'Hayward' plants in central Alabama where chilling averages 1000 to 1300 h (<7.2C) often produce inflorscences with more than one flower. However, in the southern counties where this study was conducted, single female flowers inflorscences were produced from all treatments. Occasionally, some 3-flower inflorscences were produced at the 4% Dormex rate.

It appears that the timing of the Dormex sprays resulted in a significantly higher number of undifferentiated inflorescence primordia completing their development into female flowers just before budbreak when compared to controls. Work in California (Grant et. al, 1994) indicates bud fruitfulness is dependent on the level to which leaves are exposed to sunlight as the shoots grow during Summer. Well exposed shoots produce more flower buds the following Spring. Therefore Dormex was effective in replacing lack of chilling sufficient to significantly increase flowering and fruiting of 'Hayward'. The planting used in this was study was interpolated in a pecan orchard and shading and root crowding were problems. Although pruned reasonably good in their formative years, this planting received inadequate pruning to continually generate new fruiting wood. It appears that shading and inadequate pruning reduced flowering and fruiting in this planting.

Yields were not taken until 1994, but fruit counts of selected vines were made in 1991 and 1993. yield at the 4% Dormex rate was significantly greater than the control but was considerably below the 250 to 500 fruits anticipated per vine. Fruit counts per vine made in 1991 and 1993, seasons following mild Winters, were usually 90 to 175. Bentel (1994) indicated 'Hayward' vines should produce 750 to 1000 fruits at a spacing of 4.8 m x 9.6 m (this is one-half the plant population per hectacre used in this study). It is apparent that the increased yields in this study resulting from Dormex were the result of a higher level of flower bud formation and a higher level of flowers producing fruits (nearly doubling of % fruit set).

Treatments had no effect on fruit size or shape including L/D ratio. The fruit weights produced in 1991 are considered very similar to the 90 - gm size as being ideal in California (Beutel, 1994).

Although no differences in the total soluble solids (TSS) content of mature fruits were found among treatments, several trends were evident across treatments. (1) TSS of fruits continued to increase the longer they were left on vines in both years. Attempts were made to begin harvesting when TSS content was near 6.0%. Values of TSS increased to the 11% range by the third harvest in 1993. (2) TSS after cold storage were generally significantly higher than at harvest, usually approaching or surpassing 14%, a level workers in California (Mitchell, 1994) indicate should be exceeded in ripe fruit. (3) TSS values after additional week of storage at 24C did not differ much from those attained following 4C storage in 1993. (4) Regardless of when fruit were harvested, TSS content following 4C cold storage rose to near the same values; although there was small trend toward increased TSS with the final 1993 harvest. The trend in TSS found in this study are in agreement with those reported in California (Mitchell, 1994). The data show Dormex had no material effect on total soluble solids.

There were no significant differences among treatments in flesh firmness at each harvest date in 1993 and 1994. The same was generally true for firmness values following 4C cold storage. Our firmness readings at harvest are similar to those reported in California (Mitchell, 1994). However, these same workers report that fruit firmness continue to decrease as fruits remained on vines after first harvest. This trend was not found in these studies, with average firmness decreasing only slightly. Firmness values decreased markedly following 4C storage during 1993. This agrees with other workers from California (Aparia et. al, 1994). However, following 4C storage in 1994 flesh firmness remained essentially the same as at harvest regardless of treatment. Thus Dormex had no material effect on flesh firmness values. There is a possibility that the occurrence of a light frost before the second and third harvests in 1993 may have initiated the downward trend in firmness. No frosts occurred during the 1994 harvest.

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