The date palm (Phoenix dactylifera L.) made its way to California through the Spanish missions in the late seventeenth century. In its native ranges of North Africa and the Middle East, the date provided food, fiber and shelter. As commercial date production established in the interior valleys of California and Arizona, growers sought out new varieties. Introductions of varieties from Algeria, Tunisia, Egypt and Iraq between 1890 and 1922, laid the foundations of the modern California date industry. W.T. Swingle brought offshoots of ‘Deglet Noor’ to California in 1900. Recognized as a superior variety for three hundred years in the oases of Algeria and other North African countries, ‘Deglet Noor’ is now one of the most significant cultivars in California and worldwide. In 1927, Swingle also introduced the ‘Medjool’ date to California. ‘Medjool’, probably originated in the Tafilat district of Morocco, takes its place beside ‘Deglet Noor’ as one of our most important date varieties.
From its beginnings as few thousand imported offshoots, the California date industry has grown to annual harvests totaling $23-30 million dollars, according to 2001 USDA statistics. The approximately 5,600 acres of date palms in California account for more than 95% of the U.S. industry, supplying dates for consumption and mature trees for the expanding landscape market. The varieties ‘Deglet Noor’ and ‘Medjool’, amongst a few others, remain the mainstay of this strong niche landscape industry. While there have been no important cultivars introduced into the U.S. in recent years, there are over three thousand different cultivars grown worldwide. Examining these cultivars and making comparisons with those cultivars grown in California, will allow researchers to make new introductions best suited for our industry.
The long history of cultivation has obscured the origins of the date palm, thought to be a native of western India or possibly southern Iraq. Identifying cultivars can be difficult, as this is done primarily through morphological characteristics such as the fruit, leaf bases or spines. Environmental factors such as soil or weather can influence these characteristics, which are generally only observable on mature trees of at least 3-5 years of age. The names associated with date cultivars also create some confusion. In some parts of the world, a date known by one name at an oasis might be called something completely different a few hundred miles away. To add to the confusion, dates also exhibit intra-varietal variations. These variations, such as differences in fruit size, ripening time or vegetative characteristics, were first reported in the early 1920’s. Researchers originally thought the differences between palms of one variety were due to the chance that the plant had come from a seedling instead of an offshoot thus having different varietal characteristics because it was a hybrid and not a clone from the parent plant. Those plants were grown and marketed under the name of the parent cultivar. Modern researchers, using amplified fragment length polymorphism (AFLP) markers, a polymerase chain reaction (PCR) based molecular markers, can now evaluate genetic diversity and fingerprinting of the date cultivars. In a recent study at UC Riverside, we examined 23 samples of ‘Medjool’ and 33 samples of ‘Deglet Noor’ date using AFLP markers (The Journal of Horticultural Science and Biotechnology, 2003, 78(5): 405-409). The samples for this study were collected from the USDA National Clonal Germplasm Repository for Citrus and Dates at the UC Coachella Valley Research Station, Thermal, CA and from commercial date gardens in the Coachella Valley. Our results showed that there is almost no genetic difference amongst the 33 ‘Deglet Noor’ date samples; however we identified a wide range in genetic variation within the ‘Medjool’ samples. Among 23 ‘Medjool’ date samples tested, 3 had the same genetic profiles. In total, we identified 20 different types of ‘Medjool’ dates. How can we explain such large difference found in ‘Medjool’ dates in California? The differences could have originated when horticulturists originally introduced ‘Medjool’ into California in the early 1900s. The plants could have been mistakenly identified, mislabeled, or mistakenly propagated from seedlings instead of clonal offshoots. Another possible explanation for the genetic variation could be that ‘Medjool’ palms have a high rate of mutation. The differences we found in ‘Medjool’, however, are too large to be explained by a high mutation rate, we believe another explanation is more likely. We propose that the ‘Medjool’ date that was introduced into California originated from a “landrace” variety in the Tafilalt district of Morocco, where ‘Medjool’ was initially selected. A “landrace” variety means that the variety is endemic to an area (in the case of ‘Medjool’ in the Tafilalt district of Morocco), it is a mixture of different genotypes and well adapted to the local environment. It is possible that the initial introductions of ‘Medjool’ were in reality different genotypes of ‘Medjool’ and these different genotypes exist in current plantings. On the contrary, all 33 ‘Deglet Noor’ dates that we tested are almost the same genetically. ‘Deglet Noor’ seems to exist as a pure variety without much variation.
Growers in California have observed differences in fruit quality and yield of ‘Medjool’ date in the past, but the variation always have been attributed to xenia effect, location, environmental, or management practices. Our results imply that the differences in production between ‘Medjool’ date palms may be due to genetic differences and not just cultural differences. Field testing is needed to determine if the differences in fruit quality and yield between plants is related to genetic differences between plants. For this, we need to plant a trial where we are able to evaluate genetically different strains of ‘Medjool’ palms grown under the same environmental conditions. Potentially, some strains of ‘Medjool’ may have higher yield and better fruit quality than other selections; these can be selected for propagation thereby having the potential to increase grower’s returns.
In the future, we would like to examine 30-50 different ‘Medjool’ samples collected from Morocco using the AFLP markers to confirm or dispute the possibility that ‘Medjool’ date exists as a landrace variety in Morocco. We also are collecting samples of offshoots from the same ‘Medjool’ palm to determine if high mutation rate exists in ‘Medjool’ dates. By learning more about the varieties grown in California and abroad, we hope to select varieties that would increase the vitality of the date industry in California.
