The Complex Path for New Citrus in California

Tacy CalliesCalifornia Corner, Varieties

Seedless Gold Nugget mandarins grow in California.

By Len Wilcox

Continuous research and development of new varieties is an important function of the University of California, Riverside (UCR). Efforts have led to hundreds of profitable varieties now being grown in California.

According to a report in California Agriculture, University of California’s (UC) research journal, one of those varieties has been a source of more than $14 million in licensing revenue to UC since 2006. This variety is the seedless, easy-peel Tango mandarin, bred by Mikeal Roose, professor of genetics in the Department of Botany and Plant Sciences at UCR, and UCR researcher Tim Williams. Many millions of Tango trees have been planted worldwide, with 4 million or so in California alone.

However, it is a surprisingly complex trail that a citrus variety must follow before it can be grown profitably and safely. Several questions must be answered before a new citrus variety is given a chance to succeed in California.

First, researchers must find out if it is a carrier of diseases or pests which have not been found previously in the area. Next, the variety may not be suited to the local climate or soil conditions. A Valencia orange that is exquisitely suited to Florida may change flavor or be a lackluster performer when planted in the Central Valley of California.

In addition to worries about bringing in diseased cultivars and ensuring that the plants will grow well and produce good fruit in California, the business principle of supply and demand also influences UC’s work. Changing market requirements will affect demand and profitability of any variety. During the past decade, there have been significant changes in the number, types and varieties grown in California due to changes in the consumer market for fresh citrus.

It is crucial to the California citrus industry to maintain competitiveness in the global and domestic fresh citrus market. To help meet this goal, UCR is devoting significant resources to investigating the health of new citrus varieties and their ability to thrive in the California climate. New varieties are released each year as a result of these efforts.

However, these new varieties do not just appear in the marketplace. Just getting them into the ground can be risky business, as new budwood from outside of California may contain diseases or add pest problems. That is why, in 1957, the Citrus Variety Improvement Program began. It was later renamed to Citrus Clonal Protection Program (CCPP).

The CCPP provides a safe mechanism to introduce citrus varieties from any citrus-growing area of the world for research, variety improvement or commercial purposes. This program is designed to find any problems before the plant is introduced to a commercial grove. The director of the CCPP, UCR professor Georgios Vidalakis, says that the unit “serves as a valuable and prestigious resource for scientists at UCR and throughout the world that are interested in variety improvement.”

The imported budwood of all new varieties arrives at the CCPP quarantine facility in Riverside. The arrival sets in motion a detailed process for the production and distribution of a new variety. It starts with a comprehensive testing program, to detect graft-transmissible diseases that may arrive in an imported budline. Graft transmissible diseases may be caused by viruses, viroids or other pathogens and are vegetatively propagated with an infected budline.

After successfully passing through this process, the variety is added to the UCR Citrus Variety Collection, curated by Tracy Kahn. This collection, located at the Lindcove Research and Extension Center (LREC) near Visalia, is one of the world’s most diverse assemblages of citrus cultivars and their relatives. Consisting of at least four trees each of over 1,000 different citrus cultivars and citrus relatives, this collection includes new and heirloom cultivars and relatives either developed at UCR or introduced from around the world since the collection was established over a century ago. It is one of the most important collections of citrus diversity in the world.

The collection is used for research, plant breeding and educational Extension activities. The researchers at the Lindcove collection have two main objectives:

  • Provide a demonstration block of new and existing citrus cultivars for field days, fruit displays and for growers and industry representatives to evaluate fruiting trees of different varieties
  • To serve as a resource for other citrus research projects and for California Citrus Research Board-funded evaluations for trueness-to-type and commercial potential

According to the California Agriculture report, three 34-year-old trees with inedible fruit at LREC may provide the answer to a vital puzzle facing citrus today. They are crosses of sweet orange (two) and Rangpur lime (one) with Eremocitrus glauca, the desert lime, a wild Australian citrus relative. These trees could lead to a citrus variety resistant to HLB.

A very promising mandarin called Gold Nugget was developed and released by the UCR Citrus Clonal Protection Program. It is now being established in commercial orchards. Researchers say it is truly seedless, with excellent productivity, good fruit size and a rich sweet flavor. With regular pruning, alternate bearing is reduced or eliminated. Because the variety is truly seedless, it reduces grower overhead since no netting is needed to prevent bee pollination.

It’s a good example of how the fruits of UC’s labor pay off for California’s citrus industry.

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About the Author

Len Wilcox

Correspondent at Large for Citrus Industry Magazine and AgNet West