Geneticist Craig Ledbetter examines the nuts of a self-pollinating almond selection in a California test plot.
PHOTO BY PEGGY GREB.

Native to the Middle East and South Asia, almonds have made their way into the fields of growers throughout Europe and California. For the past 25 years, California’s San Joaquin Valley has grown more than 70 percent of the world’s almond crop. Although several hundred varieties of the nut exist, California growers focus on the 25 cultivars developed within their state. Research into almond development has been ongoing at the USDA and the University of California since the early 20th century, and is responsible for the development of these cultivars.

Cultivars have been developed to produce yields of up to 3,000 pounds per acre, and to meet the tastes of consumers and the needs of processors, who prepare the nut for packaged food production and sale. For marketing purposes, California almonds are separated into grades and groups. Nonpareil is the cr<0x00E8>me de la cr<0x00E8>me almond, sold raw, sliced and processed. The California marketing group of almonds is primarily used for production and manufacturing. European and Middle Eastern varieties do not meet the needs of processors and have different properties than U.S. consumers are accustomed to eating. However, one USDA researcher is currently exploring the properties of a Spanish variety, called Tuono. Although the cultivar does not match up with any of the California marketing groups, it has a property that may be key in protecting the U.S. crop from potential threats.

A threat to California’s almond crop

Climatic stresses such as extreme cold and excessive rain can affect the almond tree and bloom, as well as the pollination process. Each year, California’s almond growers import about 1 million hives of honeybees to pollinate their crops. If cold temperatures or rain descends, the bees don’t fly, and so don’t transfer pollen among the blossoms.

A more recent concern is colony collapse disorder, a hot topic in agriculture due to its potential to devastate production on a number of important U.S. crops.

Dr. Craig Ledbetter, a research geneticist with the USDA, became concerned about honeybees a full decade before colony collapse disorder became an issue. In 1993, Africanized bees (aka killer bees) invaded the southwest U.S. Ledbetter worried that the bees would invade the San Joaquin Valley. Acting on a hunch the killer bees would affect the honeybees’ ability to pollinate the state’s almond trees, he began exploring a trait unique to the Tuono variety: self-pollination.

Self-incompatible versus self-fruitful

In order to be pollinated and bear a crop, most almond trees require trees of two or more varieties in the orchard to bloom at the same time with compatible pollen. The pollen is then transferred between the flowering trees by honeybees. These types of almonds are known as self-incompatible. Even a field with two self-incompatible trees, such as Nonpareil, will yield no almond crop. The pollen transfer between trees of the same variety is the same as pollen from a single tree going back on that tree. All of the California cultivars are self-incompatible.

A self-fruitful, or self-pollinating, almond tree is able to use its own pollen to germinate on the stigmatic surface, go down the stylar tube and fertilize the ovary to make an almond kernel. A self-fruitful tree will set a crop with a single tree in the field, even without bees. Tuono is one such variety.

The main benefit to self-fertility is assisting in consistent production. With self-incompatible almonds, like those currently grown in California, growers are dependent on many factors at bloom, many associated with bees. If the bees aren’t flying because of the weather, this could produce a zero crop for self-incompatible varieties, whereas a self-fruitful variety will yield a small crop. Although self-pollinating almonds do not need bees to set a crop, honeybee pollination does increase yield. Still, like many heirloom crops, the Tuono variety of almond is not high-yielding, even with bees present.

Something borrowed, something new

As he explored the possibility of creating a new self-pollinating almond cultivar, Ledbetter delved into material that his predecessors at the USDA had developed. This parent stock has never been released for production, but each of the germplasm samples is self-incompatible. Ledbetter has crossed this germplasm with Tuono and developed several self-fruitful varieties that will set a crop on their own pollen. The researcher reports that the new varieties are similar in look and taste to Nonpareil or some of the other California varieties. He doesn’t currently have yield data.

“In California, it’s all about how much tonnage a crop can produce in a given year,” he explains. “Our overall objective is to breed and then identify high-yielding types of self-fruitful almonds with Nonpareil-like characteristics.”

Ledbetter and his colleagues have started variety trials and have a couple years’ worth of data on some of their best selections. Before releasing any of these new cultivars, Ledbetter wants to have several years of data from a commercial setting. He needs to compare how well the new varieties yield to ensure the product he releases will benefit farmers.

Almonds without bees?

Growing self-fruitful varieties will mitigate any losses that may be created when bees are not available or available in small numbers. However, the self-fruitful varieties will not negate the need for bees. Ledbetter believes that even his highest-yielding self-pollinated selections will not produce the tonnage commercial growers are accustomed to in California without the addition of hives to the orchard. When the USDA eventually releases Ledbetter’s new cultivars, California’s commercial almond growers will still need honeybees to generate the yields they desire.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.