Dwarfing rootstocks For stone fruit

Though significant tree size differences in the three rootstock photos are difficult to see due to the pruning, the canopies of Controller 5 rootstock trees, left photo, are less dense and the trunks are smaller than Nemaguard at right, the industry stand

As the cost of labor and the availability of workers become more problematic for California tree fruit growers, the industry is looking at ways to reduce labor needs. One promising possibility is the use of dwarfing rootstocks for peaches and nectarines. Growers and researchers have been looking at several dwarfing rootstocks that produce peach and nectarine trees anywhere between 50 to 90 percent the size of normal-sized trees.

“With a regular-sized tree, you have to have workers climb twelve to fourteen feet into the air to pick fruit. As labor becomes increasingly limited, there are fewer people who can do this type of work,” said Ted DeJong, a professor of tree crops’ physiology at the University of California, Davis. “With the dwarf rootstock trees, we can keep the trees down to below seven or eight feet.”

Two varieties from the University of California—Controller 5 and Controller 9—were released for commercial use two years ago. Both can be used for peach or nectarine varieties.

Controller 5 produces a tree that’s about 50 to 60 percent the size of a standard tree, while Controller 9 produces a tree that’s about 90 percent of a tree on the Nemaguard rootstock, DeJong said.

Although trees on dwarf rootstocks produce less fruit than normal trees, growers will have a better chance of managing tree height and thus reducing the overall amount of labor used to harvest and prune the trees, DeJong said.

“The goal is not to increase yields, but to decrease labor costs.”

While trees on Controller 9 rootstocks produce yields that are comparable to traditional trees, trees on Controller 5 produce much less, DeJong said.

“But if you tighten up the spacing and make them more dense, yields will be comparable, although you’ll have to plant them at twice the density of regular trees,” DeJong said.

New strategies

Some growers might have a hard time adjusting to the idea of planting smaller trees, he acknowledged. “Growers prefer to see a lot of growth in the first year. If they don’t see a lot of vigorous growth, they feel something must be wrong.”

Some apple growers were also resistant when dwarfing rootstocks were introduced to the industry 20 or 30 years ago, he noted.

“But over time they developed orchard management practices for these rootstocks and now almost everyone is planting on size-controlling rootstocks in the apple industry.”

Peach and nectarine growers will also have to learn new management strategies for the dwarf rootstock varieties.

“Actual pruning techniques will probably vary a bit because the structure of the trees is going to be different. Growers are also going to have to really be on top of thinning. The initial fruit set tends to be heavier on dwarf rootstock trees, so you have to thin the fruit earlier in the season, otherwise it can affect the final fruit size,” DeJong said.

Some other kinks still need to be worked out of the dwarf rootstocks before they become viable for growers on a large scale, DeJong said. One of the biggest hurdles is that the dwarf rootstocks don’t have the same resistance to nematodes that the Nemaguard rootstocks do.

“There are substantial problems in the central San Joaquin Valley with nematodes, which is why we’ve had rootstocks with nematode resistance for the past 30 years and why we’re looking for dwarfing rootstocks that have nematode resistance,” DeJong said.

Nematodes

Scott Johnson, extension specialist with the University of California Department of Plant Sciences at the Kearney Agricultural Center in Parlier, said that growers with orchards on sandy, loamy soils, that have fewer problems with nematodes, are the most likely candidates to try the new dwarf rootstocks. Johnson has also been looking at other promising dwarfing rootstocks, such as the Krymsk-1 rootstock from Russia.

“It’s similar to the Controller 5 in that it’s half the size of a tree grown on Nemaguard. But the thing we like about it is that it seems to have good fruit size, which sometimes the Controller 5 does not have,” Johnson said.

The other positive attribute of the Krymsk-1 is that it, unlike others, doesn’t seem to sucker much.

Little is known yet about how it responds to various diseases and nematodes, Johnson said. “But it’s something we need to keep testing.”

Fred Hunt with Fowler Nursery in Kingsburg, California, said that his nursery has been working with several dwarfing rootstocks from Russia, Italy, France, Spain, and Germany.

Out of the 12 rootstocks the nursery has been evaluating, the most promising ones seem to be for nectarines and apricots, Hunt said. Some of the dwarf rootstocks are peach and plum hybrids, while others are plum and almond hybrids. Because these rootstocks are complex hybrids, there could be some compatibility issues with some tree fruit varieties.

“We’re working on testing these rootstocks with as many different varieties as possible,” Hunt said.

Coming up with the right rootstock for the right variety can be a delicate balancing act, he added.

“The thing about size-controlling trees is that you’re trying to make a smaller tree, but you’re also trying to make larger fruit on that tree. And when you do that, you have to be careful that you don’t produce fruit that is so large that you end up killing the tree. You’ve got to be real careful about matching tops with the rootstocks,” Hunt said.