In search of a dwarfing pear rootstock
Oregon State University is evaluating more than 400 potential pear rootstocks from the Horner series in the hope of finding some that will help pear growers to transition to higher density plantings.
Janet Turner, a technician at OSU's Mid-Columbia Agricultural Research and Extension Center in Hood River, said she's looking for a rootstock that produces a smaller tree than Old Home by Farmingdale 87 does, along with higher early yields (in the third and fourth leaf) and large, high quality fruit. But, above all, it must be easy to propagate to have any potential as a commercial rootstock.
OHxF.87 is precocious but is difficult to propagate, and with Bartlett as the scion, it can runt out after it comes into production, Turner said.
Tom Auvil, research horticulturist with the Washington Tree Fruit Research Commission, who is a lead scientist for the trial, said trees that have too little vigor often produce small fruit. As with precocious apple rootstocks, the crop load and vigor need to be managed properly.
"With Bartletts, we're looking for rootstocks that will grow big fruit and have good yields, and we haven't necessarily found the magic rootstock for Bartlett yet," he said.
The Horner rootstocks originated about 30 years ago when nurseryman David Horner developed more than 500 selections in a remote area of Oregon. They were open-pollinated crosses of OHxF.40, 51, 87, 333, and 339. Dr. Bill Proebsting, horticulturist with OSU in Corvallis, evaluated them initially for their ease of propagation. Those that were difficult to bulk up were discarded.
In 1995, horticulturists at the Hood River station began to prescreen 13 of the rootstocks for tree size, yield, and fruit size. They also looked for "fatal flaws" such as too much root suckering and susceptibility to fireblight or Pseudomonas. The scion was d'Anjou, and the trees were spaced 8 by 18 feet. The control was OHxF.97, which Auvil said is similar to seedling. During the seven-year study, two of the rootstocks—Horner 4 and 10—were more productive than OHxF.97 and produced larger fruit. OSU has propagated those two rootstocks, and Van Well Nursery in Wenatchee, Washington, will grow trees for trials in commercial orchards. The rootstocks will be compared with OHxF.87, Auvil said.
In 2004, 2005, and 2006, the Hood River station planted out 428 Horner selections with d'Anjou as the scion. The trees are five feet apart in double rows 2.5 feet apart. Turner is optimistic about the potential for finding a good new dwarfing pear rootstock. If only 5 percent of the selections show promise, that's still more rootstocks than are available today, she points out. "That's the exciting part."
She was hoping to identify some promising selections this fall from the oldest trees in the planting, so that they could be propagated for on-farm trials, but said nothing stood out yet.
Horner 4 is included. While the previous trial showed that the rootstock is extremely productive, it appears to produce a large and fairly vigorous tree, Turner said. "We were hoping it would be the size of OHxF.87 or below."
It appears to be close to OHxF.97 in size, although trees on H.4 do have flat branch angles that would make it suitable for trellised plantings.
Turner said Fred Valentine, horticulturist with Van Well Nursery, observed that H.4 might have potential for filling in spaces in an established orchard, as it grows quickly, is vigorous, and comes into production early.