Resistant varieties in the pipeline
Mildew resistance would be particularly welcome in organic sweet cherry orchards.
Dr. Jim Olmstead, manager of Washington State University's cherry breeding program at Prosser, has developed two new cherry varieties which may be resistant to powdery mildew.
So far, only a few trees of each have been grown in research orchards, but Olmstead said they will go into replicated trials in a couple of years, and he would like to find growers willing to test them in their orchards. If the selections still look promising, the fruit will be put through small-scale packing and storage tests.
Olmstead believes that if the varieties are ultimately commercialized, they would be suitable for organic production. Cherry fruit fly used to be the greatest barrier to organic cherry production, but new and effective organic pest control products are available, such as the GF 120 bait. Now, powdery mildew control is one of the greatest difficulties in organic production because growers are limited to the use of sulfur or lime sulfur.
"If nothing else, these may be really attractive for people who want to try organic production," he said.
WSU's stone fruit breeding program started out in 1949 with Dr. Harold Fogle in charge. Dr. Tom Toyama was the breeder from 1960 to 1985. Selections from the crosses Toyama made have been released since he retired, and the material is still being evaluated for potential releases.
Olmstead used the material in 1998 to make crosses while studying the inheritance of the mildew-resistance trait, which was part of a research project for his master's degree. Three of the crosses turned out to have good quality fruit and two of them are the selections with mildew resistance.
The 9816-104 DD is a late-season cherry selected because of its large fruit, excellent taste, and mildew resistance. Both the leaves and fruit are resistant to mildew. The cherries average 8-1/2 row with 22° Brix. "It would be really nice to have a late-season, powdery mildew-resistant variety to help producers in the later growing districts," he said.
The 9817-97 GG is a midseason mildew-resistant variety with excellent flavor. Average size is 9-1/2 row.
The late-season 9816-96 JJ is not resistant to mildew, but has excellent flavor. The fruit averages 8-1/2 row with 22° Brix.
Since the breeding program was revived two years ago, many more crosses have been made in an attempt to develop varieties with different harvest timings, larger fruit, better flavor, self fertility, disease resistance, and better firmness than existing varieties. New varieties should also be suited to mechanized production, Olmstead said, because it's feared that labor will continue to be scarce in the future. The breeders will be looking at how easily the cherries detach from the tree, and whether the growth habit of the tree is suitable for a trellised system.
So far, the program has about 1,700 selections from 150 different crosses, using 53 parents. Dr. Amy Iezzoni from Michigan State University and Dr. Fred Bliss from Seminis Seeds are working as consultants.
Olmstead said breeding cherries is an extended process because the trees take a long time to produce fruit for evaluation. He hopes to use molecular markers to identify traits in young seedlings so that unsuitable varieties can be quickly eliminated. Scientists are working to develop markers for powdery mildew resistance and self fertility.
Some traits, such as fruit size, are complex, and can have different factors involved. Olmstead said he is looking at differences in the number of cells in fruit flesh, which is predetermined for each variety. Screening selections for cell number, rather than just fruit size eliminates the influences of environmental and cultural effects, such as irrigation, and nutrient management. "That's something we can use for selection," he said.
He is also working to quantify the different components of flavor and how they align with what people consider to be a good-tasting cherry. "We're trying to develop a very modern breeding program, using any of the tools that are available to speed up the process and increase our efficiency," he said. "The idea is to develop varieties for growers to allow them to have sustainable and consistent production."