Apr 12, 2021
Pear rootstocks for replant in presence of armillaria

Common in soils that once held oak trees and other native forests, Armillaria mellea has a strong presence in California and the Pacific Northwest. This species is the most widely spread, is lethal to susceptible fruit trees, and currently has no method for eradication.

With this in mind, Caroline Tweedy, a master’s graduate from the University of California (UC), Davis, set out to find viable rootstocks that would be good, resistant replants in an orchard infested with armillaria. The research remained in the lab, where 185 seedling and 85 clonal genotypes were exposed to high concentrations of armillaria and analyzed.

“When the seeds were removed from the cold room, some did not germinate at all, or were contaminated, or were already dead. Some genotypes grew into great little seedlings, some had only one seedling, and there weren’t as many replicates as there were in the clonal,” said Tweedy.

Extreme example of development roots produced by the in vitro plant.

There were 20 replicates propagated for each clonal rootstock. Seedlings were based on germination success.

Tweedy used the Area Under the Disease Progress Curve, or AUDPC, to score each of the genotypes in terms of disease development – from how much media was covered in fungus to the parts of the plant that became necrotic. The plants vary throughout time, with some showing symptoms of disease as early as the first week and others several weeks later.

“Some could be green, green, green, then boom, on week six they turned completely black,” said Tweedy. “If I only looked at week six, then I’d think that plant was bad, that it took the armillaria poorly and was susceptible. But looking throughout time, it was more resistant, in a way, until week six.”

Because the curve incorporates time, Tweedy was able to assess whether the disease progressed faster or slower within a genotype.

She also used a genome-wide association study (GWAS), which is used to determine whether certain specific genetic variants are associated with particular traits. The genetics were already sequenced for all of the pear varieties she used in her study. Using both the genetic and phenotypic data she had, Tweedy was hoping to find a certain section of the genome that was associated with disease resistance.

OHxF 87 six weeks after armillaria inoculation.

“From that GWAS test, there was no section that showed significant resistance,” Tweedy said.

The AUDPC averages showed that OHxF 87 and OHxF 97 were the rootstocks most likely to have resistant qualities. Thought to be a cross between Old Home and Farmingdale varieties, later genetic studies revealed that the OHxF rootstocks are actually a cross of Old Home and Bartlett, but still use the OHxF label. OHxF 87 is less vigorous and will grow slightly smaller than its full-sized, vigorous OHxF 97 counterpart. Both rootstocks are also resistant to fire blight.

Tweedy’s research comes at what is the beginning of an increased concern focused on armillaria, and what growers can do to combat it. Most growers are unfamiliar with the fungal pathogen and don’t know if it’s in their fields nor how to control it. Though its lesser prevalence in pear has caused it to take a back seat to other diseases, such as fire blight, its lethality in a variety of crops – particularly in peach, which is highly susceptible – is causing more people to take notice.

Armillaria can cause a steady decline in tree health and, thus, yield. And while one tree may show symptoms of infection, it may take multiple years for the tree next to it to begin showing signs as well. During field sample collection, Tweedy found armillaria infection in the bark of dead tree stumps – trees that had likely been killed by armillaria – yet its neighboring replant had yet to show symptoms.

“It’s patient,” said Tweedy of the pathogen.

Line graph of a single plant’s disease development over six weeks. Each plant’s AUDPC value was calculated by area under the curve, and used to determine which genotypes performed best.

Once a tree is infected, the root system of that tree is also infected. One theory is that in an orchard system, where the trees are planted closely together, the root system of the infected tree can touch that of the neighboring uninfected tree, causing the spread of the pathogen.

How this in vitro lab study translates to the field has yet to be determined. Field studies on armillaria are difficult because plants can take years to show symptoms of infection. Though unofficial, Tweedy did stumble upon some interesting findings during her study when she decided on an impromptu side experiment: planting nearly dead plants.

Tweedy gave her armillaria-infected plants teetering on the edge of death to Wes Hackett, retired UC Davis researcher, who then planted the seedlings in soil and placed them in his greenhouse.

boxplot of average AUDPC scores
Boxplot of average AUDPC scores for all 85 clonal genotypes inoculated, ordered from most resistant to most susceptible. Image: Carolina Tweedy

“They started to grow back normally,” said Tweedy. “There are other bacteria and fungus in soil, so maybe the armillaria fungus is waiting for the plantlet to show signs of weakness or susceptibility again, or the armillaria itself is waiting to get stronger. It’s a slow-growing fungus.”

There is other research Tweedy would like to see in regards to armillaria, its effect on seeds, and if a seed’s phenolic compounds that protect it from disease early in life remain consistent throughout its lifetime.

“There are still rootstocks that are offered to growers that are in the seedling form versus the clonal form, so that would be something else nice to look at,” Tweedy said.

Until there’s further research – both in the lab and in the field – Tweedy stands by her recommendations not only for replants but for new growers considering a pear rootstock.

— Crystal Nay, Western editor




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