There is no need to push the panic button about the widely popular almond plum rootstock Krymsk86 in California’s almond industry.

A tree abnormality found in some Sacramento Valley almond orchards and a few in the San Joaquin Valley (SJV) last spring has the attention of tree nut farm advisor John Edstrom and others.

“Some trees on the Krysmk86 plum rootstock, mainly the Monterey almond variety, stopped growing last spring and the foliage turned yellow,” said Edstrom, University of California Cooperative Extension, Colusa County. “It was very obvious from a distance that there was something wrong.”

As the season progressed, the yellowed leaves turned brown, burned up, and curled. None of the impacted trees is known to have died.

“The trees grew about one-half compared to normal,” Edstrom said. “The trees set the terminal bud and just stopped growing. The leaves went from green to a bronzy-brown color and then died.”

Some growers and nurserymen tried several unsuccessful remedies including sprays of kaolin-type clays to reduce leaf stress. Impacted trees were pruned back which helped the trees partially recover.

The net cause of the problem is uncertain. It could be the Krymsk86 rootstock. Edstrom’s gut says the malady was probably, in part, a seasonal aberration caused by last spring’s cool, wet spring which slowed tree development.

“The cool, wet spring likely played a significant role,” Edstrom said. “The tree flush this coming spring should answer many questions.”

Edstrom found the problem in one to two-leaf trees in about 1 percent of the trees in the dozen or so orchards he evaluated. The phenomenon was found to a lesser degree in the Nonpareil, Winters, and Aldridge varieties on the Krymsk86 rootstock.

From a distance the tree malady resembled the effects of union mild etch which is often associated with the rootstock Mariana 2624. Union mild etch causes similar leaf symptoms, but also abnormal etching at the union graft. The affliction can be found by peeling back the bark between the scion on top and the root on the bottom.  An examination of the Krymsk86 trees last spring revealed a smooth graft union.

“The big question is will the impacted trees rebound this spring and develop normally, and will the problem occur in other one-to-two leaf trees this coming spring,” Edstrom asked.

Plausible explanation: saturated soils

Besides the weather, Edstrom offers another plausible reason for last year’s malady. Saturated soils were common in the orchards; a combination of spring rains and unnecessary irrigation.

“Some growers irrigated as usual despite the spring rain,” said Edstrom who also manages the Nickels Soil Laboratory in Arbuckle. “Some growers routinely irrigate at specific times of the season or when foliage reaches a certain maturity level on the tree.”

Edstrom convinced growers to reduce irrigation to 50 percent to 60 percent of normal during the summer months on the effected trees. No negative tree symptoms developed afterwards.

Another overlying factor is the clay and silt soils in the Sacramento Valley which are prone to retaining higher moisture levels which reduces oxygen in the soil.

Krymsk86 is of Russian origin and was brought to the United States by USDA. The University of California-Davis propagated the rootstock with the help of the nursery industry. Edstrom was among the first farm advisors to plant Krymsk86 in the early 2000s in different locations.

Today, Krymsk86 rootstock is widely popular in California almond circles.

“Krymsk is a hot item right now; growers are clamoring for the rootstock,” Edstrom said.

Among the rootstock’s top selling qualities is its excellent compatibility with Nonpareil, California’s top-planted variety. Some other existing plum rootstocks have problematic or incompatibility issues with Nonpareil.

“The compatibility of Krymsk86 with Nonpareil looks almost too good to be true,” Edstrom said. “The compatibility is nearly perfect.”

Also, Krymsk86 lacks the negative characteristic of plum suckering at the tree base often associated with other rootstocks. Edstrom has not witnessed any union etch issues in Krysmk86.

“Kyrmsk86 has more vigor than many other plum rootstocks,” Edstrom noted.

California nurseries have sold Krymsk86 like hotcakes. The rootstock is currently planted on tens of thousands of acres across the Sacramento and SJV valleys.

Another selling factor is Krymsk86’s strong, deep root system. High winds in the Sacramento Valley over the last decade uprooted or damaged tens of thousands of almond trees; many on the Lovell peach rootstock. Some rootstocks combined with root or crown diseases led to the increased tree vulnerability in high winds.

“Krymsk86 offers good anchorage for the trees,” Edstrom explained. “Krymsk86 is one of the strongest rooting trees in the California almond industry today.”

Its Superman-strong root system is partly tied to high tensile strength which makes the Krymsk86 bulkier, extensive root system harder to break, compared to weaker rootstocks including Lovell and Nemaguard.

While Krymsk86 appears to be a good rootstock for California’s almond industry, experienced nurserymen contend that 10 to 20 years under widespread planting is necessary to fairly evaluate any rootstock.

For now, Edstrom is not very worried about the problem found last spring. If there is a tie to Krymsk86, the farm advisor says it is an issue that growers can work around.

“After all, the larger issue is to develop an overall orchard canopy and not the problems of a few trees,” Edstrom said.