Consider disease resistant rootstock
Rootstock selection can be a valuable tool in managing soilborne diseases such as Phytophthora, Armillaria (Oak Root Fungus) and Prunus Replant Disease. These diseases can have varying effects on orchards, and none of the outcomes are good. Phytophthora and Armillaria attack the tree’s vascular tissue, reducing vigor and in some cases killing the entire tree. Prunus Replant Disease suppresses the growth of trees so dramatically that it often necessitates tree removal.
Field trials conducted by researchers at the University of California and USDA-ARS have examined the genetic resistance of rootstock varieties, both experimental and commercial, to soil borne diseases. Here is a summary of their findings:
Available and experimental rootstocks vary widely in susceptibility to Phytophthora
Peach-almond hybrid rootstocks, such as Empyrean and Titan, are more susceptible to Phytophthora
Marianna 2624 rootstock offers field-tested tolerance to Armillaria but is not compatible with Nonpareil varieties
Peach-almond hybrids generally offer greater Prunus Replant Disease tolerance than peach rootstocks and may reduce the need for fumigation in some soils.
Another key component to establishing an orchard is proper planting and management practices that can make your rootstock less susceptible to soilborne diseases.
“Even a disease resistant rootstock, or a semi-resistant rootstock, can become susceptible to disease based on poor management practices,” said Jamie Ott, orchard crops advisor for UC Cooperative Extension. “You really want to give your rootstock a chance to do its job for you.”
In the case of Phytophthora, where pathogens move easily in water and the disease infects through the roots and crown, growers can greatly reduce the likelihood of infection by reducing water saturation at the crown. This can be achieved by placing the graft union above the soil line when planting and moving the irrigation sources (drip or micro sprinklers) away from the trunk of the tree once established.
Choose the rootstock with more vigor
If your site has no clear limitations, Roger Duncan, pomology advisor for UC Cooperative Extension, urges growers to go with the rootstock that has the highest vigor.
“The most important reason to choose a rootstock is because of the potential problem that you have in your orchard,” Duncan advised. “If you have a salt problem or if you have a nematode problem — the first thing you want to do is choose a rootstock that works well in those conditions. However, if you don’t have any problems, choose a high vigor rootstock.”
“High vigor rootstocks may help growers increase profitability. In UC Cooperative Extension trials, the most vigorous rootstocks are consistently the highest yielding, usually the most yield efficient and produce larger kernels. In a 2022 field trial, the difference between the highest performing rootstock and the lowest performing rootstock was a yield of 1,318 pounds per acre.
“The top producing rootstocks would have brought an extra $10,000 to $11,000 back to the grower per acre compared to the standard Nemaguard over the span of the six harvests we trialed, and more than $15,000 more than the bottom performing rootstock, Lovell” Duncan explained. “You can make a lot of money from planting the right rootstock at the very beginning.”
The research is clear: peach-almond hybrid rootstocks tend to be the most vigorous, peach rootstocks are in the middle, and plum or plum hybrids are the least vigorous. The most vigorous rootstocks can offer higher profits with little to no added expense, as long as conditions allow, Duncan noted.
“These benefits are free. This is not from applying different water. It’s not from different amounts of nitrogen or potassium. It’s just putting the right tree in the ground.”