Adapted from the article “Observations on Butte County Rootstock Trials” by Joe Connell, UC Farm Advisor, Butte County in the January 2014 Sacramento Valley Almond News.
Several rootstocks have been observed in various orchard situations with local growers in Butte County over the past ten years or so. The complete results of these trials can be found in past Annual Rootstock Project Reports to the Almond Board of California. The following summarizes the most useful results and conclusions.
Replants were planted in non-fumigated oak root fungus spots to gauge their compatibility with almond and survival when exposed to the fungus. ‘Nonpareil’ on ‘Empyrean 101’ rootstock has been observed in two orchards since 2004. Nine trees of ‘Nonpareil’ on ‘Krymsk 86’ were replanted in oak root fungus spots in commercial orchards in spring 2010 and ten trees on another rootstock were similarly planted in 2012.
Oak Root Fungus Tolerance
Nonpareil’ on ‘Empyrean 101’ rootstock is similar in size and vigor to nearby trees on ‘Marianna 2624’. Although growing in an oak root fungus spot since 2004, none of these trees succumbed to Armillaria mellea. However, four out of seven ‘Empyrean 101’ rooted trees are leaning compared to trees on ‘Marianna 2624’ so poor anchorage precludes the use of this rootstock for almonds.
Nine ‘Nonpareil’ trees on ‘Krymsk 86’ planted in spring 2010 in three different oak root fungus spots have established and grown well. In spring 2013 (4th leaf), all nine trees on ‘Krymsk 86’ continue to be healthy while a ‘Lovell’ rooted replant of similar age in one of the fungus spots died. Ten ‘Nonpareil’ trees on another rootstock being screened for potential Armillaria resistance were planted in an oak root fungus affected orchard in 2012. All trees established well and have so far remained healthy.
Performance on Fumigated Soil
In a replant disease fumigation trial planted in 2004, 20 single tree replicates of ‘Krymsk 86’, ‘Lovell’, ‘Marianna 2624’, and ‘Ishtara’ rootstocks were planted in both fumigated and non-fumigated tree sites. By 2011, ‘Krymsk 86’ trunk circumference was largest while ‘Lovell’ benefited most from fumigation. After eight years, 47 percent of the ‘Ishtara’ trees and 8 percent of the ‘Lovell’ rooted trees were leaning. There were no leaning trees on the ‘Krymsk 86‘rootstock. Both ‘Ishtara’ and the ‘Krymsk 86’ rootstocks had 5 percent of the trees missing while ‘Lovell’ rootstock had 10 percent missing. ‘Ishtara’ has been killed by oak root fungus in other fields and poor anchorage made this rootstock unacceptable for almonds.
Another replicated rootstock trial was planted in March 2010 following the removal of a ‘Lovell’ peach rooted orchard containing some plum rooted replants. ‘Nonpareil’ trees on ‘Rootpac-R’, ‘Atlas’, ‘Krymsk 86’, and ‘Empyrean 1’ rootstocks are compared to trees on the standard rootstocks ‘Nickels’ and ‘Lovell’. Four of the six rootstocks established well in the first growing season with no tree losses. ‘Atlas’ suffered 10% mortality at planting and ‘Nickels’ lost 16% of the new trees. After the third growing season, trees on the ‘Empyrean 1’ rootstock were largest in trunk circumference and those growing on ‘Krymsk 86’ were the smallest. 4th leaf ‘Nonpareil’ yields were heaviest in trees on ‘Empyrean 1’ rootstock and lightest on ‘Rootpac-R’ and ‘Krymsk 86’ rootstocks. Other rootstocks produced intermediate yields. Mortality and anchorage will be noted as opportunities arise.