EVALUATION OF ROOTSTOCKS FOR TOLERANCE TO BACTERIAL CANKER, ORCHARD REPLANT CONDITIONS AND SIZE-CONTROLLING IN CALIFORNIA

Abstract

The objective of this project was to select rootstocks that are tolerant or resistant to the canker disease caused by the bacterium Pseudomonas syringae. In February 2000, twenty-three rootstocks, including the two controls, Nemaguard (susceptible) and Lovell (tolerant), grafted with the 'Ross' scion were planted in a commercial orchard near Escalon, CA. This orchard had very coarse, sandy soil and a history of bacterial canker problems. Symptoms of canker started to show on some of the rootstocks in 2003. Generally, the symptoms started to be apparent in late March and were more pronounced in mid April. Tree death due to canker disease and ratings of springtime canker infections from 2003 through 2009 indicated that trees on several rootstocks were not significantly affected by bacterial canker. These rootstocks were Guardian, Viking, P. mira, Flordaguard, HBOK 15, HBOK 1, and HBOK 17. Data from samples of soils and roots, from which live ring (Criconemoides xenoplax), root-knot (Meloidogyne sp.) and root-lesion (Pratylenchus vulnus) nematodes were extracted, showed that the highest numbers of ring nematodes were found on Nickels, K119-50, Hansen 536, Hiawatha, P. subhirtella, P30-135, Nemaguard and K146-43 rootstocks. These data, generally, correlated with the data of tree mortality, i.e., rootstocks that harbored high numbers of ring nematodes also showed high numbers of tree mortality. Dormant pruning weights indicated that growth of trees on K146-43, St. Anthony, P30-135, K119-50, Weeping Peach and Compass were the most size controlling rootstocks. Trees on Hansen 536, P. ferganensis, HBOK 15, HBOK 1, HBOK 17 and Nickels were similar to, or larger than the Nemaguard control. St. Anthony, Cadaman, Guardian and the Nemaguard control had the highest number of suckers (2.3 to 4.1 suckers per tree). The rest of the rootstocks had zero to one sucker per tree. Trees on the majority of the rootstocks tested were similar to the control with regard to the traditional measure of yield efficiency (Yield/Trunk Sectional Area). Yield efficiencies of trees on the Weeping peach, HBOK 1, Cadaman, HBOK 17, Atlas, K119-50, K146-43, and Lovell rootstocks were higher than the control. Trees on the Nickels rootstock had lower efficiency than the control. In general, all tested size controlling rootstocks were susceptible to bacterial canker.