Cultural Management of Armillaria Root Rot

RCECultural management of Armillaria root rot (ARR). Several studies have demonstrated that partial excavation of the crown of trees to expose primary roots to air and solar heating (collar excavation) can reduce Armillaria colonization and prolong the productivity of infected fruit trees. For example, in an orchard survey collar excavation prevented tree mortality at 35 sites in an 8-year-old citrus plantation infested with Armillaria for over 11 years. Since cambial temperatures and moisture contents of exposed roots never reach levels reported to directly kill Armillaria in vitro, they suggested that Armillaria fails to colonize the exposed crown of infected trees in the field due to a continuous effect of mild heating and drying.

Root collar excavation (RCE) has been applied to established citrus trees and grapevines for Armillaria root rot (ARR) control but despite its demonstrated effectiveness this cultural management system is not routinely used for ARR protection in disease-infested replant sites. One major drawback is the difficulty of excavating the below-ground root collar, the potential of excavated roots to be covered again with surrounding soil and the associated labor cost. We studied a new cultural method was investigated that resulted in trees with above-ground excavated root collars potentially eliminating many of the drawbacks. Experimental peach trees were planted in two commercial orchards (designated Landrum and Monetta) in South Carolina (SC); each tree replaced one that had declined from ARR disease the year before. Trees were planted approximately 40 cm higher than normal in open bottom Smart Pots and root collars were excavated above ground level 8 months later. Five years after planting 30 and 70% of all control trees (planted according to grower standard) had declined from ARR disease in Landrum and Monetta, respectively, whereas only 0 and 10%, respectively of trees in the above-ground root collar excavation (AG-RCE) treatment had declined. The difference in disease pressure between the two locations could not be attributed to differences in nematode pressure. Non excavated trees in Smart Pots revealed significantly less tree decline (P < 0.05) compared to the control but tree decline in both locations was greater compared to the AG-RCE treatment (P < 0.05). Trees in the AG-RCE treatment were as vigorous as the controls but produced more root suckers. In this ‘prototype’ study, we demonstrate the potential of above-ground root collar excavation for ARR management. Its potential for commercial use is discussed. A manuscript is in press in the journal 'Plant Disease'.