Efficacy of sodium tetrathiocarbonate and propiconazole in managing Armillaria root rot of almond on peach rootstock

The efficacy of sodium tetrathiocarbonate (STTC or Enzone 31.8%, a liquid f ormulation that releases carbon disulfide) and the demethylation inhibiting (DMI) fungicide propiconazole (Alamo 1.1EC) was evaluated for management o f Armillaria root rot of almond grown on Lovell peach rootstock. After 12 m onths, pre-/post-plant STTC (189 liters of 3,850 and 500 mg/liter/3 m(2) tr eatment site, respectively) or tarped pre-plant methyl bromide (Dowfume 98% , 454 g a.i./3 m(2)) soil fumigation treatments significantly (P < 0.05) re duced the recovery of Armillaria mellea from naturally infected root segmen ts at 0.3 and 1.2 m soil depths, compared with control sites. Tarped methyl bromide treatments eradicated the fungus from infested root segments at bo th depths; however, nontarped sites significantly reduced the recovery only at the 1.2 m depth. Pre-plant STTC (189 liters of 3,850 mg/liter/3 m(2)) r educed the recovery of the fungus but was not as effective as the pre-/post -plant STTC treatment. Recovery of the fungus in post-plant treatments with STTC (189 liters of 500 mg/liter/3 m(2)) was not significantly (P > 0.05) different from control sites. Additionally, mortality of almond tree replan ts from phytotoxicity was significantly higher in post-plant applications o f STTC, compared with the other treatments or with the control trees. A gel formulation of 31.8% STTC (1,800 mi of 318,000 mg of STTC per liter per tr ee stump) applied in wells that were drilled into tree stumps eradicated th e fungus from trunk and primary roots but not secondary or tertiary roots, whereas liquid formulations of STTC and metam-sodium (Vapam - 32.7% sodium N-methyldithiocarbamate) eradicated the fungus from only trunks but not roo ts. Treatments with STTC (189 liters of 500 mg/liter/3 m(2)) were not effec tive in preventing mortality of A. mellea-infected, mature, 7- to 8-year-ol d almond trees. Propiconazole (Alamo 1.1EC) was shown to be toxic to myceli al growth of A. mellea grown on potato dextrose agar with an EC50 value of 0.15 mg/liter. Therapeutic, passive injections of propiconazole into 7- to 8-year-old almond trees (Lovell peach rootstock) were successful in reducin g mortality of infected trees during two growing seasons, compared with inf ected control trees treated with water.