The action of allicin against Armillaria spp. in vitro

The effect of allicin (a stabilized garlic extract product) at five different concentrations (0, 20, 30, 50 and 100 mg/l) was studied in vitro on the growth rate of 100 isolates of Armillaria gallica and A. mellea. Isolates were obtained from 41 host genera growing in gardens located in 39 counties in the United Kingdom. Agar plugs of the actively growing Armillaria isolates were added to the centre of malt agar plates infused with allicin, and radial mycelium growth was measured on days 7, 14 and 21. The total number of rhizomorphs and length of rhizomorphs were also measured. Relative growth rates were calculated as the growth rate relative to the controls (0 mg/l). The relative growth of each isolate at each allicin concentration was used to estimate EC50 values for A. mellea and A. gallica populations as well as individual isolates. EC50 values for both Armillaria spp. increased over time. The mean EC50 values for A. mellea of 16.0, 26.4 and 102 mg/l (days 7, 14 and 21, respectively) were higher than those for A. gallica (8.8, 7.9 and 11.0 mg/l) and probably relate to the more aggressive nature of A. mellea. Isolates with higher EC50 values were also more likely to produce more rhizomorphs. At allicin concentrations of 20 and 30 mg/l, the production of rhizomorphs and the growth rates of A. mellea isolates were stimulated, when compared to the control treatments. From this study's findings, it appears that the field use potential of allicin is limited, due to better inhibition of the less virulent A. gallica, than the more aggressive A. mellea.