Chemical Control of Armillaria Root Rot

infusionChemical management of Armillaria root rot. Chemical management of Armillaria root rot has met with limited success because of the protected nature of Armillaria inoculum (i.e., being encased underneath the bark of roots and surrounded by soil). The majority of research on fumigation and soil drenches with chemicals has been inconclusive or conducted with insufficient field testing. Injections of systemic fungicides have previously been shown to reduce the mortality of Armillaria infected trees. For example, post-plant passive injections of propiconazole into 7 to 8-year old infected almond trees grown on peach rootstocks reduced Armillaria-induced mortality compared with controls over a 2-year period. This precision approach to chemical management of Armillaria has several advantages: 1) low or reduced-risk fungicides could be used; 2) off-target environmental impact compared to conventional air-blast spraying is virtually eliminated (i.e. fungicides would remain within woody tree tissues); 3) because of Armillaria’s vegetative life history strategy (reproduction events rarely occur), this technology could be safely used with fungicides otherwise at risk for resistance development.
The triazole fungicide propiconazole is effective against the Armillaria species attacking peach in the Southeast. Similar to what was reported for A. mellea (Adaskaveg et al. 1999), we found that propiconazole has a strong inhibitory effect on the mycelial growth of A. tabscens. EC50 values for A. tabescens ranged from 0.45 to 0.49 ppm. Propiconazole is registered for tree injection under the tradename ‘Alamo’. Our research has shown that spring and fall infusion of Alamo into the lower part of the tree trunk results in significant accumulation of propiconazole in the primary roots (Amiri et al. 2008; pdf).

Trunk infusion of peach with propiconazole has been suggested as a potential strategy to protect roots from colonization by Armillaria species. We investigated the persistence of propiconazole in peach roots following fall infusions and its potential for Armillaria root rot (ARR) control in a commercial peach orchard. Four 12-year-old trees were infused with either 2 liters of a propiconazole solution at 0.4 mg/ml or water in September 2007. Bark tissue collected from primary roots 48 h, 6 and 12 months after infusion and analyzed using gas chromatography mass spectroscopy contained 6.4, 1.4, and 0.9 µg propiconazole/g fresh bark in propiconazole infused trees and 0.1, 0, and 0 µg in water-infused trees, respectively. Asymptomatic trees bordering trees that had died from ARR (protective treatment) and symptomatic but alive trees (curative treatment) were infused with 1 liter of a propiconazole solution (0.4 mg/ml) in spring and fall of 2008 and 2009 at a commercial peach orchard planted in 2005. Trees were rated for ARR disease severity 6, 12, 24 and 36 months post-infusion. In the protective treatment, the survival rate in both non-infused and infused trees was 80% 36 months post-infusion. However, more (60%) of the propiconazole-infused trees remained asymptomatic compared to the control trees (30%). A significant difference was observed between the non-infused and propiconazole-infused trees following curative treatment. None of the control trees were alive after 36 months compared to 40% of propiconazole-infused trees. We conclude that fall infusion of peripheral trees of infection centers with propiconazole can slow the expansion of ARR infection centers in commercial peach orchards. This research has not been published yet.