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Armillaria root rot

ARRCultural management of Armillaria root rot (ARR). This is by far the most effective approach compared to chemical or biological options. 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 a ‘prototype’ study published in Plant Disease, we demonstrate the potential of above-ground root collar excavation for ARR management. 

Chemical 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).

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.

Biological control.  Trichoderma species have been suggested for the control of ARR. In a published paper (Cristinzio 2003) activity against oak root rot of peach was documented in Italy. Seven years after setting up the field experiment, the Remedier treated trees had 2 % disease incidence compared to 12% in the untreated control. However, the disease pressure (12%) did not seem to be high in this study and it is not known whether the product will hold up against ARR, which is caused by a different species of Armillaria.

We conducted two field experiments in 2007. In this study, Trichoderma asperellum and T. gamsii formulated as Remedier WP were drenched onto peach trees 3 to 12 days after planting (2007), and biannually thereafter in spring and fall for a total of three years in two commercial replant sites of South Carolina. All trees were planted in spots where a tree had declined from ARR the previous season to maximize disease pressure. Tree survival and trunk diameter were determined each year in the control and Remedier WP treatments. Four years after planting (2011), 50% of all control trees and Remedier WP-treated trees had died from ARR. There was no statistical significance in survival between the treatments in either location. However, three and four years after planting, surviving Remedier WP-treated trees had significantly larger tree trunks compared to control trees in the Campobello location. Not enough trees survived in the other location for meaningful analysis of tree trunk diameter data. Our results show that in soils with heavy ARR inoculum levels biannual drenches of Trichoderma formulated as Remedier WP starting at planting are ineffective for ARR control of peach. This work was published.