92 Percival and Graham: Inducing Agents and Fungicide Combinations for Pathogen Management protein and salicylic acid reduced disease symptoms by 25% to 58%, whereas liquid chitosan reduced dis- ease symptoms by 10% to 18%. These results indicate that IR agent efficacy may differ between products. The results of the present study were achieved under severe inoculum pressure, as a monoculture planting existed during field trials. Under these con- ditions, the application of a synthetic fungicide at full strength in combination with an IR agent in general provided a greater reduction in disease symptomol- ogy (2% to 20%) than application of a synthetic fun- gicide at full strength. However, this reduction was rarely significant at P < 0.05. Such a small increase in efficacy raises a dilemma. On one hand, it is unlikely to prove cost-effective to combine products. On the other, both of the fungicides used in our studies have specific modes of action and as such may lead to the rise of fungicide-resistant strains of pathogens (Gozzo 2003; Vallad and Goodman 2004; Fobert and Després 2005; Witzell and Martin 2008). As a consequence, their use requires a resistance management strategy, such as tank mixing or alternating with other fungi- cides. Combining a fungicide with an IR agent that possesses different modes of action may therefore help reduce the build-up of pathogen resistance and prolong the commercial life of an existing fungicide before it is withdrawn. Within Europe, loss of regis- tered commercial fungicides is of concern, as a stage has now been reached where fungicide withdrawal from commercial use exceeds the registration of new fungicides, placing even greater selection pressure on those fungicides still available for use (BCPC 2019). One of the key results of this investigation is that in virtually all studies, application of a fungicide at 2/3 strength plus IR agent provided the same degree of pathogen control as a fungicide at full strength. This suggests that IR agents offer the potential to control foliar pathogens using a reduced input of synthetic fungicide. Previous research using a combination of fungicides and IR agents against pathogens of crop plants supports this finding. For example, a mixture of a strobilurin fungicide and ASM was shown to be effective in controlling Albugo occidentalis and increasing leaf quality in spinach (Leskovar and Kolenda 2002), while a mixture of ASM and manco- zeb was shown to have potential to provide protection against Claviceps africana on sorghum, especially where fungal isolates resistant to the usual fungicide treatment, triadimenol, were present (Ryley et al. ©2021 International Society of Arboriculture 2003). Gent and Schwartz (2005) found that integra- tion of ASM and biological control agents with cop- per hydroxide could be used to replace less desirable fungicides without compromising effective control of Xanthomonas leaf blight on onion. Work by LaMon- dia (2009) demonstrated that the addition of the IR agent ASM was effective in a spray program that reduced lesions per plant by up to 99% compared to the non-treated control against blue mould of tobacco. Similar results have been reported by other researchers (Cole 1999; Perez et al. 2003). In addition, LaMondia (2009) also found that substituting 2 or 3 applications of ASM for the fungicides dimethomorph + mancozeb treatments in a spray program decreased blue mould lesions compared with fungicide-only treatments by 28% to 94%. In addition, systemic activity was con- sidered very desirable, as spray coverage was difficult to achieve in shade tobacco (LaMondia and Horvath 2001). In addition, as the mode of action of IR agents is indirect, selection pressure on pathogens is not exerted, and thus insensitivity does not arise, and no fungicide residues would exist within plants. This is important when using fungicides against foliar dis- eases of fruit bearing crops, i.e., the use of IR agents has little, if any, direct antimicrobial activity, presents no known human toxicity issues, and is environmen- tally benign in comparison to pesticide alternatives (Vallad and Goodman 2004; Walters 2009; Eyles et al. 2010). CONCLUSION In conclusion, 5 independent field experiments per- formed over 3 years with 1 or 2 experiments per year show that applications of IR agents alone are useful in reducing the symptomology of 2 foliar pathogens frequently encountered in urban landscapes. Combi- nations of an IR agent with as little as a 1/3 reduced dose of fungicide still significantly reduced the sever- ity of symptoms on both leaves and fruit. This suggests that an IR agent plus a reduced dose of fungicide offers a way to reduce fungicide use and may also reduce the risk of fungicide resistant pathogen strains developing by decreasing fungicide selection pres- sure. These findings would be useful to professionals involved in urban landscape management, as they indicate synthetic fungicide usage can be reduced, but adequate control retained, with the additional advantage that IR agents in general cost 40% to 80% less than conventional fungicides.
March 2021
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