Fungicide resistance analysis of major tree fruit pathogens in Michigan

Chemical and/or antibiotic resistance management is critical in order to reduce the progression or complete loss of a product while effectively controlling plant pathogen populations. This dissertation focuses on the observation for control and resistance management of four major worldwide fruit tree pathogens: Venturia inaequalis (apple scab), Monilinia fructicola (American brown rot), Erwinia amylovora (Ea; fire blight) and Pseudomonas syringae pv. syringae ( Pss; bacterial canker). Chemical fungicides have been and are still heavily relied on for management of the fungal plant pathogens V. inaequalis and M. fructicola. Single-site fungicides such as the quinone-outside inhibitors (QoIs) and succinate dehydrogenase inhibitors (SDHIs) have shown to provide excellent control but can also select for resistant pathogen populations in a relatively short time period, as in the case for both V. inaequalis and M. fructicola. Adequate control of bacterial pathogens, such as Ea and Pss has been a challenge for pome and stone fruit growers for decades. Antibiotics have been intensively used for the management of Ea while only copper formulations are registered for Pss. The current research implicates that orchards in Michigan and in other states cannot further rely on QoI fungicides for apple scab control. Further investigation of peach and cherry orchards indicates that SDHI applications have shifted Michigan populations toward resistance. Results from studies performed with Ea and Pss indicate that non-chemical applications such as acibenzolar-S-methyl (ASM) and phosphate salts have a potential to reduce the use of antibiotics and copper for control in seasons when disease pressure is low to moderate.