Effects of systemic and contact fungicides on life stages and symptom expression of Phytophthora ramorum in vitro and in planta

Nine isolates of Phytophthora ramorum Werres, de Cock & Man in't Veld were screened using a variety of systemic and contact fungicides in vitro for mycelial growth inhibition and zoospore germination inhibition, and in planta for suppression of lesion expansion on rhododendron foliage. Three isolates from each of the major clonal lineages, NA1, NA2, and EU1 were used. Systemic fungicides were the most effective at preventing mycelial growth and zoospore germination of P. ramorum, and the results from testing on host plants at the labeled rate supported the in vitro results. Development of resistance to some chemicals used for routine control of P. ramorum in the nursery should be monitored, especially in the EU1 and NA2 populations. Metalaxyl-M had the lowest EC50 for both mycelial growth inhibition and zoospore germination inhibition for all isolates. EC50 was higher for zoospore germination inhibition of the EU1 isolates by two strobilurin fungicides, indicating possible cross-resistance in this group.