Effects of Mefenoxam, Phosphonate, and Paclobutrazol on In Vitro Characteristics of Phytophthora cactorum and P. citricola and on Canker Size of European Beech.

Phytophthora citricola and P. cactorum cause bleeding cankers that lead to the death of mature European beech (Fagus sylvatica) in the northeastern United States. The effects of two fungicides and a plant growth regulator on in vitro pathogen characteristics and on canker expansion were investigated. In the first experiment, 16 single-spore isolates (11 P. citricola and 5 P. cactorum) were grown on clarified V8 juice agar amended with (i) 0 to 20 μg a.i./ml of mefenoxam, (ii) 0 to 301,429 μg a.i./ml phosphonate either with or without a bark-penetrating surfactant at 0.5 mg a.i./ml, or (iii) 0 to 25 mg a.i./ml of the surfactant alone. Radial growth, oospore production, and zoospore germination were observed to be dependent on isolate and treatment. A species effect on growth was also observed, as P. cactorum isolates were 2.5- to sevenfold less sensitive to phosphonate, but 2- to 150-fold more sensitive to mefenoxam than P. citricola isolates (based on 50% inhibition of growth). In the second experiment, bark and soil drenches of mefenoxam (50 mg a.i./ml and 19 μg a.i./ml, respectively), phosphonate (301,429 and 101 μg a.i./ml, respectively), and a soil drench of paclobutrazol (21 mg a.i./ml) were evaluated for their efficacy as curative or preventive treatments against bleeding canker. None of the treatments (curative or preventive) were able to stop canker expansion or prevent infection. However, saplings inoculated with P. citricola and treated with the phosphonate bark drench as either a curative or preventive treatment had cankers that were 36 to 82% shorter than those of inoculated control stems treated with water. For saplings inoculated with P. cactorum, the phosphonate bark drench was only effective when applied as a preventive (38% shorter than inoculated control stems treated with water), and not as a curative treatment. No other treatment was effective at limiting canker expansion.