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ARS Home » Pacific West Area » Corvallis, Oregon » Horticultural Crops Research Unit » Research » Publications at this Location » Publication #232220
Title: Effects of Mefenoxam, Phosphonate, and Paclobutrazol on In Vitro Characteristics of Phytophthora cactorum and P. citricola and on Canker Size of European Beech

Author
item Weiland, Gerald - Jerry
item NELSON, ANGELA - CORNELL UNIVERSITY
item HUDLER, GEORGE - CORNELL UNIVERSITY

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2009
Publication Date: 7/1/2009
Citation: Weiland, G.E., Nelson, A., Hudler, G. 2009. Effects of mefenoxam, phosphonate, and paclobutrazol on in vitro characteristics of Phytophthora cactorum and P. citricola and on canker size of European Beech. Plant Disease. 93(7):741-746.

Interpretive Summary: Phytophthora citricola and P. cactorum are pathogens that cause cankers (regions of dead bark), which eventually kill mature, and often historic, specimens of European beech in the northeastern United States. Because of the economic value placed on these trees, experiments were conducted to investigate the effects of two fungicides and a plant growth regulator on pathogen development in petri plates and on canker expansion in saplings. In the first experiment, sixteen pathogen isolates (11 P. citricola and 5 P. cactorum) were compared on a growth medium in petri plates amended with increasing concentrations of: 1) the fungicide mefenoxam (Subdue MAXX); 2) the fungicide phosphorous acid (Agri-Fos 400) either with or without a bark penetrant (Pentra-bark – to help phosphorous acid penetrate into tree bark); or 3) the bark penetrant alone. Increasing the concentrations of both fungicides and the bark penetrant alone led to decreased growth, spore production, and spore germination. It was also found that the growth of all tested P. cactorum isolates was less sensitive to phosphorous acid but more sensitive to mefenoxam than P. citricola isolates. In the second experiment, bark and soil drenches of both fungicides and a soil drench of the plant growth regulator paclobutrazol (Cambistat) were evaluated for their ability to slow or stop canker expansion (curative treatments) in inoculated trees and for their ability to prevent infection (preventative treatments) by both pathogens. Curative treatments were applied one month after inoculation of beech saplings with a single isolate of either P. cactorum or P. citricola. One year later, the same treatments were applied as a preventative two weeks prior to inoculation. None of the treatments (curative or preventative) were able to stop canker expansion or prevent infection. However, saplings that were treated with the phosphorous acid bark drench usually had shorter cankers than those measured from the other treatments.

Technical Abstract: Phytophthora citricola and P. cactorum cause bleeding cankers that lead to the death of mature European beech in the northeastern United States. Because of the economic value placed on these trees, experiments were conducted to investigate the effects of two fungicides and a plant growth regulator on in vitro pathogen development and canker expansion. In the first experiment, sixteen single-spore isolates (11 P. citricola and 5 P. cactorum) were compared on clarified V8 juice (CV8) agar amended with: 1) 0, 0.1, 1, 5, 10, and 20 µg ai/ml of mefenoxam (Subdue MAXX); 2) 0, 1, 5, 10, 50, 100, 300, 500, 1000, 1600, 3090, and 301,725 µg ai/ml phosphorous acid (Agri-Fos 400) either with or without a bark penetrant (Pentra-bark) at 0.5 mg ai/ml; or 3) 0, 0.5, 2.5, 5, 10, and 25 mg ai/ml of the bark penetrant. Pathogen growth, oospore production, and zoospore germination were evaluated and found to be dependent on isolate and treatment. A species effect on growth was also observed as P. cactorum isolates were 2.5 – 7x less sensitive to phosphorous acid, but 2 – 150x more sensitive to mefenoxam than P. citricola isolates (based on EC50 values). No species effect was observed for either oospore production or zoospore germination. In the second experiment, bark and soil drenches of mefenoxam (5000 and 20 µg ai/ml, respectively) or phosphorous acid + bark penetrant (301,725 and 100 µg ai/ml, respectively), or a soil drench of paclobutrazol (Cambistat at 20 mg ai/ml) were evaluated for their efficacy as curative or preventative treatments. Curative treatments were applied one month after inoculation of beech saplings with a single-spore isolate of either P. cactorum or P. citricola. One year later, the same treatments were applied as a preventative two weeks prior to inoculation. None of the treatments (curative or preventative) were able to stop canker expansion or prevent infection. However, saplings that were treated with the phosphorous acid bark drench usually had significantly shorter cankers than those from the other treatments.