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Degree

Doctor of Philosophy, Ohio State University, Plant Pathology, 2006.

Abstract

Efficacy and physical mode of action (pre- and post-infection activity) of azoxystrobin, pyraclostrobin, mefenoxam, and phosphorous acid for control of strawberry leather rot (LR), caused by Phytophthora cactorum, was evaluated on a commercial farm and under greenhouse conditions. In addition, the distribution of the sensitivity of P. cactorum isolates to azoxystrobin was determined in vitro. In field trials all fungicide treatments had significantly less LR than the untreated control and there were no significant differences in LR incidence among any fungicide treatments during both years of testing. To evaluate the physical mode of action of the previously mentioned fungicides strawberry plants (‘Honeoye’) and attached fruits were sprayed with fungicides at 2, 4 and 7 days before inoculation with a zoospore suspension or 13, 24, 36, or 48 hours after inoculation. Azoxystrobin and pyraclostrobin provided protectant activity for up to 7 days, but only slight curative activity when applied 13 hours after inoculation. Phosphorous acid and mefenoxam also provided protection for up to 7 days and curative activity of at least 36 hours. No significant differences in protectant activity between tested fungicides were observed. Sensitivity distributions of P. cactorum isolates, causing crown rot (CR) and LR, to azoxystrobin were determined based on mycelial growth and zoospore germination. Radial growth of mycelia on lima bean agar amended with 0.001, 0.01, 0.1, 1.0, 10 and 30 ìg/mL of azoxystrobin plus 100 ìg/mL of salicylhydroxamic acid (SHAM) was measured after 6 days. Effects on zoospore germination were evaluated in aqueous solutions of 0.005, 0.01, 0.05, 0.1, 0.5, and 1.0 ìg/mL of azoxystrobin after 4 h of incubation in 96-well microtiter plates. Effective dose (ED50) for mycelial growth ranged from 0.16 to 12.52 ìg/mL for LR isolates, and 0.1 to 15 ìg/mL for CR isolates. The Kolmogorov-Smirnov test showed significant differences (P<0.001) between the two distributions. Estimated ED50 values for zoospores ranged from 0.01 to 0.24 ìg/mL. Sensitivities of mycelia and zoospores were not strongly correlated (r=0.31). The sensitivity distribution obtained in these studies could be used as a baseline sensitivity for monitoring fungicide resistance development in P. cactorum to this fungicide.

Subject Headings

Agriculture, Plant Pathology

Keywords

Physical mode of action; Fungicide sensitivy; Plant disease management; Strobilurins

Advisor

Michael A Ellis

Pages

136p.

Document number: osu1155221342
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=osu1155221342

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