| Summary: | Citrus greasy spot, caused by <i>Zasmidium citri-griseum</i> ( = <i>Mycosphaerella citri</i>) is the most prevalent fungal disease of citrus in Panama, causing premature defoliation and yield reduction. Fungicide applications are generally needed for the control of greasy spot. In this study, the sensitivity to fenbuconazole of 34 isolates of <i>Z. citri-griseum</i> from Panama was determined by calculating the effective concentration needed to reduce mycelial growth by 50% (EC<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>50</mn></msub></semantics></math></inline-formula>). Two field trials were conducted from 2011 to 2013, to evaluate the efficacy of fenbuconazole to reduce disease severity and yield loss. The EC<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>50</mn></msub></semantics></math></inline-formula> values for fenbuconazole ranged from 0.09 to 7.62 <inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">μ</mi></semantics></math></inline-formula>g mL<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></semantics></math></inline-formula>, with a mean EC<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mn>50</mn></msub></semantics></math></inline-formula> value of 2.66 ± 0.36 SE <inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">μ</mi></semantics></math></inline-formula>g mL<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></semantics></math></inline-formula> for mycelial growth. These data can be used for monitoring sensitivity shifts in <i>Z. citri-griseum</i> to fenbuconazole and reduce risk of fungicide resistance in Panama. In the field trials, sprays with fenbuconazole significantly reduced (<i>p</i> < 0.0001) the severity of greasy spot on leaves compared with the non-treated control. Greater disease control was obtained when three sprays of fenbuconazole were applied instead of one. Nevertheless, no significant differences (<i>p</i> > 0.05) were detected in yield.
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