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Alternaria Leaf Spot, one of the major fungal diseases of pistachio trees, is caused by different species of the genus Alternaria. This disease can exert significant negative impacts on the growth and yield of pistachio trees. Alternaria leaf spot can spread rapidly in susceptible areas with high humidity and, if not properly managed, may result in severe yield losses. To address this issue, the efficacy of the systemic fungicide Signum (pyraclostrobin + boscalid) was evaluated as the proposed treatment, in comparison with the fungicides Tilt (propiconazole), Elite (fosetyl-aluminium), and Mishocap (copper oxychloride), under laboratory, greenhouse, and orchard conditions in the provinces of Kerman (Soughan, Orzueeyeh), Yazd (Ardakan), and Semnan (Damghan). Laboratory assessments indicated that all fungicides significantly reduced mycelial growth, with the highest efficacy observed in the following order: Signum, Tilt, Elite, and Mishocap. Fungicide concentrations, Alternaria species, and their interactions had significant effects on mycelial growth and inhibition. Among these factors, fungicide concentrations were the most influential, although species differences and their interactions with fungicide concentrations also played a considerable role. Greenhouse evaluations demonstrated that Signum, at 0.5 g L⁻¹, achieved 95.8% efficacy, and at 1 g L⁻¹, 98.8% efficacy, making it the most effective treatment in controlling the disease. Elite and Tilt exhibited moderate efficacy, whereas Mishocap showed the lowest efficacy. The untreated control exhibited a disease severity of 69%, which was significantly higher compared to all fungicide treatments. Field experiments revealed that Signum, at concentrations of 0.5 and 1 g L⁻¹, achieved the highest efficacy in controlling Alternaria leaf spot, with control rates ranging from 67% to 77%. Tilt, Elite, and Mishocap followed, respectively, in descending order of efficacy. Analysis of variance indicated that treatment and location had significant effects on disease severity, while their interactions were not significant. In all trials, the control treatment consistently exhibited the highest disease severity. Overall, the results of this study indicate that the application of Signum at 0.5 g L⁻¹ during early May and June is recommended for the management of Alternaria leaf spot in pistachio orchards. Furthermore, Elite, which has been previously registered for the management of Phytophthora root rot in pistachio orchards, can be applied in rotation with Signum as part of an integrated disease management program.
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