Fruit susceptibility and critical period for protection of Persian lime against citrus canker with copper sprays

In this study, we investigated the susceptibility period of Persian lime ( Citrus × latifolia Tanaka) to Xanthomonas citri subsp. citri associated with fruit diameter and assessed the effectiveness of an optimized copper spray program for the control of citrus canker. All experiments were carried out in a commercial orchard located in São Paulo state, Brazil. The susceptibility was assessed by spray-inoculating X. citri on fruit with 1–5 cm in diameter. Complementary, the fruit size of different blooms was monitored in spring–summer, summer-autumn, and winter-spring. The efficacy of copper sprays was assessed during two consecutive seasons by comparing the incidence of leaves and harvested fruits with citrus canker in treated and non-treated plots. Results showed that fruits were highly, moderately, and less susceptible to X. citri until 2.9 cm, 3.9 cm, and 4.9 cm, respectively. Fruits with ≥ 5 cm were resistant. Over 70% of fruits with 1 to 2.9 cm developed citrus canker lesions. The average number of lesions on these fruits was ≥ 3.4. In contrast, the incidence and severity of the disease remained as low as 1.1% and 0.3 in fruits with ≥ 5 cm. Regression models estimated that fruits took 82 days and 126 days to reach 4 cm and 5 cm in spring to late autumn, when weather is favorable for disease outbreaks. Citrus canker was efficiently controlled during this period using optimized rates and spray volumes of copper. In non-treated trees, the AUDPC* for the incidence of affected leaves and fruits in the two seasons ranged from 12.2 to 24.8%. By contrast, the AUDPC* did not exceed 2.1% in copper-treated plots.