Effect of citrus leaf-miner damage, mechanical damage and inoculum concentration on severity of symptoms of Asiatic citrus canker in Tahiti lime

Abstract

Citriculture in São Paulo State, Brazil, is threatened by Asiatic citrus canker ( Xanthomonas axonopodis pv. citri). The introduction of the Asian citrus leaf miner ( Phyllocnistis citrella [CLM]) has resulted in an increase in the number of disease foci and has changed the spatial pattern of citrus canker symptomatic trees from strong aggregation to intermediate aggregation and random patterns. We evaluated the effect of inoculum concentration (10 1, 10 2, 10 4 and 10 6 colony forming units (cfu) ml −1) on infection of leaves of Tahiti lime ( Citrus latifolia) with three distinct treatments: (1) intact leaves, (2) mechanically wounded leaves, and (3) leaves exhibiting injuries from CLM at the egg stage, first instar, third instar, and pupal stages. The minimum inoculum concentration to cause symptom development in intact leaves was 10 4 cfu ml −1; in mechanically wounded leaves and in leaves with CLM injury at the egg stage and first instar stage, 10 2 cfu ml −1; in leaves with CLM injuries at the third instar and pupa stage, 10 1 cfu ml −1. The injuries from the third instar and pupa stages resulted in greater disease severity than other treatments at all inoculum concentrations (five times higher than in the intact leaf). Disease severity in leaves with mechanical wounding and with CLM injuries caused at the egg and first instar stages did not differ from intact leaves.