Phytophthora-Diaprepes Weevil Complex: Phytophthora spp. Relationship with Citrus Rootstocks.

Diaprepes abbreviatus (Coleoptera: Curculionidae) is a root weevil, introduced from the Caribbean Basin into Florida in 1964. The larval stage feeds on fibrous and structural roots of citrus, predisposing the injured root system to infection and girdling by Phytophthora spp. In citrus orchards, the rootstocks trifoliate orange (Poncirus trifoliata) and hybrid 'Swingle' citrumelo (Citrus paradisi × P. trifoliata) are resistant to the complex of P. nicotianae with D. abbreviatus, while 'Cleopatra' mandarin (C. reticulata) is susceptible to this complex. When Phytophthora palmivora is coincident with P. nicotianae in fine-textured, poorly drained soils, Swingle citrumelo is more vulnerable to attack by the complex with P. palmivora than is Cleopatra mandarin. Infestation of 9-month-old seedlings with zero, two, or five neonate larvae resulted in a wide range of fibrous root and taproot damage of trifoliate orange and Cleopatra mandarin. Leakage of reducing sugars increased sharply as injury to the roots exceeded 75%. The relationship between feeding damage and root leakage was similar for the two rootstocks. Thus, reduced root damage was not based on host resistance to larval feeding. Root infection and rhizosphere populations of P. nicotianae were higher on Cleopatra mandarin than on trifoliate orange. Root rot by P. nicotianae did not increase with severity of feeding injury on either rootstock. Root infection and root rot by P. palmivora was more severe on trifoliate orange than on Cleopatra mandarin and increased with severity of larval damage. P. palmivora infected and rotted the taproot of both rootstocks if predisposed by larval feeding, but P. nicotianae did not. Commercial rootstocks are severely damaged by larvae of D. abbreviatus; therefore, tolerance of the Phytophthora-Diaprepes weevil complex should be based on resistance of rootstocks to each Phytophthora sp.