Characterization of Phytophthora citrophthora and P. inundata associated to foot and root rot of citrus trees in Chile

Abstract

Citrus production has increased considerably in Chile to about 17300 ha mainly planted with lemon, mandarin, and sweet orange. Although irrigation managements have improved and tolerant rootstocks have been commonly used, Phytophthora foot rot (gummosis) and root rot occur frequently. Symptoms included slow decline, moderate leaf chlorosis, reduced growth, lack of tree vigor and dieback. These symptoms were associated to extensive canker lesions and gummosis at the base of the trunk, and root rot, often extending from the main roots into the feeder roots. The frequent outbreaks of Phytophthora diseases in Chilean citrus groves motivated this survey to characterize the species of Phytophthora associated to foot and root rot. A total of 27 pathogenic isolates of Phytophthora were obtained in 2003 and 2004. Morphologically, 20 isolates were identified as P. citrophthora ; however, they were distinctively divided into sterile and fertile isolates. The latter were morphologically similar to P. citrophthora sensu lato but, they induced oospore production when pairing against P. cinnamomi A2. These fertile isolates exhibited a 98% match with several P. citrophthora strains and were considerably less aggressive than sterile isolates. Phytophthora inundata , a relatively new described species, was identified on the basis of analysis of the ITS regions of rDNA. Isolates of P. inundata were weakly pathogenic on citrus and, possibly were found as secondary pathogen without importance on foot and root rot development on citrus trees. Therefore, P. citrophthora was the cause of foot and root rot found on Chilean citrus groves, planted in temperate and relative dry weather conditions. The isolation and pathogenicity of P. citrophthora on Citrus macrophylla and Carrizo citrange , considered resistant rootstocks, could be suggesting that these isolates have overcome resistant genes associated with Phytophthora resistant citrus rootstocks. La produccion de citricos ha aumentado considerablemente en Chile, con alrededor de 17300 ha, principalmente destinadas a la produccion de limon, mandarin y naranjo. La pudricion del pie (gomosis) y la pudricion de las raices ocurren frecuentemente. Esto a pesar que el manejo del agua de riego ha mejorado y se han introducido portainjertos tolerantes. Los sintomas de esta enfermedad consisten en un lento y progresivo decaimiento, clorosis foliar moderada, menor crecimiento, escaso vigor y muerte parcial de ramas y ramillas. Estos sintomas estan asociados con la presencia de cancros y gomosis en la base del tronco y podredumbre radical, que compromete raices primarias y secundarias. La ocurrencia relativamente frecuente de focos de esta enfermedad en huertos citricos en la zona central de Chile, motivo esta investigacion que tuvo el proposito de caracterizar las especies de Phytophthora asociadas a este sindrome. Se obtuvo 27 aislamientos de Phytophthora entre 2003 y 2004. Morfologicamente, 20 aislamientos se identificaron como P. citrophthora entre los cuales se identificaron aislamientos esteriles y fertiles. Los aislamientos fertiles fueron morfologicamente indiferenciables de P. citrophthora sensu lato pero, desarrollaron oosporas al aparearlos con P. cinnamomi A2 y presentaron 98% de similitud al compararlos molecularmente con P. citrophthora . Los aislamientos fertiles fueron relativamente menos agresivos que los esteriles. Phytophthora inundata , corresponde a una especie recientemente descrita en el mundo, identificada sobre la base de la caracterizacion de la region ITS del rADN. Los aislamientos de P. inundata fueron debilmente patogenicos en citricos y posiblemente tienen secundaria importancia en el desarrollo de esta enfermedad. Por lo tanto, segun los resultados obtenidos en esta investigacion P. citrophthora es la causa de la pudricion del pie y de las raices en huertos citricos chilenos, generalmente plantados en zonas templadas y relativamente secas. El aislamiento y patogenicidad de P. citrophthora obtenida desde Citrus macrophylla y Carrizo citrange, considerados como portainjertos resistentes, sugiere una posible perdida de resistencia de estos portainjertos.