Genetic variation and host specificity of Phytophthora citrophthora isolates causing branch cankers in Clementine trees in Spain

Abstract

Considerable tree losses have been observed during the past few years in Spain due to Phytophthora branch canker of clementines caused by Phytophthora citrophthora. The emergence of this disease led to the speculation that either the pathogen has evolved increasing its aggressiveness or specificity to clementines. A total of 134 isolates of P. citrophthora collected from 2003 to 2005 in 135 citrus orchards in Spain and 22 reference isolates were analyzed genotypically and phenotypically to determine the structure of the population. Genotypic diversity was evaluated by means of Inter-Simple Sequence Repeat (ISSR) markers. Among the phenotypic characteristics examined, sporangial characters, sexual behavior, growth rates and colony morphology of the isolates at different temperatures were studied. The aggressiveness and host-specificity of selected isolates were evaluated by pathogenicity tests on sweet oranges and clementines under field conditions. Phytophthora branch canker of clementines was associated mainly with one genotype (P-1), which included 88% of the isolates obtained from branches. Strains isolated years before the first disease outbreak clustered also with this major genotype, thus it may be considered as a predominant population. Thirteen other minor genotypes were determined, but most contained only one isolate. Although there was wide variation in the morphological and physiological characters, all Phytophthora isolates obtained from branch cankers were sexually sterile and showed a characteristic petalloid colony pattern. As in previous greenhouse studies, pathogenicity tests under field conditions demonstrated that clementines and their hybrids were more susceptible to P. citrophthora than sweet oranges. However, no evidence was found to support the hypothesis that the emergence of the disease was associated with more aggressive or host-specific forms of P. citrophthora.

Phytophtho citophthrotrunk and branch canker on Clementine mandarins in the Western Cape province of South Africa

“…and four out of seven citrus nurseries surveyed were contaminated with the pathogen (Wehner et al, 1986). Snails and ants can also serve as dispersal vectors of Phytophthora citrophthora (Alvarez et al, 2009). In South Africa, brown snails [Helix aspersa (Müller)] and dune snails [Theba pisana (Müller)], were identified in Clementine mandarin trees and their role as possible vectors are proposed as targets for future research.…”

Two clonal lineages of Phytophthora citrophthora from citrus in South Africa represent a single phylogenetic species

“…These two putative taxa were reported to be reproductively and geographically isolated (Cohen et al 2003). Alvarez et al (2011) studied a collection of 134 P. citrophthora isolates from Spain with the same RAMS markers as Cohen et al (2003) and found 14 different genotypes (P-1-P-14) of which P-1, P-9 and P-12 respectively corresponded to G1, G3 and G2 of Cohen et al (2003). Multivariate analyses of morphological and physiological characteristics of isolates from the 14 genotypes indicated that P-12 (G2) isolates clustered separately from the remainder of the P. citrophthora genotypes, adding further support to the suggestion that the G2 group of Cohen et al (2003) represents a distinct species.…”

“…Phytophthora citrophthora (Smith and Smith) Leonian is a serious pathogen of citrus worldwide, causing root rot, trunk and branch cankers and brown rot of citrus fruit (Erwin and Ribeiro 1996, Maseko and Coutinho 2002, Dirac et al 2003, Schutte and Botha 2010, Alvarez et al 2011. Phylogenetically the pathogen fits into Phytophthora clade 2a sensu Blair et al (2008) based on the analyses of several nuclear (internal transcribed spacers [ITS], b-tubulin, 28S ribosomal DNA, 60S ribosomal protein, enolase, tigA, heatshock protein 90 and elongation factor 1-a) and mitochondrial (cytochrome c oxidase [COX] 1 and 2 and NADH dehydrogenase subunit 1) regions (Cooke et al 2000, Kroon et al 2004, Blair et al 2008.…”

“…Phytophthora citrophthora studies focusing on citrusassociated isolates reported low genetic variation among isolates and the presence of only a few genotypes (Cohen et al 2003, Alvarez et al 2011. Cohen et al (2003) studied 114 P. citrophthora isolates collected from citrus trees in eastern Corsica with restriction fragment length polymorphisms (RFLPs), internal transcribed spacer (ITS) sequences and random amplified microsatellites (RAMS).…”

“…Multivariate analyses of morphological and physiological characteristics of isolates from the 14 genotypes indicated that P-12 (G2) isolates clustered separately from the remainder of the P. citrophthora genotypes, adding further support to the suggestion that the G2 group of Cohen et al (2003) represents a distinct species. In eastern Corsica and Spain the G1 genotype dominated and represented 70% or more of the studied isolates (Cohen et al 2003, Alvarez et al 2011. Despite reports of intraspecific morphological and genetic variation in P. citrophthora, no study has validated the taxonomic status of the different groups in this species with multigene phylogenies.…”