Phytophthora heterospora sp. nov., a New Pseudoconidia-Producing Sister Species of P. palmivora.

Bruno Scanu,Benedetto Teodoro Linaldeddu,Reza Mostowfizadeh-Ghalamfarsa,Thomas Jung,Josef Janoušek,Santa Olga Cacciola,Marília Horta Jung,Mario Riolo,Hossein Masigol,Andrea Brandano

doi:10.3390/jof7100870

Abstract

Since 1999, an unusual Phytophthora species has repeatedly been found associated with stem lesions and root and collar rot on young olive trees in Southern Italy. In all cases, this species was obtained from recently established commercial plantations or from nursery plants. Morphologically, the Phytophthora isolates were characterized by the abundant production of caducous non-papillate conidia-like sporangia (pseudoconidia) and caducous papillate sporangia with a short pedicel, resembling P. palmivora var. heterocystica. Additional isolates with similar features were obtained from nursery plants of Ziziphus spina-christi in Iran, Juniperus oxycedrus and Capparis spinosa in Italy, and mature trees in commercial farms of Durio zibethinus in Vietnam. In this study, morphology, breeding system and growth characteristics of these Phytophthora isolates with peculiar features were examined, and combined mitochondrial and nuclear multigene phylogenetic analyses were performed. The proportion between pseudoconidia and sporangia varied amongst isolates and depended on the availability of free water. Oogonia with amphigynous antheridia and aplerotic oospores were produced in dual cultures with an A2 mating type strain of P. palmivora, indicating all isolates were A1 mating type. Phylogenetically, these isolates grouped in a distinct well-supported clade sister to P. palmivora; thus, they constitute a separate taxon. The new species, described here as Phytophthora heterospora sp. nov., proved to be highly pathogenic to both olive and durian plants in stem inoculation tests.

Highlights

The genus Phytophthora de Bary (Peronosporaceae, Peronosporales, kingdom Stramenipila) is one of the most important groups of plant pathogens, causing a range of diseases in agricultural, horticultural, forest, and natural ecosystems worldwide [1,2,3]
The downy mildews with colored conidia (DMCC), graminicolous downy mildews (GDM), and brassicolous downy mildews (BDM) clade resided in sister position to a large cluster comprising Phytophthora clades 1–5, 12, and 14 and the downy mildews with pyriform haustoria (DMPH), whereas the latter clustered together with the obligate biotrophic Phytophthora cyperi from clade 14 in a sister position to clade 1 (Figure 2)
Since no specimens or DNA sequence data linked to the original description are available and no further reports of this variety have been published, it remains unclear whether P. heterospora and P. palmivora var. heterocystica belong to the same taxon, it cannot be ruled out

Summary

Introduction

The genus Phytophthora de Bary (Peronosporaceae, Peronosporales, kingdom Stramenipila) is one of the most important groups of plant pathogens, causing a range of diseases in agricultural, horticultural, forest, and natural ecosystems worldwide [1,2,3]. Airborne Phytophthora species, on the other hand, produce almost exclusively caducous sporangia and primarily infect aerial parts of plants, causing leaf necroses, shoot blights, fruit rots, and bleeding bark cankers on stem and branches [1,3,5]. In this case, infections occur through detached sporangia spread by wind and rain splash that can either germinate directly to produce mycelia (which in turn can differentiate further sporangia) or indirectly by releasing zoospores [3,7,9]. There are some Phytophthora species with a mixed epidemiological strategy, having both caducous and persistent sporangia, thereby behaving as both soil- and airborne pathogens [3,5,7,10]

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Conclusion