Taxonomic studies on Cronartium species
Cronartium species are plant parasitic rusts that can infect branches, cones and stems of Pinus species, resulting in economic losses and ecological damage in temperate regions of the Northern hemisphere. Cronartium species are difficult to identify, have complex life cycles, and alternate between two types of taxonomically unrelated host plants to complete their life cycle (aecial/telial hosts). Over the years, the numerous and different taxonomic classifications, either based on morphology or host specificity, have created confusion and uncertainties. As some of these Cronartium species are of regulatory interest, it is essential to have a clear understanding of the species boundaries.
Studies have been conducted (Zhao et al., 2022) to revise the taxonomy of Cronartium using 1032 herbarium specimens (235 from Pinus – 797 from other plant families: Asclepiadaceae, Fagaceae, Gentianaceae, Myricaceae, Paeoniaceae, Santalaceae, Saxifragaceae and Scrophulariaceae). Species delimitation was established based on morphological examination and molecular phylogeny studies. As a result, 26 Cronartium species including 7 new species were recognized. The EPPO Secretariat has extracted below new data concerning species that are of regulatory interest to the EPPO region.
- Cronartium coleosporioides (EPPO A1 List) was confirmed as a distinct species based on the study of specimens from Honduras* (collected from Castilleja integrifolia), Canada (Melampyrum lineare), USA (Castilleja miniata, Pinus contorta var. latifolia), and Japan* (Pinus pumila). It was also noted that specimens of C. coleosporioides included in the USDA fungus collection had been collected from Costa Rica* (Castilleja tenuiflora), and Guatemala* (Lamourouxia dependens, L. rhinanthifolia, Orthocarpus luteus).
- Cronartium comandrae (EPPO A1 List) was confirmed as a distinct species based on the study of specimens from Canada (Comandra umbellata, C. umbellata subsp. pallida, Pinus banksiana), USA (Comandra livida, C. umbellata, C. umbellata subsp. pallida, Pinus contorta var. latifolia, P. contorta var. murrayana, P. eldarica, P. ponderosa var. scopulorum, P. ponderosa). It was also concluded that Cronartium comandrae is distinct from C. pyriforme (with which it was previously synonymized).
- Cronartium comptoniae (EPPO A1 List) was confirmed as a distinct species based on the study of specimens from Canada (Comptonia peregrina, Pinus banksiana) and USA (Comptonia peregrina, P. banksiana).
- Cronartium fusiforme (EPPO A1 List) was confirmed as a distinct species based on the study of specimens from China* (Pinus massoniana, P. sylvestris, Quercus emoryi, Q. fabri, Q. variabilis), Japan* (Q. serrata), Sweden* (Vincetoxicum nigrum), USA (Q. emoryi, Q. fabri, Q. nigra, Q. palustris). Previously, it had been suggested that C. fusiforme was a forma specialis of C. quercuum (C. quercuum f.sp. fusiforme) infecting P. elliottii and P. taeda, but Zhao et al. (2022) confirmed that morphological differences could be observed between C. fusiforme and C. quercuum, and that C. fusiforme formed a well-supported clade, distant from C. quercuum.
- Cronartium orientale (not listed by EPPO but see C. quercuum below) was confirmed as a distinct species based on the study of specimens from China (Pinus hwangshanensis, P. densata, Quercus aquifolioides, Q. glandulifera, Q. liaotungensis, Q. spinosa, Q. semecarpifolia, Q. variabilis) and Japan (Q. crispula). Previous studies from Kaneko (2000) had shown the presence of C. orientale in China, Japan, the Republic of Korea, and the Russian Far East on various other hosts (Castanea crenata, Pinus densiflora, P. thunbergii, Quercus acutissima, Q. aliena, Q. fabri, Q. glauca, Q. phellos, Q. rubra, Q. serrata, Q. spinosa), and had also concluded that C. orientale was a species distinct from C. quercuum.
- Cronartium quercuum (EPPO A1 List) is a rust alternating between Pinus and Fagaceae (mainly Quercus spp.) and has a complex taxonomic history. It has been considered to be a species complex, and some authors have proposed that it comprised several formae speciales. The present study confirmed the following points:
- C. quercuum is distinct from C. fusiforme, C. strobilinum and C. conigerum (other North American species which were considered as synonyms in the past).
- C. harknessii (EPPO A1 List - an asexual and autoecious form found only on Pinus spp., not alternating on Quercus spp.) belongs to the C. quercuum clade, and therefore should be considered as a synonym (as already suggested by other authors).
- Records of pine-oak rust in Asia (China, Japan, Republic of Korea, Russian Far East) which had previously been attributed to C. quercuum, most probably correspond to other species (e.g. C. orientale- see above).
These results illustrate the importance of taxonomy in plant quarantine. More studies using molecular tools are clearly needed on Cronartium species, in particular to clarify their geographical distribution, host range, and economic impact (as most herbarium specimens used in the present study were collected many years ago). Up-dated information is needed to be able to re-assess the risks that Cronartium species may present to the EPPO region in the light of this new taxonomy.
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* New country records. However, as these geographical records are based on herbarium specimens and not on recent observations or surveys made in the field, these records need to be confirmed by other studies. For the moment, the EPPO Secretariat has considered them as ‘doubtful’ in the EPPO Global Database, awaiting more information.
Sources
Zhao P, Liu F, Huang JE, Zhou X, Duan WJ, Cai L (2022) Cronartium rust (Pucciniales, Cronartiaceae): species delineation, diversity and host alternation. Mycosphere 13(1), 672–723.
Kaneko S (2000) Cronartium orientale, sp. nov., segregation of the pine gall rust in eastern Asia from Cronartium quercuum. Mycoscience 41(2), 115-122.
USDA Fungal Databases. Fungus-Host. https://fungi.ars.usda.gov/