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Taxonomy of Cronartium Quercuum and C. Fusiforme

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Taxonomy of Cronartium Quercuum and C. Fusiforme TAXONOMY OF CRONARTIUM QUERCUUM AND C. FUSIFORME HAROLD H. BURDSALL, JR Center for Forest Mycology Research, Forest Products Laboratory, Forest Service, U. S. Department of Agriculture, Madison, Wisconsin 53705 AND GLENN A. SNOW Southern Forest Experiment Station, Forest Service, U. S. Department of Agriculture, Gulfport, Mississippi 39501 SUMMARY Cronartium fusiforme is considered conspecific with C. quercuum based on morphological studies, but maintained as a distinct forma specialis of C. quercuum because of its restricted host range on Pinus taeda and P. elliottii. Three other special forms of C. quercuum are proposed for physiological entities that selectively attack P. banksiana, P. echinata, or P. virginiana. The importance of recognizing the con­ specificicity and genetic plasticity of these organisms when breeding resistant pines is emphasized. Fusiform rust, caused by Cronartium fusiforme Cumm., has long been recognized as an important disease of southern pine (Hedgcock and Siggers, 1949; Czabator, 1971). However, there is still question as to the taxonomic status of the causal organism (Hedgcock and Long, 1914; Arthur, 1915, 1934; Hedgcock and Hunt, 1918; Rhoads et al., 1918; Hedgcock and Siggers, 1949; and others). It was not until Cummins (1956, p. 603) provided a description for the perfect state that thc combination C. fusiforme was validly published. Questions are still being raised concerning the relationship between C. fusiforme and the causal organism of the eastern gall rust, C. quercuum (Berk.) Miyabe ex Shirai (Gooding and Powers, 1965; Peterson, 1967; Dwinell, 1969a : Czabator, 1971; Jewell, 1972; Kais and Snow, 1972; Powers, 1972). The relationship is accepted as being close because the two “species” have comparable life cycles, attack most of the same alternate host species. although with different intensities (Dwinell, 1969a, 1971, 1974), and possess almost identical morphological characters. 503 504 MYCOLOGIA, VOL. 69, 1977 Peterson (1967, p. 518), discussing the aecial state of C. quercuum (as Peridermium cerebrum Pk.), indicated “Peridermium fusiforme ap­ pears to be indistinguishable microscopically . ” and (1973, p. 216) found nothing to distinguish C. quercuum and C. fusiforme at the species level, but awaited further studies of “cultured specimens” before rele­ gating C. fusiforme to subspecific rank. Scanning electron microscopic studies of acciospores (Grand and Moore, 1972, p. 1742 ; Antonopoulos and Chapman, 1976, p. 289) and of urediniospores and teliospores (Grand and Moore, 1972, p. 1742), exposed no morphological differences be­ tween these two species. Comparing hyphal width and haustorial length of the two taxa, Jewell (1972, p. 767) found that the size differences between C. fusiforme and C. quercuum were not statistically significant. Using these characters to support other similarities, he states “ . pos­ sibly they should not be considered separate species.” Our studies compared size and shape of aeciospores of C. quercuum on Pinus banksiana Lamb. (jack pine) and P. echinata Mill. (shortleaf pine) with those of C. fusiforme on P. taeda L. (loblolly pine) as well as comparing size and shape of urediniospores and teliospores of C. quer­ cuum (inoculum originating on jack pine and shortleaf pine) with those of C. fusiforme (inoculum originating on loblolly pine). Basidiospores of C. quercuum (inoculum originating on shortleaf pine) were compared with those of C. fusiforme (inoculum originating on loblolly pine). We find no consistent morphological differences between the two taxa, indi­ cating that separation of the taxa as species on the basis of sizes and shapes of these structures is not warranted. The most common means for distinguishing the two is the difference in the shape of the galls formed on the host plant. The galls caused by C. fusiforme are said to be spindle shaped (fusoid) while those caused by C. quercuum are supposed to be globose to subglobose and possess bark collars at each end (Hedgcock and Siggers, 1949, p. 20). Neither of these characters is entirely dependable since C. fusiforme sometimes produces subglobose to globose galls (Snow et al., 1969; Powers, 1971) which may also possess bark collars, at least on the lower end (Hedgcock and Siggers, 1949, p. 20). Cronartium quercuum, on the other hand, occasionally produces galls like those usually caused by C. fusiforme. Serological differences (Gooding and Powers, 1965) and differing pathogenic interactions of C. fusiforme and C. quercuum on Quercus species (Dwinell, 1969a, 1971, 1974) as well as a statistically significant aeciospore size difference (Dwinell, 1969b) have been demonstrated. These differences do not seem to warrant separation because they are so slight as to need statistical methods for detection. BURDSALL A N D SNOW: CRONARTIUM 505 The findings of Kais and Snow (1972) and Powers (1972) demon­ strated the existence of “races” of C. quercuum as indicated by vastly different host-pathogen interactions with different species of pine. It appears to us, however, that these differences are similar to those sepa­ rating C. quercuum from C. fusiforme. In view of this, it seem appro­ priate that C. fusiforme be reduced to the same status as the “races” of C. quercuum. Rather than applying the designation “race” to this level of pathologi­ cal specialization, we suggest that the term forma specialis (special form) is more appropriate. This concept of forma specialis (f. sp.) fits exactly the situation we have at hand. It is defined by the International Code of Botanical Nomenclature, Art. 4 (Stafleu, 1972), as follows, “In classifying parasites, especially fungi, authors who do not give spe­ cific, subspecific, or varietal value to taxa characterized from a physi­ ological standpoint but scarcely or not at all from a morphological stand­ point may distinguish within the species special forms (formae speciales) characterized by their adaptation to different hosts, . .” Robinson (1969) indicates that formae speciales should only be used to designate pathogens differentially pathogenic on different species or taxa of higher order. This applies perfectly to the physiological entities called races by Kais and Snow (1972, p. 502) and Powers (1972, p. 509). As a result of this study, we reduce C. fusiforme to the rank of forma specialis of C. quercuum and erect three other formae speciales on the basis of their pathogenic interactions on different pine species. We propose the following new formae speciales: Cronartium quercuum (Berk.) Miyabe ex Shirai f. sp. banksianae: primarily pathogenic on Pinus banksiana Lamb. (jack pine). C. quercuum (Berk.) Miyabe ex Shirai f. sp. virginianae: primarily pathogenic on Pinus virginiana Mill. (Virginia pine). C. quercuum (Berk.) Miyabe ex Shirai f. sp. echinatae: primarily pathogenic on Pinus echinata Mill. (shortleaf pine). C. quercuum (Berk.) Miyabe ex Shirai f. sp. fusiforme: primarily pathogenic on Pinus taeda L. (loblolly pine) and Pinus elliottii var. elliottii Engelm. (slash pine). This nomenclature provides a workable system of naming those entities that produce different responses on given species of pine. Since nomenclature of formae specialis is not governed by the Code (Stafleu, 1972, Art. 4), authority citations, type specimens, and Latin diagnoses for these new taxa are not necessary and are not provided. 506 MYCOLOGIA, VOL. 69, 1977 FURTHER CONSIDERATIONS Studies by Snow et al. (1969), Snow and Kais (1970), and Snow et al. (1975) demonstrate substantial variability in pathogenicity to slash pine by isolates of C. fusiforme. These isolates react differentially on different “families” (a population from seed from a single wind- pollinated tree) of a pine species. The families possess different levels and possibly different types of resistance to attack by C. fusiforme and arc attacked by specific pathogenic strains of the fungus. It is at this level that the term “race” would be more appropriately applied. Robin­ son (1969), however, points out several disadvantages of the term “race” and strongly suggests the use of the term “pathotype” in its place to designate physiological entities that exhibit differential inter­ action between host varieties (in this case, pine “families”). Therefore, instead of using “race” to designate these pathogenic entities, we support the use of the designation “pathotype” as defined by Robinson (1969, p. 604) feeling that it leads to a more exact and practical nomenclature for such plant pathogens. DISCUSSION Recent and future evolution of C. quercuum is worthy of some speculation in view of the extensive work in progress to obtain resistance to C. quercuum f. sp. fusiforme in slash pine and loblolly pine. Morpho­ logical similarities of the special forms of C. quercuum suggest a common ancestry and pathological differences are likely a result of adaptation to the different pine species involved. The geographic ranges of these pines overlap in many parts of North America as do the distributions of the special forms of C. quercuum. Selection pressure by pine species must, therefore, be a continuing process. More sampling of the C. quer­ cuum population would likely reveal some forms that are more distinct and others that are less distinct in terms of pathogenicity than those dis­ cussed here. For example, no definitive pathogenicity studies have been done with P. palustris Mill. (longleaf pine) which is also attacked by Cronartium species. Such studies may lead to recognizing other formae speciales. Currently, tree breeding programs
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