Taxonomy, Distribution, and Pathology of Phomopsis Occulta and P. Juniperovora

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Abstract
SUMMARY Because of the close morphological similarity between the saprophyte, Phomopsis occulta (Trav.) Sacc., and the destructive nursery parasite, P. juniperovora Hahn, on eastern red cedar (Juniperus virginiana L.), the distribution of the latter as a possible endemic pathogen has been confused, particularly in cases where diagnoses of Phomopsis on wildling cedars are concerned. Comparable wound inoculation tests carried on in an unheated greenhouse with monospore cultures of Phomopsis occulta and P. juniperovora, the latter serving as a “check” to establish the suitability of the environmental conditions, demonstrated that P. occulta was unable to attack any of the 14 wildling eastern red cedar saplings used for the tests. The P. occulta inocula were obtained from both wildling and nursery stock. Results of these tests corroborate similar ones obtained previously with P. occulta on Arizona cypress and Chinese arborvitae. All of the 7 red cedars inoculated with Phomopsis juniperovora became infected. Three of the Wildlings, which were bluish-green in color, were girdled and killed back toward the base of the plant, whereas two saplings, which were decidedly green in color, showed blight resistance. Infections on one of the latter, which was lighter and brighter in color, were confined to nongirdling cankers on the trunk, and the terminal and lateral branches did not show discoloration. Reisolations of P. juniperovora on synthetic malt agar produced the yellow coloration and flaming orange crystals first reported in 1917. These characters are typical of the growth of the pathogen on a number of media. Neither the yellow color nor crystals appear in cultures of P. occulta, and they serve to differentiate distinctly the two Phomopsis species, the spore size ranges of which overlap. In a discussion of the two related Phomopsis species it is pointed out that the results obtained by the writer in earlier studies demonstrated that Phomopsis occulta could be distinguished morphologically from P. juniperovora, that P. occulta is distributed widely on conifers both in western Europe and throughout North America on numerous host genera, and that Diaporthae on conifers, including the perfect stage, Diaporthe conorum (Desm.) Niessl, of P. occulta, although common tin Europe, are exceedingly rare in the Western Hemisphere. Unpublished data are given on experiments conducted in Great Britain during 1926 to 1929, in which the perfect stage, Diaporthe conorum, was obtained culturally on twigs of English elm (Ulmus procera Salisb.) from monopycnidiospores of the fungus originally isolated from cultures of monoascospores of D. conorum collected on Douglas-fir. These results tend to support Wehmeyer's treatment of a large Diaporthae complex on both hard and soft woods belonging to the type species, Diaporthe eres Nit. on Acer, although admittedly, the homothallic D. conorum holds priority. Phomopsis occulta is to be regarded as a secondary fungus on cedars following injury by other factors. Previous studies carried on by the writer showed that under artificial conditions the organism at most was weakly parasitic on the coast form of Douglas-fir. The pathogen, Phomopsis juniperovora, under natural conditions parasitizes only genera of the Cupressaceae. Its Diaporthe stage is unknown. A revised host list is given, which includes all of the host species known to be attacked up to the present time. Early studies by the writer demonstrated that under artificial conditions saplings of Douglas-fir, Coast form, were highly susceptible to P. juniperovora. Although nursery stock of Douglas-fir is not known to be attacked naturally by the cedar-blight parasite, the potentialities of P. juniperovora on juvenile stock of this host merit consideration. Phomopsis juniperovora is reported to occur in Europe as a nursery parasite on the continent. As in the case of P. occulta, its perfect stage (Diaporthe) may be present, and there is the possibility that it may be discovered eventually on some obscure European host. Although we have considerable information on the distribution of the cedar-blight parasite in commercial and Federal nurseries in this country, there is not a great amount of data available concerning the occurrence of the organism on trees and reproduction of eastern red cedar in native stands. A small amount of fruiting bodies of Phomopsis juniperovora collected by Dr. Wright and the writer on suppressed red cedar Wildlings, was confirmed later by culture tests and is reported for the first time. Additional information on the occurrence of Phomopsis juniperovora on Wildlings is highly desirable from the standpoint of increasing our knowledge concerning the dissemination of cedar blight. In regions where cedar blight has not been reported and where there is a possibility of its nonoccurrence, this information should prove of practical value in disease control.

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