Graduate Thesis Or Dissertation
 

A leaf spot diseases of English holly caused by an undescribed species of Sclerophoma

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  • English holly culture in Oregon has been threatened by the development of red leaf spots which prevent sale of cut holly. Chemical, mechanical and insect injury and boron deficiency have been shown in the past to cause different types of red leaf spotting, but the cause of holly scab, a widely prevalent type of red leaf spotting, had not been determined. The objectives of this study were to determine the cause of holly scab and to investigate the factors that influence its development. Holly scab spots are irregular, translucent, or red-black swollen areas principally on the lower leaf surface, although some varieties show spotting on both leaf surfaces. Observation of scab development on trees in a variety plot showed that the size and color of holly scab spots varies considerably on different varieties of holly. Green stem hollies appear to be as susceptible to scab as the brown or blue stem varieties but form less pigmentation. No promising resistant varieties were observed. Nutritional vigor of the host did not affect susceptibility to infection. When nitrogen, calcium or boron deficient plants and normal plants were exposed to natural inoculum in the field all plants became infected. Over 100 different organisms were isolated from leaves affected with holly scab and inoculation attempts were made with 28 of the isolates. Only a species of Sclerophoma produced holly scab. Attempts to identify the fungus repeatedly lead to Phoma and Phyllosticta. Because of significant differences from characteristics of these genera, however, cultures were sent to B. C. Sutton of the Commonwealth Mycological Institute for identification. Sutton acknowledged that the fungus was not a Phoma or Phyllosticta. He pointed out its similarity to Sclerophoma pithyophila (Cda.) Hohn. and listed it as a Sclerophoma species with accession number IMI 107649. The fungus in culture has pycnidia that are non-ostiolate, spherical to pyriform, ranging in size from 75 microns to 200 microns. Spores are hyaline, ovoid-oblong, guttulate, 2-4 x 5-7 microns with no visible sporophores at maturity but are embedded in a gelatinous matrix. The fungus grows slowly on streptomycin-PDA over a range of 15° to 25° C but does not grow at 30° C. It grew best on media with an initial pH from 4 to 7. The holly Sclerophoma did not sporulate under normal laboratory conditions, but sporulated readily when KNO₃ was used as a nitrogen source or following exposure to prolonged periods of near-ultraviolet. Sporulation has not been observed in infected leaves. Comparison of the holly Sclerophoma with descriptions of the Sclerophoma species listed by Grove showed distinct differences in host range and morphology which would support the establishment of a new species. Unfortunately cultures of other Sclerophoma species were not available for comparison, and further attempts to clarify the taxonomic status of the holly Sclerophoma are pending receipt of collections from Europe. Attempts to control the disease, with spring and summer sprays were inconclusive due to a lack of natural infection within the test plots, but provided information on three other non-pathogenic leaf-spots. Purple blotch is a physiological imbalance that shows only with the ripening of the berries and colder weather. Copper injury occurred when fixed copper was used at normal recommendations in spring and fall spray applications. Leaf spotting associated with summer application of nabam was observed and is being studied further.
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