Physiology and Pathology of Septoria Species on Chrysanthemum

SUMMARY

In the study of collections of diseased foliage from chrysanthemums considerable variation was found to exist in the amount of disease manifested on plants in scattered locations and among the varieties, as well as differences in the number of pycnidia and the size of the pycnidiospores. Isolations from diseased material indicated 2 Septoria spp. Each included a wide host range within Chrysanthemum morifolium. Our measurements of pycnidia of Septoria obesa are 60 to 160 μ in diam and the pycnidiospores are 44–108 μ in length. In regard to S. chrysanthemella, our measurements are pycnidia 40–124 μ and pycnidiospores 22–70 μ. These comparisons, although variable, establish the fact that the pycnidiospore length is distinct enough for identifying these fungi. The width is also significant, although not so easily applied. Septations are significant, but the danger of immaturity of spores results in an unstable factor. In artificial culture, the fungi are distinct in rate of growth, but develop best at the same optimum temperature on potato-dextrose agar. They show no distinguishing significance as to cardinal temperatures although no growth was observed at 32 C, at which temperature Hemmi and Nakamura (11) found better growth than at 20 C.

Each of the 2 fungi grew and sporulated on each of the carbon sources used, with lactose producing the poorest growth. The 6 different nitrogen sources resulted in markedly varied cultured development of the fungi. Septoria obesa cultures were very dark or black on all nitrogen sources used; in contrast, S. chrysanthemella was more or less zonate, compact or fluffy and showed no sporulation when ammonium sulfate was used or where nitrogen was lacking.

Significant data were not obtained relative to the nitrogen source requirements of these fungi in relation to their selective pathogenicity on the chrysanthemum varieties inoculated.