An investigation of a fungus disease of fathen (Chenopodium album)

1. A fungal disease complex causing a leaf and stem disease of Chenopodium album in New Zealand is described. Because little was known about the disease initial investigations were of an exploratory nature. Variation in fungus morphology necessitated a more intensive study of the systematics to clarify the identity of the fungi involved. 2. Suitable isolation and inoculation techniques were developed and optimum environmental conditions for culturing and inoculation determined. These were utilized in subsequent systematic studies. 3. In culture optimum mycelial growth and sporulation were produced on Chenopodium extract agar, at 23.5°C. The reaction of the medium in the range PH 5.2-7 had little influence on growth, and light had no discernible effect on sporulation. 4. Pathogenicity was proved. Satisfactory infection was obtained at ambient summer temperatures, and 100% R. H. 12 hrs before and at least 12 hrs after inoculation. Wound inoculation was required for Group III. 5. Experiments confirmed that inoculum in and on soil provided a source of infection with Pleospora calyescens (group III), infection developing via dead spidermal tissue at the base of the stem. Slugs were shown to disseminate the disease and initiate leaf infection. Field observations showed both of these factors to be important in disease epidemiology. 6. Perithecial formation could not be induced in cultures but the pleomorphic relationship of the perithecial form and the Group III pycnidial form was demonstrated in the systematic studies. 7. Three Groups were differentiated with pychidiospore morphology on host material and in culture, host identity, and life history criteria. The groups are descrbed and their more important characteristics illustrated. 8. The taxonomy of the Sphaeropsidales and the nomenclature of the present fungal groups is discussed. Pleospora calvescens (Fr.) Tul. Is concluded to be the valid name for Group III. Group I is identified as Septoria Chenopodii Westd., and Group II tentatively as Ascochyta Chenopodii Rostr. If Group I proves to be synonymous with Group II S. Chenopodii has priority. 9. The implications arising from this investigation extend beyond the particular disease problem on C. album. An important method of controlling plant diseases is to interrupt the life cycle of the pathogen at the carry over phase. With a number of fungi this involves a weather resistant sexual stage. In practice a sexual form should not be assumed top belong to the life cycle of a pathogenic fungus even if shown to possess a conidial form morphologically indistinguishable from that of the pathogen. The present studies have served to show the difficulty that can be encountered in distinguishing a pycnidial pathogen from the pycnidial stage of an ascogenous fungus that acts as a saprophyte or weak pathogen.