Some studies on Graminicolours Didymella spp. in New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The aim of this thesis was to elucidate aspects of the biology of Didymella species present on Graminicolous hosts in New Zealand. The taxonomy of Didymella spp. and their anamorphs was investigated. Four main types were found and these corresponded to Didymella exitialis D. phleina, D.graminicola and Didymosphaeria loliina. The anamorph states were identified as the Ascochyta state of D. exitialis, Ascochyta phyllachoroides Sacc. and Malb. Forma melicae, Ascochyta state of D. graminicola and the Ascochyta state of D. loliina. Two of the isolates identified as the Ascochyta state of D. graminicola could also be classified as A. sorghi. The production of the teleomorph in culture was not achieved on a consistent basis. The teleomorph was produced by four isolates that were plated on CD-V 8 agar. Only pseudothecia and ascii were produced; no ascospores could be found. The teleomorph was produced on one other occasion, when isolates on PDA had been dried down slowly. Conidia of Didymella germinate on leaf surfaces between three and five hours after inoculation at 25°C and 98% RH. Production of appressoria occurs approximately 30 hours after inoculation at 20°C and 98% RH. The method of penetration of the leaf surface is direct with no preferred sites for penetration. A subcuticular, intramural mycelium then develops between the cuticle and the epidermis until the host becomes stressed, or the leaf material starts to senesce. Symptoms affecting less than one percent of the leaf area developed on inoculated plants, no increase in the rate of leaf senescence of infected plants compared to uninoculated plants was observed. Didymella spp. are frequently isolated from symptomless surface sterilised leaf material during the wheat-growing season. Didymella spp. are initially isolated after the first leaf unfolds and can be isolated sporadically from other leaves throughout the growing season, until the leaves senesce. During the 1995/96 growing season the application of azoxystrobin significantly reduced the levels of Didymella spp. isolated from leaves and the amount of sporulation on leaf material. In 1996/97 azoxystrobin had no significant effect upon the level of Didymella spp. cultured from leaf material, however, the level of sporulation on leaf material was less than those leaves treated with tebuconazole or a water control. The levels of Didymella spp., bacteria and Sporobolomyces spp. cultured from leaf tissue were not significantly correlated with weather conditions. Fungicide sprays did not affect the level of Sporobolomyces on leaf tissue. In field surveys Didymella spp. were the most common fungi on senesced leaf tissue. The levels of Septoria nodorum and Septoria tritici were found to be very low in both the 1995/96 and 1996/97 field surveys. No cultivars resistant to Didymella spp. were identified.