Studies on the Efficacy of Three Trichoderma atroviride Isolates for Controlling Take-all (Gaeumannomyces graminis var. tritici) in Grasses : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

Take-all, caused by the fungal pathogen Gaeumannomyces graminis var. tritici (Ggt), is an internationally important widespread root disease of cereals and grasses. The disease is suspected as the main cause of the increasing occurrence of light seeds in ryegrass seed crops. This study was carried out to investigate its impact on grass seed yield using prairie grass (Bromus wildenowii) as a model system, and examined the interaction responses of Ggt, biocontrol by Trichoderma and moisture stress. Another study was used to assess if Trichoderma isolates could deliver effective biocontrol activity and growth promotion of perennial ryegrass grown in a soil naturally infected with Ggt. Take-all delayed the time to first seedhead emergence (TFSE) of prairie grass by 1.5 days, reduced the number of seeds, total seed weight, and the weight of machine dressed seeds (MDS) by 14%, 14%, and 13%, respectively. Seed yield loss occurred mainly through a reduced number of seeds, instead of weight of the individual seeds. However, seed yield reduction did not occur in plants with a restricted access to soil nutrients. The disease also caused a substantial root infection and reduced root dry weight. Soil inoculation with Trichoderma isolates increased all seed yield components. Trichoderma isolates also accelerated plant growth and continuously increased shoot dry weight for up to 7 months after their introduction. Trichoderma also enhanced drought resistance of the plants by increasing leaf relative water content under moisture stress conditions. This experiment confirmed that moisture stress decreased all seed yield components, including the number of fertile tillers, the number of seeds, the total seed weight, the weight of MDS, and thousand seed weight by 13%, 32%, 41%, 46%, and 11%, respectively. The stress also severely reduced shoot dry weight throughout the experimental period. Take-all did not reduce seed yield in the plants protected by Trichoderma. On the other hand, growth promotion effects by Trichoderma isolates occurred only in the presence of Ggt. Moisture stress reduced the root disease severity, but did not affect the biocontrol efficacy of Trichoderma against Ggt. Instead, a greater increase in the weight of MDS occurred in moisture stressed (42%) than non-stressed (24%) plants. Without protection by Trichoderma isolates, take-all combined with moisture stress exacerbated the seed yield loss, reducing the weight of MDS by 51%. Moisture stress was the primary determinant of Trichoderma-plant-pathogen interactions, reducing the shoot and seed yields in the absence/presence of Trichoderma or Ggt. The second study confirmed that in the presence of Ggt all Trichoderma isolates significantly increased perennial ryegrass shoot dry weight by 46-73%, which was strongly correlated with an increase in root dry weight by 42-62%. Trichoderma isolates also significantly decreased root disease severity. This study did not support the hypothesis that a combination of the three Trichoderma isolates could improve the biocontrol efficacy over that of any individual isolate. However, the mixtures of isolates and isolate LU140 produced a greater root dry weight than the control, which could possibly give the plant a greater ability to cope with adverse environmental conditions than the two other isolates.