Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2007
Publication Date: 4/15/2007
Citation: Chilvers, M., Peever, T., Akamatsu, H., Chen, W., Muehlbauer, F.J. 2008. Didymella rabiei primary inoculum release from chickpea debris in relation to weather variables in the Pacific Northwest of the United States. Canadian Journal of Plant Pathology. 2007. 29: 4, 365-371. 33 ref. Interpretive Summary: Ascochyta blight is an important disease of chickpea in the US Parcific Northwest and elsewhere. The primary inoculum of the disease is the ascospores which are produced on overwintered chickpea debris. The timing of ascospore release was investigated during a five year period by employing potted trap plants. The results showed that ascospore release began in mid-March, and continued through the beginning of June when numbers of trapped ascospores dropped off dramatically. Rainfall events are the primary driver of ascospore release. Results of this study have allowed the development of a simple model for predicting ascospore release from overwintered debris, and will allow us to target control strategies at this critical period.
Technical Abstract: Didymella rabiei (anamorph: Ascochyta rabiei), the ascomycete fungus that causes Ascochyta blight of chickpea, produces pseudothecia on overwintered chickpea debris. Ascospores released from pseudothecia are thought to constitute an important primary inoculum source for Ascochyta blight epidemics in the Pacific Northwest of the United States and other parts of the world. The timing of ascospore release from overwintered chickpea debris was investigated in the field using potted susceptible chickpea plants as trap plants. The experiment was repeated over five years and was replicated at two sites in the first and latter two years. In the first three years of study, ascospores were trapped by placing trap plants adjacent to a Ascochyta blight screening nursery or into a commercial chickpea field. In the last two years of study, chickpea debris, heavily infected with D. rabiei, was spread in a 1 square meter area approximately 5 cm deep, secured with netting and allowed to overwinter in a natural setting. Ascospores were trapped the following spring by placing 27 trap plants adjacent to D. rabiei infested chickpea debris. Following exposure in the field for 2-3 days trap plants were returned to the greenhouse, misted with water and covered to maintain leaf wetness for 24 h to promote infection. Ascochyta lesions were counted 2 weeks later with each lesion assumed to represent infection by a single ascospore. Ascospore release began in mid-March, coinciding with previously published data on the maturity of pseudothecia, and continued through the beginning of June when numbers of trapped ascospores dropped off dramatically. Rainfall was the variable which best described ascospore release in regression analysis, suggesting that rainfall events are the primary driver of ascospore release. Results of this study have allowed the development of a simple model for predicting ascospore release from overwintered debris, an increased understanding of ascospore release periods and will allow us to target control strategies at this critical period.