Resistance to early blight in potato and genetic structure of the pathogen population in Southeast Sweden

摘要

Potato early blight caused by the necrotrophic fungus Alternaria solani is a common foliar disease in many potato-growing regions. Application of fungicides is commonly used to effectively control the disease, although they are undesirable due to environmental consequences. Use of resistant cultivars would be the most optimal solution, but there are no cultivars with high level of resistance available on the market. In the present thesis, assessments of early blight resistance both in leaves and tubers of potato cultivars/clones were performed by applying different screening methods (field and greenhouse). Plant defence signalling in response to A. solani infection with main emphasis on salicylic (SA) and jasmonic acid (JA) hormones, was also studied. Furthermore, the genetic variability in A. solani populations from different potato growing regions of southeast Sweden was investigated. The fungal isolates were analysed for the F129L substitutions, which are associated with loss of sensitivity to QoI fungicides. In addition, field experiments were conducted to determine the occurrence of the F129L substitution and genetic shifts in the population during one growing season in response to two different fungicide strategies. ududCultivars/clones revealed significant differences in resistance to A. solani both in leaves and tubers irrespective of screening method. Results from field and intact plant inoculation experiments were significantly correlated but there were no correlations observed between these two methods and detached leaf assays. Some cultivars/clones showed relatively higher level of resistance to the pathogen. Results from the data suggested that SA appears to be responsible for regulating symptom development while JA dependent COI1 defense signaling is important to inhibit fungal growth during early stages of infection. Microarray analysis showed rapid defense responses to A. solani infection mediated by partially overlapping SA and COI1 dependent jasmonic acid (JA) signaling. It was also observed that JA/ethylene signaling responses dominate at later time points. ududThe genetic variability was relatively high among isolates of A. solani and significant genetic differentiation was found among populations from different locations in southeast Sweden. Two mitochondrial genotypes (GI and GII) were found among the isolates but the F129L substitution was only detected in GII isolates. Results from the field experiment showed that application of azoxystrobin (QoI fungicide) alone did not control the disease; better disease control was achieved with boscalid combined with pyraclostrobin. Similar results were obtained for yield. Moreover, results of sensitivity tests showed that isolates with the F129L substitution were less sensitive to azoxystrobin. AFLP analysis indicated within season changes in the A.solani population, especially at the end of the season.