Adaptation of Fusarium Species causing Head Blight to Quantitative Resistance in Wheat: Field Evidence for Increased Aggressiveness in a New Pathogen Population

The use of quantitatively resistant wheat cultivars is an essential component of a sustainable management strategy of Fusarium head blight (FHB), caused by several Fusarium species. However, little information is available on the variation of aggressiveness of the newly emerging FHB collection compared to old one. It is therefore important to determine to what extent FHB populations can be selected for increased aggressiveness by wheat cultivars with several levels of quantitative resistance. To this end, FHB populations were sampled in 2005 (old population) and in 2015 (new population) from one of the major Syrian wheat production regions, chosen as a location where head blight occurs regularly. New and old FHB isolates were characterized for aggressiveness by single-floret inoculation under controlled conditions on eight durum and bread wheat cultivars of contrasting susceptibility to FHB, and molecularly distinguished using DNA markers. Results showed the new population caused a higher disease severity (ranging from 55% to 67%) than the old population. Thus, their aggressiveness increased between early and late samplings, suggesting that wheat plants cultivated over 10 years selected for increased aggressiveness during epidemics. Our comparative population genetic analyses with analyzed markers showed that the new population had more polymorphic loci compared with the old one. The information obtained in this study indicated that FHB populations adapt to prevailing wheat cultivars, irrespective of their resistance levels, and can therefore overcome polygenic, quantitative resistance. Adaptation to wheat resulting in increased pathogen aggressiveness that was not specific may render quantitative resistance nondurable if not properly managed