NH4 fertilization increases susceptibility of sweet basil (Ocimum basilicum L.) to grey mould (Botrytis cinerea) due to decrease in Ca uptake

Sweet basil (Ocimum basilicum L.) is an economically important herbaceous annual plant of the Labiaceae family. One of the main pathogens of sweet basil is grey mould (Botrytis cinerea). Previous work showed that susceptibility to grey mould increased with increasing N availability. The purpose of this work is to determine the role N form and Ca in this phenomenon. First, data from previous experiments were reexamined in order to determine whether there was a correlation between shoot Ca and basil plant susceptibility to B. cinerea. Then, basil plants were grown in a complete randomized design with 12 treatments and 5 repetitions in which N concentration, N type, and Ca concentration were varied. Plants were harvested 7 times, and shoot Ca and N were measured. Cuttings and full plants were inoculated with B. cinerea in order to determine their susceptibility to grey mould. We found that increases in N availability only increased the susceptibility of the basil plants to grey mould when the N was provided as a mix of NO3 and NH4, whereas in treatments where N was provided solely as NO3, N availability had no effect on plant susceptibility. Furthermore, we found that NH4 inhibited Ca accumulation, and shoot Ca concentration was negatively correlated to plant susceptibility. Taken together, this shows that N availability per se did not increase basil susceptibility to B. cinerea, rather it was the increase in NH4 fertilization which caused a reduction in shoot Ca which in turn was responsible for this effect.