The effect of powdery mildew inoculum pressure and fertilizer levels on disease tolerance in spring barley

The effect of inoculum pressure on disease tolerance was studied on a range of spring barley genotypes by manipulating the quantity of spores being deposited on field plots from surrounding plots. Genotypes included previously selected tolerant and non-tolerant types and the modifying effects of fertilizer levels were also studied. Under low fertilizer and 'normal' inoculum pressure and especially high fertilizer and high inoculum pressure conditions, where host and pathogen development were likely to be near equilibrium, designation of barley genotypes as tolerant or non-tolerant was more reliable than under the other conditions tested. Single trials were not reliable for designating tolerance.