In laboratory tests, treatment of the first leaves of barley (Hordeum vulgare L cv Golden Promise) with
potassium phosphate led to significant reduction in
infection of the second leaves with the powdery mildew fungus Blumeria graminis f sp hordei Marchal, with a 25 mM treatment giving 89% reduction in
infection. Although the optimal interval between
phosphate treatment of the first leaves and mildew inoculation of the second leaves was 2 days, significant protection was still obtained if the interval was increased to 12 days. Protection against powdery mildew
infection was not as effective when the
potassium phosphate was applied as a seed treatment or root drench.
Phosphate treatment of the first leaves led to significant increases in activities of
phenylalanine ammonia lyase (PAL),
peroxidase and
lipoxygenase in second leaves.
Enzyme activities, especially PAL and
peroxidase, were increased further when second leaves of
phosphate-treated plants were inoculated with powdery mildew.
Phosphate treatment of the first leaves did not adversely affect plant growth and, in a field trial, 25 mM
potassium phosphate provided 70% control of mildew and gave a small increase in grain yield.