Potassium fertilization in relation to downy mildew disease incidence in grape leaves

Influence of different sources of potassium and their method of application was studied on incidence of downy mildew infection in grape leaves of 'Sharad Seedless' (syn: 'Kishmish Chorni'). The total phenolic content and individual phenolic acids were analysed from healthy and downy mildew infected leaves of 'Sharad Seedless' and the degree of downy mildew infection was well correlated with potassium content in the petioles. Different sources and method of potassium application had significant effect on Phenylalanine ammonia-lyase (PAL) enzyme activity and preformed phenols in healthy vines. Significant increase in PAL enzyme activity, total phenols and individual phenolic acids was registered with increase in disease severity. Magnitude of percent change in PAL enzyme activity and total phenolic content was highest in vines with maximum disease infection. Among the phenolic acids, o-Coumaric acid, p-Coumaric acid have a definite role in disease resistance. We could also observe the variation in disease severity in vines which received different sources of potassium which was supplied in different quantities whether through soil and/or through fertigation.