Epidemiology of blue mould (Peronospora tabacina Adam) of tobacco in the Ovens Valley district of Victoria. II. Disease severity

Abstract

Tobacco fields in Victoria were rated for severity of blue mould disease during six consecutive seasons. The disease was most severe on plants infected when less than 9 inches in height and slight on plants infected when near or at the flowering stage. Disease severity on leaves increased rapidly during the 3-week period commencing about December 25. The disease can be destructive in leaves, especially on plants growing in soils recently under pasture. The basal stem infection phase of blue mould is most destructive. Avoidance of stem infection is the major requirement for control. Basal stem infection was not of importance in years when fungicides were applied for control of leaf infection. Soil factors promoting lush growth, and weather conditions favouring sporulation and infection early in the season, were the main factors influencing disease severity. Resistant varieties need to have leaf resistance to reduce spore numbers, and stem resistance to prevent loss of entire plants.