Tobacco blue mould (Peronospora tabacina Adam) in north Queensland 2. Epidemiological studies bearing on the development and control of the disease

Abstract

In North Queensland, blue mould (Peronospora tabacina Adam) persists on out-of-season hosts comprising plant crops, standover crops, volunteer plants and the native host Nicotiana glauca Graham. There is some evidence of seedborne infection. The oospore stage of the fungus appears to be absent.
A uniform distribution of inoculum does not exist early in the season. Early mould outbreaks are predictable following a study of the location of sources of infection. The
build-up of mould in early-planted crops ensures a high intensity of inocuh1m by the time the major field plantings are made.
Initial aerial infections are slight and may be expected three weeks after field plantings where a source of inoculum is close. Economic loss and coincidental build-up of mould to an intensity where widespread aerial d;spersion of spores takes place may be expected some three weeks after initial infections. The relationship of rate of spread to intensity of inoculum is discussed.
Stem mould is correlated with leaf mould severity at an early age and is found to increase as time from leaf infection increases.
A north-south row direction was found to alleviate mould losses, probably because of better penetration of sunlight.
The relationship between rainfall and the development of the disease is discussed. Dew alone is sufficient to ensure the spread of blue mould throughout a crop but the increase
in intensity of infection to epidemic proportions depends on adequate rainfall.