Effect of inoculum spore load, length of infection period, and leaf washing on occurrence of Peronospora tabacina Adam, (blue mould) of tobacco

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Abstract
Leaves of tobacco plants cv. Virginia Gold were spray-inoculated with different concentrations of spores of Peronospora tabacina strain APT1 and kept wet for 2 to 16 hr. Incubation period and occurrence of the disease on leaves were related to spore concentration, length of the infection period, temperature, and leaf position. The length of the incubation period increased from approximately 3 to 8 days with decrease in spore number. There was a similar increase with decrease in the length of the infection period and with low temperatures. The number of disease lesions increased with spore number in the inoculum and with increase in the length of the infection period up to 8 hr; it was about 100% higher on washed than on unwashed leaves. All leaves of washed plants had the same number of lesions but with unwashed plants, the lowest leaf had the most lesions and the uppermost the least. In a limited series of experiments with APT2 on cv. Virginia Gold and cv. Sol, the results were similar to those reported for APT1. The host–pathogen reaction was modified by spore load and temperature, the result being a range of symptoms; this indicates the need for caution in the use of symptoms as a means for distinguishing between different strains of the pathogen and between levels of resistance. High spore concentrations and infection periods of 4 hr or longer were necessary for extensive occurrence of the pathogen in leaves.