FURTHER STUDIES ON QUANTITATIVE METHODS WITH TWO PLANT VIRUSES

The arrangement of experiments for intercomparison of a number of virus samples is discussed and the use of randomised blocks or Latin squares, combined in certain cases with half-leaf comparisons between all samples or against a standard, is recommended.
Improvements in the ground-glass spatula method of inoculation have been suggested.
Provided there is sufficient inoculum to cover the leaf when the spatula is rubbed over it, the amount, of inoculum makes no difference to the number of lesions produced.
The conditions to which test plants are subjected shortly before inoculation were found to influence the number of lesions produced by a given inoculum.
The number of lesions produced by otherwise similar inocula is influenced by their pH value and electrolyte content. The optimum pH . range for the virus of tomato spotted wilt is very limited (roughly pH 6.0–8.5). Tobacco mosaic virus in a potassium phosphate buffer at a pH value about 7 produced the maximum number of lesions in the concentration range 0.05-0.2 M . It is recommended that for most quantitative work viruses should be buffered at a definite pH value.
When effects due to varying pH value and electrolyte content were excluded, the virus of tomato spotted wilt was still found to be inactivated by certain oxidising agents and preserved by certain reducing agents.