356 Hcieniifie Proceedings, Royal Dublin Society. 



The reason for the development of such a severe attack late in the 

 season seems to be twofold. The climate in late summer and autumn is 

 almost always such that blight is present, and yet in many cases a balance 

 is maintained, so that both plant and parasite go on developing until np to, 

 or nearly up to, digging time. There is frequently no frost in the coastal 

 districts until late October or even early November ; and the season is so 

 short (June 1st to October loth) that the stalks do not die from natural 

 maturity. In the second place, some of the varieties grown there are 

 standard American potatoes, which, as is well known, are on the whole very 

 susceptible to blight, and are not suitable for cool coastal regions. Due to 

 this combination of circumstances, the results about to be described have a 

 significance for portions of Eastern Canada which is not, perhaps, equalled 

 elsewhere; but, as will be shown, the same factors also operate in Ireland to 

 some extent. 



The observation was made in 1915, the first season spent in Canada, that 

 a conspicuously small amount of tuber disease followed a most severe blight 

 attack, which completely killed all the foliage early in September. As it turned 

 out, this was ihe most serious blight epidemic which occurred in the five-year 

 period 1915-1919, yet the amount of tuber rot which followed was far lower 

 than in any of the other years mentioned. The total amount of Phytophthora 

 tuber rot developed by the crop in that season, including that found in 

 storage up to the following April, was only 1'2 per cent., while the corre- 

 sponding average for the succeeding four years^ was 263 per cent. In none 

 of the latter years was the blight sufficiently severe as a rule to kill the 

 foliage completely before the crop was dug. The attack, therefore, persisted 

 longer, to which fact is generally attributed the greater amount of decay in 

 the tubers, because of their being exposed to infection for a greater length 

 of time. 



It is generally agreed that a long period of spore discharge (that is, a 

 moderate and long-continued attack of blight) and severe tuber rot are 

 associated ; and the prevalent view is that the latter is a direct consequence 

 of the former. The assumption must then be made that more spores find 

 their way to the soil when the bliglit never (or certainly not for a long time) 

 develops sufficient intensity to kill the plants, than happens when the blight 

 attack is much more severe but less protracted ; or, alternatively, that under 

 the former conditions the spores reach the tubers in greater numbers or 

 under more favourable conditions for infection. These are hypotheses which 



' The figures for 1919 do not record the amount of tuber rot developed after 

 November, but they include the severe outbreak which generally occurs within the first 

 month after harvesting, subsequently to which little or none develops, 



