44 



INJURIOUS INSECTS AND FUNGI. 



[June 1895. 



Advice is freely given, that care must be taken to avoid 

 planting tubers having disease germs within them. In theory, 

 this is correct and vahiable ; in practice, it is most difficult to 

 carry out. No exteinal sign may be present of the existence 

 of the disease. Large quantities of stored potatoes have been 

 examined which outwardly were clean, sound, and bright. Upon 

 cutting some of the tubers, the brown spots were seen, while 

 other tubers in the clamps were quite free from spots. In 

 various clamps the per-centage of spotty tubers ranged between 

 10 and 40 per cent. 



The only possible mode of avoiding infection in such circum- 

 stances would be to reject all tubers too small to be cut for seed, 

 and to cut all the larger tubers and put aside all that were 

 found to be spotty. It should be stated, however, that even 

 if this selection were made there would still be danger, as in 

 some of the tubers that were examined only the slightest traces 

 of disease could be discovered. In some cases, the discoloura- 

 tion was only discernible with the help of a strong glass, and it 

 was noticed that as the shoots grew from the eyes, the disease 

 spots grew larger and more numerous. It was also seen that, 

 when tubers were cut in two, disease spots that were then 

 almost imperceptible, and would probably have been unnoticed 

 by the selectors of sound seed, increased in size remarkably after 

 36 hours' exposure to air and warmth. Although it has been 

 shown by experiments extending over several j^ears made by M. 

 Girard in France, and by M. Caluwe in Belgium, as well as by 

 experiments made by the Technical Education Committee of the 

 Wilts County Council, and by the experiences of many practical 

 potato growers, that whole potato seed of a medium size gives 

 better results than cut seed, it will be desirable to ma.ke this 

 comparatively slight sacrifice rather than to run the risk of 

 having the crop damaged by disease. 



Where this precaution of cutting the suspected seed is not 

 adopted, and where the per-centage of spotty potatoes does not 

 in the opinion of the growers necessitate such action, it is most 

 important that preventive spraying with sulphate of copper 

 solutions should be adopted directl}^ the plants are high enough. 

 This will arrest the progress of the disease in any plants that 

 have become infected through the tubers, and save the healthy 

 plants from the infection by arresting the development of 

 conidia. 



In view of the fact that planting medium-sized whole seed 

 ensures the best crop, and that clean-looking, smooth, and appa- 

 l^ently sound seed may nevertheless contain the disease germs, 

 even though the greatest care is taken in selecting cut seed, it 

 seems to be necessa-ry, in order to avoid any risk of infection, to 

 spray potato plants when tliey are 2 feet high, as a preventive 

 measure. The results of the experiments in spraying with sul- 

 phate of copper, especially in the last season in this country, in 

 Ireland and in foreign countries, prove that this treatment is most 



