238 INSECTICIDES, FUNGICIDES, AND WEED KILLEES. 



and the same method of dissemination. They may be treated with 

 bouilHe bordelaise. 



Phytophthora infestans, De By. (potato disease). — To avoid the 

 phytophthora sound tubers which do not bring the first germs into 

 the field must be planted in the first instance, and secondly sprayed 

 on the stems to prevent the propagation of the disease by plants ac- 

 cidentally attacked. 



Treating the Tubers. — Beyond scrupulous sorting disinfection 

 may be of great service. Of all the treatments recommended, that of 

 bouillie bordelaise is the most energetic and the most certain. The 

 tubers are steeped in a 2 per cent bouillie bordelaise ; 10 gallons of 

 bouillie suffice for 100 lb. of tubers. The duration of the steep is 

 twenty-four hours, after which the tubers are steeped for twenty-four 

 hours in pure water, then they are dried in the open air. The treat- 

 ment should be carried out five to six weeks before planting. The dis- 

 infection has no objectionable effect on the germinative capacity of the 

 tuber if the process be conducted as recommended by Frank. Godefroy 

 found no alteration even after several days' steeping in the bouillie. 

 Prank and Kruger found the treatment, on the contrary, favourable to 

 the future evolution of the potato ; it springs up quicker and yields a 

 more vigorous plant and also a greater number of tubers in the ratio 

 of 3 to 2. The good effects of the bouillie on the germinative capa- 

 city are not felt, except on condition that the tuber has no forward 

 sprouts at the time of treatment, for the bouillie is very injurious to. 

 them. That is the reason why it is essential to disinfect potatoes 

 sometime before planting them, when they have not yet sprouted. 



Treatment of the Stems. — So long as treatment by the bouillie 

 bordelaise was limited to the aerial part of the plant the results were 

 not always favourable, especially as regards the yield in tubers, and 

 opinions as to the efficiency of the treatment were divided. In fact it 

 will be seen that where the phytophthora has deeply invaded the tissue 

 of the tubers a treatment of the stems with bouillie bordelaise can pre- 

 vent the evolution underground of the disease, and that failure is 

 inevitable in such conditions. If all underground evolution is, on the 

 other hand, prevented by a rational treatment of the seed potatoes, the 

 effects of the aerial treatment will not fail to give satisfaction. It is 

 not, therefore, surprising that certain experimenters have obtained, in 

 some cases, a considerably diminished yield. Lieber found 20 per 

 ■cent and Brunner 47 "5 per cent, and that this treatment, which was 

 first tried by Jouet in 1885, did not become general so rapidly as that 

 for the mildew of the vine. However, Fasquelle's trials in 1886, 

 Prillieux's in 1888, and Girard's in 1890, showed that a scientific ap- 

 plication of bouillie bordelaise killed the phytophthora without injuring 

 the plant. The diminution in the number of tubers, and in the 

 starch which they contain, seems due to the poisoning of the leaves 

 by imperfect bouillie, inducing a degeneration of the leaves, which 

 renders them less fit for assimilation (Sorauer). The bouillies used 

 at first contained too much copper and caused burns. Sempotovsky 

 threw light on this point by showing that the weaker the bouillie 

 the more active it is. 



