176 



Chocolate Spot Disease. 



[May, 



defoliated, the remaining leaves showing a good deal of blacken- 

 ing. On 10th July the canes in the central portion of the field 

 were beaten down by rain, all the leaves had fallen except a 

 bunch at the top of each stalk, and the whole plants were being 

 rapidly rotted by Botrytis, which in all cases observed followed 

 rapidly after the " Streak " disease. In many instances the 

 plants in the outer parts of the field were observed to be less 

 severely attacked than those in the centre, the conditions in 

 the outer more exposed portions being naturally drier than at 

 the centre and hence less favourable to the spread of the disease. 



Cause of the Disease. — As stated above, the organism causing 

 this disease is the same bacillus which causes " Streak " in 

 sweet-peas and " Stripe " in tomatoes — a small yellow bacillus 

 named by Manns and Taubenhaus Bacillus lathyri. The entry 

 of the organism into the plant may be through the stomata of 

 the leaf; the apparent spread of the disease eastwards during 

 1920 would seem to suggest wind dispersal of the causative 

 organism and entry into the leaf in this way. At the same time 

 there is evidence that the organism is carried on the seed of 

 winter beans, and especially, on those which have been bored by 

 the bean beetle Bruchns rvftmanus. In its attack upon the 

 young pod this beetle may inoculate the plant at the time of 

 laying its eggs, and the young larvaa which develop in the pod 

 may infect the seed when they bore their way in. Foreign 

 Bruchids, e.g., Bruchus obtectus, can continue to breed in 

 stored beans, and this species is not infrequently introduced on 

 Canadian Wonder bean seed, though at present there are no 

 records of this beetle having been found amongst field beans. 



Control. — It should be remembered that the extent of the 

 disease seems to be markedly influenced by weather conditions. 

 The year 1920, when the disease was specially prevalent, was 

 followed by the exceptionally dry season of 1921, in which the 

 disease made its appearance in the early spring, but the plants 

 soon recovered from the attack and a month later showed no 

 sign of disease. 



Where disease has occurred to a serious extent it would be 

 well, before another crop of beans is sown, to dress the land 

 well with potash, since it has been shown that this treatment 

 has successfully checked the ravages of the bacillus upon tomato 

 plants. 



Further, it would be well to examine the seed carefully and 

 to reject any showing an excessive amount of boring by beetles, 

 and to sterilise the seed by soaking for 10 minutes in weak lysol 



