189 



cilia and come to rest. Each then develops a very 

 fine tube which grows out into a hypha. The latter 

 dissolves its way into the cells of the leaf, or stalk, 

 among which it then grows and spreads and soon 

 reproduces the mycelium above described. See 

 Plate XVI (3)-(6). 



(2) The conidium instead of forming zoospores itself puts 



out a germ-tube which directly enters the plant tjs- 

 sues and forms the mycelium. See Plate XVI (7). 



(3) The conidium forms a germ-tube as before, but this 



instead of entering the plant tissues forms a secondary 

 conidium at its apex which in its turn falls off and 

 gives rise to zoospores or to another germ-tube which 

 directly enters the plant-tissues. 



The first of these processes is the most frequent especially 

 with conidia which come in contact with water shortly after 

 falling. Conidia lose their power of germinating in about three 

 weeks, and dry weather is unfavourable to their production and 

 germination. Conidia which are washed by rain upon tubers can 

 infect the latter directly, the germ-tube penetrating the tissue 

 and forming a mycelium there. The mycelium may also pass 

 down the potato stalks and infect the tubers in this way. The 

 mycelium having reached the tubers may proceed to destroy 

 them in the same way as the leaves, the numerous bacteria in the 

 soil assisting in the decomposition of the diseased tissues, or the 

 mycelium may remain in the tubers in a dormant condition. 

 Such infected tubers on being cut open usually exhibit dark 

 patches and when stored frequently rot. If infected tubers are 

 planted out the next season the mycelium again becomes active 

 and growing along with the young shoots sent out by the tuber 

 reproduces the disease. 



The plants may be sprayed with Bordeaux mixture which Preventive 

 has been described as " the most effective and cheapest fungi- Measures - 

 cide known ". This does not poison the tubers or make them 

 unfit for consumption, but it kills the conidia and zoospores and 

 prevents their germination. In addition to its action on the fun- 

 gus this mixture has actually proved beneficial to the potato 

 plant and results in an increased yield of tubers. The copper 

 in the mixture in some way which is not clearly understood 

 increases the amount of chlorophyll in the leaves and conse- 

 quently the assimilating powers of the foliage.* 



Care must be taken not to use infected tubers for seed. 



* For instructions regarding the preparation and application of Bordeaux 

 mixture see Potato Diseases of India by Dr. E. J. Butler, published in the Agr - 

 cultural Ledger, 1903, No. 4, pp. 108-109. 



