330 . PLANT DISEASES 



immense numbers and germinate quickly, hence the very 

 rapid spread of the disease can be readily understood. 



It is somewhat remarkable that the young rootlets of 

 the tomato are the only part of the plant through which 

 the fungus can gain admission to its interior. All attempts 

 to inoculate above-ground portions with either Diplo- 

 dadium or Fusarium spores have failed. 



The probable explanation of this is that the fungus 

 commences life as a saprophyte, and afterwards becomes 

 parasitic. Numerous strands of mycelium are also present 

 in the soil, where they spread, and in due course produce 

 a third form of fruit, known as resting-spores. These 

 remain dormant for a season and then germinate, forming 

 a mycelium capable of attacking the rootlets of tomatoes. 

 By this means the disease is communicated to a new crop 

 of tomatoes the following season. 



PREVENTIVE MEANS. Owing to the mode of attack, 

 spraying with fungicides is of no avail. It is considered 

 fortunate by growers if even a small quantity of fruit is 

 obtained from a diseased crop ; but this is obviously un- 

 sound reasoning, especially where tomatoes are grown in 

 the same house and in the same soil year after year, for 

 the simple reason that when a diseased crop is allowed to 

 stand and mature its fruit, resting-spores innumerable are 

 produced in the soil, ready to attack the succeeding crop, 

 which in turn can only yield a very small quantity of 

 fruit. 



Plants should be removed on the first symptoms 

 of disease, and the soil mixed with quicklime. Where the 

 plants in a house have been badly diseased, all the soil 

 should be removed, and the walls, etc., drenched with a 

 solution of iron sulphate. 



