83 



tissues of diseased asters. The writer,* however, has 

 proved that a species of Phytophthora is the primary cause 

 of the Black-Leg disease and that Fusarium only appears 

 later as a saprophyte living upon tissues previously killed 

 by the other fungus. The Phytophthora is always present 

 in the diseased asters even in the earliest stages; it has 

 been isolated and grown separately from all other organ- 

 isms, and the disease has been produced artificially from 

 such growths. Other organisms found on diseased asters, 

 including Fusanum, are unable to infect and produce wilt- 

 ing in healthy plants ; such saprophytes only succeed after 

 the tissues have been killed by the Phytophthora. 



The insidious nature of this disease renders it extremely 

 difficult to deal with, for it is usually almost impossible 

 to detect diseased plants until the wilting actually sets in. 

 Whilst sufficient scientific trials of remedial measures have 

 not yet been made to warrant promise of complete success 

 in all cases still some precautions may be indicated. 

 The soil of the seed-bed should be partially sterilised 

 by steam or hot water and asters should not be planted in 

 ground which produced diseased plants the previous sea- 

 son. All diseased material should be removed and 

 burned and the infested soil thoroughly drenched with 

 Formalin (i pint per 10 galls, water) and covered with 

 sacking for a few days. Many growers raise aster seed- 

 lings on hot-beds of stable manure, but these conditions 

 should be avoided since they are much more favourable 

 to the fungus than when alkaline artificial manures are 

 used. 



As a first example of the common diseases of leaves, 

 the Tomato-leaf rust or mould caused by Cladosporium 

 fulvum may next be considered. This is more strictly a 

 disease of the leaves than is the Late-Blight of potato, 

 but even in this case the whole tomato plant suffers 

 because of the attack on the leaves. The disease, which 

 has been known in this country for over a quarter of a 

 century, first appears on the leaves in the form of small, 

 yellowish spots. These gradually increase in size and 

 often run together, and the under surface of the leaf 

 in the diseased areas becomes covered with a rusty, velvety 

 growth. This is the reproductive part of the fungus pro- 



*" Annals of Applied Biology. 7 ' Vol. II., Nos. 2 and 3, July, 

 1915, pp. 125-137. 



