DISEASES OF VEGETABLES. 281 



of proper manures to neutralise the injurious qualities 

 predominant in the soil, are the most eligible means 

 to be followed to preserve trees from its ravages. 



Canker also appears on some sorts of herbs. 

 Blanched celery, endive, and lettuce, are all subject 

 to be disfigured and spoiled by something like canker. 

 The curl on the leaves and the scabs on the tubers 

 of the potato, are deemed to be diseases of the same 

 kind. So is that on the cucumber and melon ; but on 

 these last the cause is well known, namely, a want 

 of sufficient heat in conjunction with too much humi- 

 dity. Some of the former may be the work of invi- 

 sible insects 9 fungi 9 &c., and others, as the brown spots 

 on celery and the curl on potatoes, appear to be ende- 

 mial, as they are only known in certain districts for 

 a year or two, and then disappear. Our reason for 

 averring this much is founded on the history of the 

 disease, and on what we have gleaned from experi- 

 ence. At one time the curl was very prevalent in 

 the south of England, now it is rarely and only par- 

 tially seen. Some writers attribute the disease to the 

 use of over ripe tubers as sets. Many cultivators 

 in the Lowlands of Scotland prefer potatoes for sets, 

 grown on cold moor earthy districts in the Highlands. 

 In Wales it is said to be an insect ; but in most of 

 the central counties of England, as it is seldom seen, 

 no precautions are taken to prevent it, except occa- 

 sionally changing the kind, or procuring sets from a 

 distant place ; and this not to prevent curl, but to 

 obtain better crops. From many published reports 



