DISEASES OF VEGETABLES. 



13D 



find destroy the tender shoots of the plant, by 

 stopping the current of the juices. The leaves 

 which ai-e thus deprived of their due nourish- 

 ment, wither and fall; and the juices that are 

 now stopped in the passage, swell and burst the 

 vessels, and become the food of innumerable 

 little insects, that soon after make their appear- 

 ance. Hence they are often mistaken for the 

 cause of the disease, instead of the consequences 

 of it; the farmer supposing they are wafted to 

 him on the east wind, while they are only fos- 

 tered in the superabundant and obstructed juices, 

 which form an appropriate nursery for their eggs 

 and young. These propagating, will undoubt- 

 edly contribute to the extension of the disorder, 

 as they increase in proportion to the supply of 

 suitable food. 



A similar disease is also occasioned by the 

 early frosts of spring. If the weather is pre- 

 maturely mild, the blossom is forced before its 

 time, a circumstance which, though hailed by 

 the inexperienced with pleasure, is yet viewed 

 by the judicious with fear. For it frequently 

 happens tliat this premature blossom is totally 

 destroyed by subsequent frosts, as well as both 

 the leaves and shoots; which consequently wither 

 andfall,and injure, if they do not actually kill the 

 plant. This evil is also often augmented by the 

 unskilful gardener, even in attempting to pre- 

 vent it, that is, by matting up his trees too 

 closely, or by keeping them covered in the course 

 of the day, and thus rendering the shoots so 

 tender, that they can scarcely fail to be destroyed 

 by the next frost. 



The second kind of blight generally happens 

 in summer, when the gi-ain has attained to its 

 full growth, and when there are no cold winds 

 or frosts to occasion it. Such was the blight 

 that used to damage the vineyards of ancient 

 Italy, and which is yet found to produce great 

 destruction in the hop plantations and wheat 

 fields of Britain. 



The Romans had observed that it generally 

 happened after short but heavy showers, occur- 

 ring about noon, and followed by clear sunshine 

 about the season of the ripening of the grapes; 

 and that the middle of the vineyard suffered the 

 most. This corresponds pretty nearly to what is in 

 England called the fire-blast among hops, which 

 has been observed to take place most commonly 

 about the end of July, when there has been rain, 

 with a hot gleam of sunshine immediately after. 

 The middle of the hop ground is also the most 

 affected, whether the blight is general or partial; 

 and is almost always the point in which it ori- 

 ginates. In a particular case, minutely observed 

 by Hale, the damage happened a little before 

 noon, and the blight ran in a line, forming a 

 right angle with the sunbeams at that time of 

 the day. There was but little wind, which was, 

 however, in the line of the blight. Wheat is 



also affected with a similar sort of blight, and 

 about the same season of the year, which totally 

 destroys the crop. "In the summer of 1809," 

 says Dr Keith, " I had watched the progi-ess 

 of the growth of a field of wheat on rather a 

 light and sandy soil, merely from having had 

 occasion to pass through it every Sunday, in 

 going to serve at church. It came up with every 

 appearance of health, and also into ear, with a 

 fair prospect of ripening well. I had taken par- 

 ticular notice of it on a Sunday about the be- 

 ginning of Jul^', as exceeding any thing I should 

 have expected on such a soU; but on the follow- 

 ing Sunday, I was surprised to find a portion of 

 the crop on the east side of the field, to the ex- 

 tent of several acres, totally destroyed, being- 

 shrunk and shrivelled up to less than one-half 

 the size of what it had formerly been; with an 

 appearance so withered and blasted, that I for 

 some time imagined I had got into the wrong 

 field; the rest of the field produced a fair crop." 

 The third kind of blight seizes on the leaves 

 and stem, both of herbaceous and woody plants, 

 such as euphorbia m/parissus, berberis vulgaris, 

 and rhamnus catharticus; but more generally 

 gi-asses, and particularly our most useful grains, 

 wheat, barley, and oats. It generally assumes 

 the appearance of a nasty-looking powder, that 

 soils the finger when touched. "On the 25th of 

 March, 1807," says Dr Keitli, " I examined some 

 blades of wheat that wei-e attacked with this 

 species of blight ; the appearance was that of a 

 number of rusty-looking spots or patches, dis- 

 persed over the surface of the leaf, exactly like 

 that of the seeds of dorsiferous ferns, bursting 

 their indusium. Upon more minute inspection, 

 these patches were found to consist of thousands 

 of small globules, collected into groups beneath 

 the epidermis, which they raised up in a sort of 

 blister, and at last burst. Some of the globules 

 seemed as if imbedded even in the longitudinal 

 vessels of the blade. They were of a yellowish 

 or rusty brown, and somewhat transparent. But 

 these groups of globules have been asceiiained, 

 by Sir J. Banks, to be patches of a minute fun- 

 gus, the seeds of which, as they float in the air. 



a, stem of mildewed wheat ; b. The fungus magnified. 



enter the pores of the epidermis, of the leaf 



particularly, if the plant is sickly; or they exist 



u 



