14 BOARD OF AGRICULTURE. 



maintain that they are prolific sourcps of disease, others that they may excite 

 it in a few instances, and others doubt whether they produce any at ail, but 

 the weight of evidence preponderates^ in fiivor of the conclusion that insects 

 produce a few diseases, aggravate others and convey morbid contai;ious mut- 

 ter ; th.it fungi or parasitical plants, although they principally atJV-ct dec ly- 

 ing vegetation, sometimes prey upon debilitated living tissues, which they 

 destroy by their attacks ; hence, practically, we may consider them in many 

 cases as exciting, or at least secondary ciuses, and may more rationally take 

 measures to prevent or destroy them. 



Tlie ravages of army norms, grasshoppers, and all the larger insects which 

 eat the growing crops, thereby preventing their growth, are not diseases, but 

 mere accidents, like tlie depredations of cattle. 



The ditease of the potato, which, even during the past season haa destroyed 

 more than half the crop in Ireland, deserves particular consideration, on ac- 

 count of its wide prevalence, and the injury it has caused to an article of food 

 of very general consumption. It may be defined an ejiidemic and contagious 

 disease, in which black spots appear on the stems and gradually extend to the 

 tubers, many of which are aflF.-cted by putrid degeneration. It first appeared 

 in the Islands of the Hebrides, but did not widely spread before 1844. In 

 that, and the two following years, it extended over Ireland and other parts of 

 Great Britain, and prevailed also in the United States, every where making 

 great havoc in a crop which before had been healthy and abundant. It rarely 

 commences hefore the end of July, and sometimes not until October. In some 

 instances it has not been discovered until the potatoes have been dug and 

 stored ; hut generally it attairks the plants towards the end of August, often 

 blackening and killing the leaves and stems of the most luxurious vegetation 

 in a few hours. It may affect the foliage and tubers simultaneously, or the 

 tubers may be diseased, while the foliage has a healthy appearance ; but usu- 

 ally, the " potato tops" first look faded and sallow, then the dark spots are 

 seen on the leaves and stems, and after an interval of some days, spots of a 

 brownish color begin on the skin of the tuber, and gradually affect its sub- 

 stance, resulting in ulceration, with an offensive watery discharge, and ter- 

 minating at length in gangrene. As potatoes were mainly propagated from 

 tubers, and the same variety was continued for a long series of years, the idea 

 at first was, that it had grown old, and the disease was the effect of advanced 

 age, and that new varieties just originated from seed might escape ; but it was 

 soon ascertairied that the new varieties equally fell victims. It was also at- 

 trihuted to clianges of temperature, hut it was found to prevail in all vicissi- 

 tudes, in heat and cold, in rain and drouth ; only long continued wet weather 

 seen ed to promote its progress and dissemination. Certain kinds of soil are 

 peculiarly liable, such as damp clay, wet land, gardens pampered with guano 

 and annual excrement, and cultivated fields that have long been robbed of 

 valuaMe ingredients by annual crops of grain and vegetables. 



When decay or putrefaction lias once been induced in the plant, various 

 parasitical growtlis fasten upon it. At least six fungi have been noticed, of 

 which particularly the kind called botrytis infestans penetrates the foliage 

 and tuhers, contributing to their rapiil destruction. They are, however, mere 

 attendants, not primary causes of the disease. 



It is Will known, when the Asiatic cholera swept so rapidly over the world, 

 that it geiit^rally selected for its victims those who<e constitutions had been 

 impaired by previous diseits^ intemperance or debilitating causes; so the 

 potato epidemic has always bjen most destructive to those plants which have 

 degenerated and acquired a predi-position to it by long cultivation and im- 

 proper treatment, it has been ascertained by careful investigation, that many 

 soils have been exhausted of those elements essentially necessary to the healthy 

 growth and robust vigor of the potato. Among the mineral subelances, it 

 especially needs large supplies of potash, of which ordinary soils possess much 

 lees than one part in the hundred ; and yet a farmer, who cultivates five acres 



