EDGAR TULLIS 



51 



By means of the works of the authors mentioned and the discoveries of 

 practical horticulture, as well as the great sensation called forth by the tree 

 wax for injured trees which was discovered by William Forsyth in 1791 and 

 universally overestimated, the conviction of the agricultural significance of 

 plant diseases was extended over so wide a circle that special books could 

 now be published for this branch of knowledge. 



The year 1795 makes us acquainted with three such works. The first 

 one written by Plenk^ treats of the diseases of all cultivated plants 

 of importance at that time and is based on thorough observations. He de- 

 scribes thus : — "d. spongy large outgrowth at some place on the trunk from 

 which exudes, even in the most scorching weather, a caustic moisture which 

 corrodes the whole extent of the swelling." Thus Pyriis Cydonia, standing 

 near a swamp, was attacked by tree canker while other quince trees planted 

 in a higher place were healthy. The sap, it seems, becomes so caustic from 

 the acidity of the standing water that it eats up the ducts. There are two 

 kinds of tree canker determined by the diflFerence in the location of the dis- 

 ease; first, open tree canker, when the canker knots appear on the external 

 surface of the bark ; second, hidden canker, when a sharp cancerous pus 

 collects between the bark and the wood but does not escape from the bark 

 in any place. In both cases the tree becomes incurably wasted, when the 

 parts attacked by canker are not cut out at once and covered with wound 

 wax. In this blight Plenk distinguished between a dry and a moist blight. 

 By the first he means "a black and dry wilting of the leaves or of some other 

 part of the plant" and by "moist blight" he designates the "moist and soft 

 degeneration of the plants into a putrid pus." 



We find almost the same terminology in the explanation of canker in 

 Schreger's- book which otherwise gives many of his personal obser- 

 vations. In regard to the phenomena of blight in which the bark or other 

 parts of the tree appear black and soft and are consumed, he says, "Such 

 black spots of the bark grow further and further round about themselves 

 even attacking the wood so that the bark itself at last splits off, as if dead, 

 and the wood appears dry and black, as if burned." This explanation cor- 

 responds exactly with the phenomena which we perceive when frost causes 

 considerable injuries to the bark. In fact this observer arrives at the same 

 conclusion as we do in regard to the cause. "Bruises from hailstones give 

 rise to its production and also cold frosts. This frost is more injurious in 

 low and moist regions than in high dry ones. For this reason there is less 

 injury from frost on windy nights than on clear, cold ones. If the trees 

 freeze in winter and die, the cause of their death is usually a blight induced 

 by this freezing. This happens sometimes when the severe cold comes too 

 early in the autumn while the sap is still flowing actively ; sometimes in the 

 spring when the sap, so to speak, has begun to run. The latter case is the 



1 Plenk, Physiologie und Pathologie der Pflanzen. Wien 179.5. ^ 



2 Erfahrungsmassige Anweisung zur richtigen Kenntnis der Krankheiten der 

 Wald- und Gartenaume. Leipzig 1795. 



