GROWTH OF PARASITIC FUNGI. 



&c., predispose plants to an attack of fungi, independent of atmospheric influence; thus 

 we read respecting the diseases of wheat — "it rarely happens that blight, rust, and mil- 

 dew are felt in sunny seasons, except in confined enclosures, or marshy ground, where 

 the evening dews stagnate, and fogs are generated." Again, " in looking over a blighted 

 field of wheat, we may observe that the lowest and richest parts, or where it stands thick 

 upon the ground, are more affected than those which stand higher." Wheat to which 

 manure is directly applied, is found more subject to rust and mildew, than that which is 

 grown upon a clean fallow. An interesting instance of the predisposing influence of an 

 excess of manure, is mentioned in the Agricultural Gazette. Wheat which had been top- 

 dressed with guano, " was a good crop, and free from symptoms of blight, excepting in 

 the spots where the sacks were put down; here the straw was blighted, evidently from re- 

 ceiving an overdose." Now, all these difierent circumstances would tend to make wheat 

 plants unhealthy, hence the attack of fungi — it cannot be attributed to atmospheric influ- 

 ences in these cases; since wheat growing in large open fields, or on the higher parts of 

 fields, or on clean fallows, instead of on land recently manured, or if plants growing at 

 proper distances apart, escaped. The atmospheric conditions under which the healthy and 

 blighted plants were growing, must have been the same in most of these instances, but 

 other conditions which affected the health of the plants, were not the same; the inference 

 therefore, is, that the latter are most likely to be the conditions which led to the attack of 

 fungi. 



Andrew Knight considered that one of the principal causes of mildev/ was the want of 

 sufficient moisture in the soil, more especially if excessive humidity in the air, and low 

 temperature succeeded warm bright weather. The pea when cultivated late in the fall is 

 very liable to be attacked by mildew, and Mr. Knight found that by deepening the soil 

 and by copious watering he could prevent its appearance. In a forcing house he found it 

 equally easy of appropiate management to introduce or prevent the appearance of mildew. 

 " When he had kept the mould very dry, and the air in the house damp and unchanged, 

 the plants soon became mildewed, but when the mould had been regularly and rather 

 abundantly watered, not a vestige of the disease has appeared." The development of 

 fungi in these cases also was obviously not dependent solely on certain atmospheric con- 

 ditions, but partly on the state of plants consequent on the moisture or dryness of the soil 

 in which they were growing. 



We have further evidence that the growth of parasitic fungi does not depend exclusive- 

 ly on atmospheric influences. Varieties of the same species of plant growing under the 

 same circumstances may off"er greater resistance to fungi than others, owing apparently to 

 some peculiarity of constitution, or to greater constitutional vigor, or to the stage of growth 

 the}' have reached when exposed to the external influences which favored the devolopment 

 of the parasite, When examining crops of potatoes on the first outbreak of the latest 

 form of disease of that plant, known as the blight, or murrain, I have observed early va- 

 rieties with the foliage destroj^ed, many of the tubers having the peculiar rotten-apple-like 

 appearance while the parasite had only just commenced its ravages on some adjoining 

 patches of a later variety. Instances have occurred in which tubers of a late variety had 

 been introduced with the manure into a plot of any early variety, and the late plants con 

 tinned to grow vigorously, while the foliage of the early ones was nearly destroyed and 

 the tubers decaying. If the growth and increase of fungi depended solely upon certain 

 atmospheric conditions, should not the attack have been simultaneous in these cases.' The 

 plants were growing under the same circumstances, living in the same soil, breathin 

 same atmosphere, and whether the germs of the parasite are admitted into plants 



