350 



5 degrees C, while the air temperature in the shade not infrequently was 

 higher than 19 to 22 degrees C. Such differences in temperature between 

 the air and the soil result directly in the excessive transpiration of the aerial 

 parts of the plant, while the roots kept in a state of inactivity because of the 

 cold soil, are incapable of taking up the soil water, or not to the amount 

 necessary to replace the aerial loss. Thus the young pines dry up even 

 when the soil is abundantly wet. 



The greater the difference between the soil and the air temperatures in 

 direct sunlight, the more abundant and devastating is the leaf casting. On 

 the other hand, the more frequently conditions arise which raise the soil 

 temperature, such as warm spring rains, or which prevent a greater lowering 

 of it, i. e. masses of long unmelted snow or of mulch, the less the disease 

 appears. This lessening of the disease will take place also if the temper- 

 ature of the air and the intensity of the sunlight are decreased as, for ex- 

 ample, by a very cloudy sky, by a position on northern slopes, or under 

 the protection of larger trees, high grasses or bushes, or by the artificial 

 screening of the seed beds during the day. 



That older plants suffer less often from leaf casting is explained, in the 

 first place, by the more strongly developed wood which in all plants may be 

 considered as a water reservoir; in the second place, by a more abundantly 

 developed, deeper reaching root system, which possesses more organs for 

 absorption in its greater number of fibrous roots. 



Holzner^ has raised an objection to this theor}^ In leaf casting, dis- 

 coloration appears within 2 to 3 days, while, in an actual process of dr}'ing, 

 the pine needles redden only gradually. He considers the cause a direct 

 effect of frost. It is a well established fact that frost will also cause leaf 

 casting. Baudisch- protected seedlings by a brush covering one meter deep 

 above the surface of the soil. The plants which had remained healthy until 

 the protection had been removed then suffered from the April frosts. 



Many authors ascribe an injurious influence to autumn frosts^. The 

 theory most generally accepted at present considers the disease to be para- 

 sitic and, accordingly, recommends that it be treated with fungicides. Ac- 

 cording to V. Tubeuf's* experiments, it cannot be doubted that there are 

 also cases of parasitic leaf casting"'. However, the fact must be taken 

 into consideration, that the fungi of leaf casting are present in abundance 

 on older pines, firs, spruces and larches, without calling forth the specific 

 phenomena. Therefore, unless some conditions especially favorable for the 

 much dreaded juvenile disease are present, it cannot become epidemic. . 



1 Holzner, Georg. Die Beobachtungen iiber die Schiitte der Kiefer oder Fohre 

 und die Wintei-farbung- immergruner Gewachse. Freising, 1877. Here bibliography 

 of 145 studies on leaf casting. 



1! Centralbl. f. d. ges. Forstwesen VII, 1881, p. 362. 



•■! Alers in Centralbl. f. d. ges. Forstw. 1878, p. 132. Nordlinger ibid p. 389. 

 Dammes and others, .Tahrbuch d. schles. Forstvereins 1878, p. 40 ff. 



4 V. Tubeuf, Studien iiber die Schiittekrankheit der Kiefer. Arb. d. Biolog. Abt. 

 am Kais. Gesundhcitsamt. Part II, 1901. 



■• Cf. Vol. II, p. 268. 



