349 



Leaf Casting Diseases. 



The leaf casting diseases form the most significant case of premature 

 dropping of the leaves. We speak here in the plural, although it is custo- 

 mary generally to call a sudden dropping of the needles of young pines 

 "leaf casting." All plants can "cast their leaves" which are capable 

 in any way of pushing off their dying leaf apparatus. The only concern, 

 then, is wliether the leaf body in its entirety suddenly becomes functionally 

 weakened, or even functionless. It is only because it appears so uncom- 

 monly abundantly among pines and is accompanied by severe results that 

 the dropping of the pine needles is cited especially often for "Leaf Casting." 



This form of disease manifests itself most frequently and severely in 

 seedlings two to four years old, the needles of which suddenly become 

 I'rownish-yellow or brownish-red in the spring and fall after a short time. 

 The considerable spread of this phenomenon dates only from the general 

 change in the cultural methods ; instead of sowing the seed and of the 

 Femel management, the raising of plants in seed beds has been introduced. 



Since that time it has been observed that in the months from March to 

 May. often within a few days, great areas of seedling plants look as if they 

 had been burned. In this, however, it can be noticed that young plants 

 protected by a not ver\' close conifer forest, or one of mixed trees, or, in 

 nurseries protected by trees of an earlier seeding, do not cast their needles, 

 while exposed areas in the open or in enclosed places are extraordinarily at- 

 tacked by the disease. Specimens with pruned roots suffer more than those 

 with long, vigorous ones, while plants on wet soil suffer most intensely. 

 Mountain plantations are less attacked than those on plains and a northern 

 exposure seems to be almost entirely spared, while a southern or western 

 one suffers greatly. 



The disease does not appear every year, but generally only after wet, 

 cold winters with but little snow, and alternating sharp frosts. The plants 

 cast their needles most extensively if dry springs, March and April, are 

 distinguished by bright warm days followed by cold nights. Often the 

 phenomenon occurs in stripes or spots. It has been obsen^ed further, that 

 plants protected from the noonday sun by neighboring woods, etc., general- 

 ly did not become diseased. Plants in seed beds, which were left covered 

 until after the time of spring frosts, did not cast their needles while ad- 

 jacent, unprotected seedlings did so. Seedlings grown between older 

 covered plants or between broom plants, even those protected by high grass, 

 did not develop the disease, while others where the broom plants had been 

 dug out in the spring were attacked. 



Ebermayer^ in explanation of these facts, states that observations of a 

 forestr}^ experimental station, made for several years, showed that in March 

 and April the soil temperature down to 1^4 meters was scarcely more than 



1 Ebermayer, Die physikalischen Einwirkungen des Waldes auf Luft und 

 Boden etc. Resultate der forstl. Versuchsstat. in Bayern. Aschaffenburg-, 1873. 

 Vol. I, p. 251. 



