Plant Sanitation* 



410 



May, 1910. 



cent. ; but on the plot without a wind- 

 belt, the increase is 131 per cent, in 1904 

 and 37 per cent, in 1905. Hence the im- 

 provement is greatest on the plot roithout 

 a wind-belt. Both plots are much below 

 the average of the whole estate for 1905, 

 viz., 7,117 pods, though of course the rest 

 of the estate had no wind-belts. Another 

 point which must be taken into con- 

 sideration is that the method of count- 

 ing the crop by the calendar year gives 

 as real information about the annual 

 improvement of a given plot. For the 

 crop year is from June to June, and the 

 heaviest yielding months are about the 

 change of the calendar year ; consequent- 

 ly one calendar year may have only two 

 main crop months, while the next calen- 

 dar year may have four. The latter 

 may then appear twice as good as the 

 former, although the actual crops are 

 really equal. Unfortunately it is not 

 possible to correct the figures of the 

 experiment quoted above, since the num- 

 bers of the plots are unknown, 



THE MISTLETOE PEST IN THE 

 SOUTH-WEST. 



(Prom the U- S. Department of Agricul- 

 tural Bureau of Plant Industry, 

 Bulletin No. 166.) 



Summary- 

 (1.) In general, the American mistletoe, 

 like its European prototype, is more 

 cherished because of its biologic interest 

 and historic setting than feared for its 

 harmfulness to trees. In some districts, 

 however, notably in Central Texas, its 

 destructiveness as a tree parasite out- 

 weighs other considerations in its behalf. 



(2.) The region in which mistletoe is 

 most destructive coincides with the 

 transition from a humid climate favour- 

 able for forest growth to a dry climate 

 less favourable for trees, and where the 

 effects upon tree growth are such as 

 to furnish the parasite more favourable 

 light conditions than in the closer stands 

 and denser foliage ot humid climate 

 forests. 



(3.) The harmfulness of mistletoe is 

 due in part to its mechanical injury 

 to trees (deformity of branches and 

 trunk, wounds followed by decay), but 

 more especially to its drain upon the 

 trees' vitality by withdrawing water 

 and nutriment substances from them. 

 The sinkers which connect the parasite 

 with the water-transporting vessels of 

 the wood and the cortical roots which 

 ramify in the soft bark are the means 

 by which the parasite withdraws sub- 

 stances from its host. 



(4.) The first infection of a tree by 

 mistletoe takes place only through the 

 agency of a germinating seed placed upon 

 the body or branch of a tree by birds 

 (mostly mocking birds, wax wings or 

 cedar birds, and robins), except in the 

 rare case where berries fall upon a 

 branch from a bunch of mistletoe in 

 an overtopping adjacent tree. The sub- 

 sequent spread of infection upon a tree 

 may take place by the falling or wash- 

 ing of berries upon other parts of the 

 tree from the previously established 

 mistletoe shrubs, or by the spread of 

 cortical roots from which new mistletoe 

 shoots arise. Spreading by cortical roots 

 occurs more readily upon some species 

 of trees than others, and is especially 

 stimulated by the injury or removal of 

 the original shoot, # 



(5.) The mistletoe seed and seedling 

 exhibit unusual powers of resistance to 

 drying out, and are thereby enabled 

 to survive in considerable numbers the 

 critical period from the time the berry 

 is placed upon a branch until the para- 

 site plant becomes established. This 

 period may extend beyond the first 

 growing season. 



(6.) A tree may become infected at 

 any point where living tissue is exposed 

 or covered only by a thin layer of cork 

 with breathing pores, but the most 

 vulnerable points are the young branches 

 and sometimes buds. The sinker of 

 the mistletoe seedling is able to pene- 

 trate certain tissues by dissolving the 

 walls of cells living in its path. It is 

 uncertain whether cutinized or corky 

 cell walls can be so dissolved, but the 

 writer believes that they can, 



(7.) The trees most liable to infection 

 are those which occur singly or in 

 clumps or rows along streets and high- 

 ways, in vacant lots and parks, along 

 the border of fields, and narrow strips 

 of timber along streams. The damage 

 to trees in forest stands is negligible. 

 Shade and ornamental trees suffer most. 



(8.) While it is not certain that any 

 broad-leaved tree is wholly immune to 

 attack from the American mistletoe, 

 some are practically so, although free- 

 dom from infection seems to vary with 

 locality. In the choice of trees for plant- 

 ing the question of the ability of a tree 

 to resist infection might profitably be 

 considered. It is believed that any tree 

 subject to infection may be infected 

 by seed from mistletoe growing upon 

 any other species ; e.g., the hackberry 

 may be infected by seed of mistletoe 

 grown on the elm, the live oak from 

 those on the mesquite, etc. 



