622 INSECTS AFFECTING VEGETATION 



other side be wilted, but that is not usual. Frequently even a single 

 leaf will show one-sided infection. The wilted leaves soon die, dry 

 up and eventually the whole stalk dies. It then remains standing 

 with its dead leaves still hanging. It is thus not to be confounded 

 with temporary wilting due to lack of moisture, excessive heat, etc. 



At the stage of earliest wilting a section across the stem shows 

 a yellowish discoloration of the woody portion. In more advanced 

 stages, or in sections taken lower on the stem, the w r ood is found 

 either on its inner or outer parts to be penetrated longitudinally by 

 black streaks, varying in size from that of a cambric needle to that 

 of a knitting needle. These streaks are so abundant in stages im- 

 mediately preceding death that the whole or nearly all of the wood 

 seems to be so affected. Frequently similar streaks penetrate the pith, 

 though this is only in the most extreme cases. The black streaks in 

 the wood are usually more abundant adjacent to the cambium than 

 to the pith, and simply removing the bark from near the base of 

 sick plants, discloses them in abundance. The blackening often 

 progresses from the wood outward through the bark, producing 

 shrunken, blackened patches on the surface of the stem. 



In more advanced stages when all the leaves are wilting the 

 wood and bark at the base of the plant are blackened nearly through- 

 out and the pith has decayed leaving the stem hollow or filled with 

 a soft, rotten residue. The bark near the level of the ground turns 

 black, and becomes dry and hard. The pith and wood in the upper 

 portions of the plant usually dry up before decay overtakes them 

 resulting in the collapse of the upper portions of the plant in ir- 

 regular longitudinal folds in parts where the woody layer is too soft 

 to maintain the shape of the plant when the support of the dis- 

 tended pith is withdrawn. If a badly diseased plant be cut off near 

 the ground, a dirty yellowish exudate issues from the cut wood, 

 accumulating in a layer one or two millimetres thick. This exudate 

 is slightly viscous, hanging together in strands two to four milli- 

 metres long when picked with a knife point. 



The root seems to be the seat of the original infection, and any 

 plant in a stage of disease advanced enough to show symptoms in its 

 foliage will be found to .possess roots already in an advanced stauv 

 of decay. In early stages one root or more may be diseased; in 

 later stages all succumb ; in the more advanced stages of disease in 

 any root the bark is black, soft and dry, a spongy mass of fibre left 

 by the decay of the more watery parts. In the worst cases even this 

 spongy covering may drop off leaving the wood of the root bare and 

 dry. Usually, however, the bark remains as a spongy layer, sur- 

 rounded by a dry papery jacket more or less cracked transversely, 

 the remains of the epidermis. The decay is characteristically a dry 

 one, although if the soil be wet with rain the decayed residue may 

 become slimy, wet, and mushy. 



The wood of the root undergoes changes similar to those of the 

 stem. In the root as in the stem the disease manifests itself earlier 

 in the wood than in the bark, appearing first as longitudinal streaks 

 of black, in that portion of the woody cylinder lying close to the 



