422 Journal of Agricultural Research voi. v, No. lo 



HONEYCOMB HEART-ROT IN WHITE OAK 

 MACROSCOPIC CHARACTERS 



The first indication of this honeycomb heart-rot in white oak (Q. alba) 

 is a slight discoloration of the heartwood, which assumes a water-soaked 

 appearance. This "soak" may extend from i to 6 feet beyond the 

 actually rotting region where delignification is occurring. When dry, 

 the water-soaked heartwood becomes tawny in color. 



Light-colored, isolated areas appear in the discolored wood. These 

 areas, which are the beginnings of the pockets, usually originate in the 

 region of the large vessels and often have a small medullary ray in their 

 centers. The rot then spreads in all directions into the surrounding 

 tissue, but moves more rapidly in the summer wood of the annual ring 

 of the preceding year. This results in the bulk of the pocket lying in the 

 summer wood of one year and the spring wood of the succeeding year. 



The next stage of the rot is one of delignification in which very small 

 irregular patches of delignified wood fibers appear in the light-colored 

 areas. This delignification, which seems to begin in the wood fibers of 

 the preceding year's growth of summer wood immediately adjacent to a 

 large vessel, proceeds rapidly until white, oval to circular pockets appear 

 PI. XLI, fig. 3). In radial section these lens-shaped pockets range from 

 5 to 15 mm. long by i to 5 mm. wide, with their main axes parallel to 

 the grain of the wood. These pockets are at first filled with white cel- 

 lulose (PI. XLI, fig. 3 and 4), which later is gradually absorbed, leaving 

 cavities lined with the remnants of the cellulose (PI. XLI, fig. 5). Some- 

 times long lines of cellulose fibers extend longitudinally through sev- 

 eral adjacent cavities, but, as a rule, the cellulose is limited to each indi- 

 vidual pocket. 



The attacked area increases in size until the pockets reach a large 

 medullary ray on either side (PI. XLI, fig. 6). These large rays seem 

 to check the activity of the enzyms and therefore become the boundaries 

 of the radial walls of the pockets. They are very evident even in the 

 badly diseased heartwood (PI. XLI, fig. 6). This is especially notice- 

 able in tangential and cross-sectional views. Each pocket usually does 

 not involve more than two annual rings of growth, unless the rings are 

 very narrow, in which case several may be included. In cross section 

 the rot shows as irregular to circular holes from i to 5 mm. in diameter 

 lying between the large medullary rays. 



All the cellulose finally disappears (PI. XLI, fig. 2 and 7), leaving the 

 pockets either (i) empty, (2) containing the shrunken white membranes 

 of the included vessels, or (3) more or less filled with mycelium. 



In the last stage of the rot the wood is very light and of a honeycomb- 

 like structure (PI. XLI, fig. 2 and 7). The pockets are longer than they 

 are broad, and all of the wood has disappeared, except the thin walls 

 surrounding the pockets, which remain distinct and usually involve the 

 heartwood uniformly. The rotted wood is therefore in the shape of a 

 cyUnder. 



