A Further Study of the Dry-Rot Disease of Opuntia

Abstract

As pointed out in an earlier note on a dry-rot disease of Opuintia 2 the spots tend to run together so that the infected side is completely destroyed if the spots are not too far apart, and are rather inumerous (FIG. 1). The parasite must grow rather rapidly at first. WVhen plants like the one shown in the figure were brought to the laboratory for study it was noticed that the spots did not seem to increase very much more in size even after several weeks. Sections extending from healthy through to the diseased tissue disclosed the reason for this. After a certain period following infection the fungus becomes completely cut off from the healthy tissue on all sides by a well marked callus tissue consisting of three or four layers of cells (FIG. 2, a). Wolf3 described a similar reaction of the host against the advance of the parasite lHeIdcrsonia Opuntiae, the cause of sun scald of the prickly-pear, the mlain difference being that in the scald disease the suberized layers are laid down parallel to the hypodermis, while in our disease the callus cuts in obliquely from the epidermis, beginning just beyond the limits of hyphal growth, extending down under the diseased tissue and up obliquely again on the opposite side of the spot. Because of the callus, the disease does not usually extend completely through the segment. This differentiates it from the Splhacrella disease described by Wolf and others, a