162 



A BACTERIOSIS OF PRICKLY PEAR PLANTS 

 (OPUNTIA spp.). 



By Professor T. Harvey Johnston, M.A., D.Sc. University of x^delaide, 

 and L. Hitchcock, Commonwealth Prickly Pear Laboratory, Brisbane. 



. [Read Jmie 14, 1923.] 



Whilst the senior author was in Miami, Southern Florida, in 1920, on behalf 

 of the Commonwealth Prickly Pear Board, making inquiry regarding diseases 

 and insect enemies of prickly pears, his attention was drawn by Mr. Simmonds, 

 officer in charge of the Plan Introduction Garden belonging to the Federal 

 Department of Agriculture, to a few very sickly Opuntia plants. Their appear- 

 ance suggested a bacteriosis. The infected specimens were O. tomentella, from 

 Guatemala, in Central America, and O. ficus-indica, from Columbia, South 

 America. Prior to his departure from U.S.A. he arranged for material to be 

 sent across later by one of his colleagues, Mr. J. C. Hamlin. Specimens collected 

 by the latter in Florida in March, 1921, were eventually received at the Prickly 

 Pear Laboratory in Brisbane as dried fragments, complete disintegration having 

 occurred en route. Cultures were made and the various bacteria obtained were 

 sorted out, the causal organism being ultimately obtained. 



The disease is first recognizable by the appearance of a rounded blackish 

 area, a bright purple margin being commonly seen adjacent to a narrow chlorosed 

 region separating it from the uninvaded plant tissue. The lesion is at first 

 generally much more obvious on the one surface, i.e., the originally infected 

 surface, but soon the disease reaches the opposite portion of the cladode and the 

 lesion spreads. The underlying parenchyma becomes completely disintegrated 

 and the cuticle may sink somewhat. In other cases, the gas formed as a result 

 of the organism's activity causes the cuticle to bulge out, the gas collecting 

 below it. 



As the disease progresses the distinct purplish colouration close behind the 

 advancing edge of the lesion becomes very marked. The parenchymatous tissue 

 ultimately disintegrates into a greenish-brown, or even dark-brown, slimy, foetid 

 liquid. The disease does not spread along the vascular bundles, but evidently 

 advances through the breaking down of parenchyma cells. Fully disintegrated 

 segments may resemble dull-green or grey-green cushions containing liquid and 

 gas, but as there is frequently some opening by which these products of putre- 

 faction can escape, only a dried cuticle surrounding the vascular strands may 

 remain. 



The disease will not travel from an infected segment to that above or below 

 it, the organism being unable to utilize the vascular bundles. It is then limited 

 to the parenchyma of the inoculated segment. Under certain conditions of 

 climate the progress of the malady is so slow that the protective mechanism of 

 the invaded cladode is able to limit the lesion, the diseased portion then drying 

 out and collapsing. Such happens under cold and dry conditions. \\'hen 

 temperature and moisture are suitable (as occurs in Queensland during the hot, 

 moist, summer months, January to March) the disease advances too rapidly 

 for the plant to circumscribe it. A lesion which has become limited by the 

 activity of the plant cells not infrequently may subsequently extend through 

 the surrounding tissues if weather conditions be favourable, when the destruction 

 of part or whole of the segment may result. The effect on the stems is somewhat 

 similar, but the disease spreads very slowly unless assisted by some inoculating 

 agent such as a borine insect larva. 



