6S 



FtJNGOlD ENEMIES OF THE PRICKLY 

 PEAR. 



"Black Rot" or "S>hothole." 

 These are two different manifestations of a 

 disease produced by the fungus Glceosporium 

 lunatum, Ellis and Everhart (1891, p. 82). It 

 has been briefly referred to by Dr. Griffiths 

 (1908 a, p. 28) and by Messrs. Hunter, Pratt, and 

 Mitchell (1912, p. 17), the causal fungus being 

 indicated by them as a Perisporium. It was the 

 most destructive prickly-pear malady noticed by 

 the Commission, though its full effect was not 

 seen, as its attacks are particularly dangerous to 

 its host-plant during spring, more especially if 

 weather conditions be humid during that season 

 of the year. Dr. Griffiths, Mr. Hunter, and Mr. 

 Mitchell, all of whom have had long experience 

 in prickly-pear matters, regard this Glceosporium 

 as being capable of doing more damage than any 

 other fungus which they have seen attacking 

 Opuntias. 



The disease, which occurs quite commonly 

 in Texas, was first seen by the Commission on 

 Mr. W. Sinclair's farm, near San Antonio, 

 Texas, where it was found to attack the various 

 Texan species of Opuntia, 0. lindheimeri, 0. 

 ferruginispina, and their allies. Here, as else- 

 where, a gummy exudate was commonly found 

 associated, but it does not seem to be a necessary 

 symptom or effect of the malady. No serious 

 effects were seen during the visit of the Com- 

 mission (October, November, 1913). 



It usually manifests itself by the presence 

 of greyish areas of dead plant tissue extending 

 through the joint, these portions being cut off 

 from the living tissues of the Opuntia by a layer 

 of corky material. Not infrequently the diseased 

 piece drops out, and then one observes the "shot- 

 hole" effect produced on the attacked segment. 

 These holes may be few or numerous; they may 

 be quite small or they may reach an inch or more 

 in diameter.* The malady may be associated 

 with the presence of small spots which are gener- 

 ally confined to one surface. Each of such spots 

 is seen as a rounded, sunken, grey area on whose 

 dry surface there may usually be seen abundant 

 tiny, dark, fruiting bodies (acervuli) of the fun- 

 gus. When these "spots" are very numerous 

 they do not extend deeply into the plant tissue, 

 whereas when they are relatively few they pene- 

 trate the joint, the fungus mycelium readily 

 finding its way through the segment, its lateral 

 ramifications being usually limited by the layer 

 of corky material which is produced 'by the host 

 tissues in response to the injury, t In some cases 

 the fungus was found to have developed too 

 rapidly for the plant tissues to prevent the 

 advance, and in such eases the "black rot" stage 

 had taken place. It is this condition which is 

 produced in humid weather during spring. 



Some dead and dried joints were placed 

 under moist conditions, and it was found that 

 the diseased areas soon produced abundance of 

 salmon-coloured conidia or spores, differing from 

 those generally met with in Gloeosporium in that 

 they were uniseptate. The mycelium is appar- 

 ently able to survive in the dead tissues, and to 



* Dr. Griffiths's photograph of O. pachona shows the 

 presence of a large " shot-hole" evidently produced by 

 this Gloeosporium (1910, plate 22). 



t The development of this protectivei layer is brought 

 about by mechanical injury also. 



produce crops of spores when the more favour- 

 able conditions of springtime return. 



Dr. Wolf has published a good account of 

 the malady, which he had under careful observa- 

 tion for a long time (1912a, p. 116). He men- 

 tions that the disease at first causes the forma- 

 tion of soft, sunken, brownish areas on which in 

 a few days abundance of flesh-coloured acervuli 

 appear. A few weeks later these areas become 

 dry and hard and the acervuli blacken. When 

 infection occurs late in the season or upon old 

 joints, the formation of conidia in these acervuli 

 does not occur. He goes on to say that under 

 favourable conditions of temperature and mois- 

 ture young segments are readily attacked, rot- 

 ting setting in very rapidly, and from a single 

 centre of infection only a few days may be 

 necessary for the fungus to produce a decay 

 extending through the joint. Infection was found 

 to occur in the spring, and to depend entirely 

 on the climate and on the age of the segment. 

 He did not observe the occurrence of new infec- 

 tion in the field on old joints, and stated that the 

 fungus was able to gain admission only while the 

 plant tissues were quite young and scarcely 

 differentiated. Rainy or cloudy days in spring 

 were found to be favourable to the growth of the 

 fungus as well as the Opuntia, and it is on such 

 days that the Gloeosporium is able to invade the 

 host and produce a serious rotting. The effect 

 is thus to destroy the young growth. Even on 

 old infected segments, the fungus is capable of 

 developing further during rainy weather, and 

 may bring about the destruction of the joint. 



Wolf made repeated attempts in greenhouses 

 and in the laboratory to inoculate segments of 

 various ages by planting mycelium on the surface 

 of the joint, as well as by applying conidia by 

 means of a spraying apparatus. These were all un- 

 successful, but if a wound were made with a 

 scalpel or needle infection followed, the fungus 

 developing very rapidly, and penetrating through 

 the segment in the case of young joints, but 

 spreading slowly and not penetrating deeply in 

 the case of older segments. He also enclosed 

 some cactus bugs Chelimdea vittigera, Uhler, 

 under a bell jar along with healthy and diseased 

 young segments, but there was no evidence that 

 the insects had infected the former. However, 

 he thinks that they may be the cause of some of 

 the infection in the field, especially since they 

 are abundant during the spring. Mr. Hunter 

 (1912, p. 17) also believes that Chelinidea may 

 act in this way. 



Dr. W. Orton and Miss B. C. Field of the 

 Division of Pathology, Bureau of Plant Industiy, 

 Washington, D.C., kindly made cultures of this 

 Gloeosporium from specimens forwarded by the 

 Commission from San Antonio, Texas, and found 

 the cultural characters of the fungus to fully 

 agree with the account given by Dr. Wolf (pp. 

 118, 119). The latter failed to attain the ascig- 

 erous condition in his cultures, even after three 

 years, but commonly found it in the field about 

 a month after the formation of the areas (p. 

 122), the perithecia occurring on both surfaces 

 of the joint, and arising from the stroma of the 

 old acervuli. This stage was recognised as 

 SpJmrella opuntim, Ellis and Everhart. Miss 

 Field was unable to find the ascigerous or Glome- 

 rella stage of the Glceosporium, but foimd a 

 species of Colletotrichum fruiting in abundance 

 in one of the specimens sent from Texas. 



