100 A PLANT-DISEASE SURVEY IN TEXAS. 



PRICKLY PEAR. 



Anthracnose (Gloeosporiumsip.). — The prickly pears {Optintia&pp.) 

 of this region are frequently attacked by this fungus, which produces 

 generally circular or subcircular depressed areas, with grayish cen- 

 ters densely covered with minute black acervuli which become less 

 abundant toward the periphery, which is limited by a zone of brown. 

 The spots are most commonly 1.5 to 2.5 cm. in diameter, but they 

 may be much larger in some cases. The isolated spots may coalesce 

 and cause the complete death of a stem segment. The diseased area 

 always extends completely through the stem segment, and the old 

 dead tissue may persist or it may fall out. In all cases of spots of 

 the average size mentioned the advance of the fungus is limited by 

 a development of corky tissue at the periphery of the area. This 

 same fungus produces on young fronds, under favorable conditions 

 of temperature and moisture, a severe rotting, and the fimgus may 

 advance through an entire segment in a few days, starting from a 

 single center of infection, and thus leave the segment brown and com- 

 pletely dead. In this stage an abundance of pale acervuli is gener- 

 ally produced at the center of the infected area. In such cases the 

 frond is chlorotic some distance beyond the advance of the fungus, 

 and there is frequently a marked gummy exudation from the spot. 



Specimens collected: Austin, 562; Llano, 1773; Hondo. 2252; Georgetown, 

 2393; Round Rock, 2406. 



Black-spot {Perisporium wrightn B. and C). — This fungus pro- 

 duces superficial black spots, generally circular in outline and 5 mm. 

 to 1 cm. in diameter, on the stem segments. They may be few in 

 number, or they may be sufficiently abundant to coalesce and nearly 

 cover the surface. The black color is due to the large numbers of 

 spore fruits or perithecia. No instances have been observed where 

 the fungus was causing any material injury. 



Specimens collected : Austin, 1293 ; Elgin. 1875 ; Round Rock, 2408. 



Scald {Hendersonia (?) opitntlae Ell. and Ev.). — Probably the 

 most general and the most severe disease of the prickly pear is what 

 is popularly called " sun scald " in this territory. The whole sur- 

 face of the older fronds becomes covered with a yellowish-brown, 

 scaly growth of a corky character. In this scaly growth may be seen 

 numerous minute black specks, the fruits of the fungus. Large 

 plants may bo killed, but the fungus remains superficial and the 

 injurious effect is apparently due to the corky covering which cuts 

 off the light in part and prevents the aeration of the underlying 

 tissue. The network of dark-brown fungous filaments may be found 

 just beneath the epidermal layer and also to some extent deeper down 

 iind inclocod in the corky layers. 



226 



