64 



in forwarding cultures of the fungus grown 

 on string-bean agar, Miss Field wrote as 

 follows : — 



" I have grown this organism on the follow- 

 ing media :—Coru-meal mush, corn-meal agar, 

 Irish potato cylinders, Irish potato agar, rice, 

 synthetic agar, and wheat-heads. I find that it 

 fruits best on string-bean agar, wheat-heads, and 

 Irish potato agar. The following is the formula 

 we use in making up the string-bean agar : — 



" To each 500 grams of clean string-beans 

 (rinsed in tap and distilled water) add 1,000 c.c. 

 of distilled water. Steam one hour. Filter 

 through paper. Let cool, add 20 grams of 

 agar-agar to each liter of solution, let stand till 

 powder is moist, stir and steam for one-half hour. 



" Stir, tube, plug with cotton and sterilize 

 for 15 minutes at 11 lb. pressure. 



" The wheat-heads are simply placed in 

 tubes, water added and sterilized for 20 minutes 

 at 11 lb. pressure. ' ' 



These cultures have been handed over to 

 Dr. White, the officer in charge of the Prickly- 

 pear Experiment Station at Dulacca. 



Sun Scald. 



A disease very commonly met with in 

 prickly-pear regions in Texas is that known as 

 Sun Scald, which is usually confined to the older 

 parts of the plant. Affected joints give rise to 

 quite healthy segments. 



The symptoms, which are described by Wolf 

 (1912a, p. 128), are as follows: — Discolouration 

 commences around the areoles as well as else- 

 where, and then a series of more or less concen- 

 tric brownish zones makes its appearance around 

 each of these areas, frequently extending in such 

 a way as to meet other "spots" so that quite 

 commonly the whole segment may be "scalded" 

 and greyish-brown in appearance. The surface 

 becomes scaly, and on some of these scales one 

 may recognise small dark areas indicating the 

 presence of fungi. A cursory examination made 

 by us revealed the presence of dark-brown 

 hyphse as well as a few spores of a Cladosporium 

 or Macrosporium type of fungus, but it appears 

 doubtful whether the condition is due to the 

 presence of fungi as Wolf suggests. It seems 

 probable that the popular idea is correct — 

 i.e., that the disease is due to unfavourable 

 climatic conditions. These would tend to cause 

 the development of corky tissue below the epi- 

 dermis in order to prevent excessive loss of mois- 

 ture. The fungoid infection would thus be 

 secondary. 



Wolf (1913a, p. 128) found on 0. lind- 

 heimeri a fungus associated with this condition, 

 and has identified it as Hendersonia opuntice, 

 Ellis and Everhart, which was originairy 

 described in 1902 from 0. ficus-indica from 

 Alabama. This fungus, which he regards as 

 beiing the cause of the disease, is said to gain an 

 entrance through the stomata, ramifying through 

 the epidermal cells and eventually blocking the 

 stomata, giving rise to the scalded appearance. 

 As a result of the presence of the parasite in the 

 epidermal tissue system, the cuticle and the 

 upper part of the epidermal wall become 



ruptured and pushed up, allowing air to entef— 

 hence the grey colour and flaky appearance o± 

 the diseased areas. Besides, there is a forma- 

 tion of corky issue by the plant,* the fungus 

 thus being prevented from invading the deeper 

 portions of the joint. The pycnidia are 

 developed in the aggregations of fungoid tissue 

 in the stomata. He goes on to say that this is a 

 very serious disease of the prickly-pear, since 

 the fundamental tissues gradually disintegrate, 

 and the segment dies as the result of the for- 

 mation of fungoid plugs in the stomata and of 

 the corky layers below the epidermis, all 

 photosynthetic activities being prevented. 



The fact must not be overlooked, however, 

 that unfavourable weather conditions may 

 account for all the symptoms of sun scald, the 

 fungi being then regarded as secondary. In a 

 later paper, Wolf (1912 b, p. 295) seems to have 

 modified his opinions slightly, as he states that, 

 though Sorauer is of opinion that it may be due 

 to unfavourable atmospheric conditions, it "may 

 also be due in part at least. ... to the presence 

 of a fungus, Hendersonia opuntice. ' ' 



Heald and Wolf (1912, p. 101) briefly refer 

 to sun scald, and mention that a species of Rhab- 

 dospora may sometimes be associated with the 

 Hendersonia, but that it is apparently only of 

 secondary importance. 



Black Spot Disease. 



This malady, which is due to the presence of 

 a fungus, Perisporium wrightii, Berk, and Curtis, 

 is not serious, nor is it as commonly met with as 

 the other two diseases of prickly-pear. It was 

 seen on various species of flat Opuntias near 

 Robbstown, Brownsville, and San Antonio, in 

 Texas. As already mentioned. Dr. Griffiths 

 (1908a, p. 28), as well as Messrs. Hunter, Pratt, 

 and Mitchell (1912, p. 17), refer to the Glceo- 

 sporium disease under the name of Perisporium. 



In its early stage the black spot malady 

 resembles that caused by Gloeosporium, both 

 appearing as small, rounded, smoky areas below 

 the epidermis of the joint. Later on these spots 

 become quite dark, on account of the formation 

 of abundance of black-fruiting bodies or peri- 

 thecia which become elevated above the general 

 surface of the joint. These perithecia may be 

 readily scraped off the surface, when a smoky 

 area is seen below, indicating the presence of dark- 

 coloured mycelium within the plant tissues, the 

 fruiting portion of the fungus having developed 

 through the stomata. Surrounding each "black 

 spot " is a yellowish or chlorosed zone. Later 

 on, there appears in the diseased area a brown 

 discolouration somewhat similar to that seen in 

 sun scald, and this is no doubt due to the forma- 

 tion in each ease of a cork layer below the in- 

 jured tissues. As a rule by this time the black 

 spot condition has disappeared. One may find 

 all stages on the same segment, and even when 

 abundant no appreciable damage appears to be 

 done to the plant. 



* Wolf allowed some segments to partially dry out in 

 the laboratory, and found that a cork layer was developed 

 as a means of checking the loss of water, this layer being 

 similar to that found in joints affected by " Sun Scald." 

 He suggests that the fungus, by destroying part of the 

 cuticle and epidermis, exposes the underlying tissue, and 

 in order to prevent excessive evaporation the plant 

 protects itself by interposing a layer of corky tissue. 



