222 



THE AGRICULTURAL NEWS. 



July 8, 1911. 



FUNGUS NOTES. 



MISCELLANEOUS POINTS OF INTEREST. 

 In the following article is contained intoiination on 

 several points, either contributed by various observers in the 

 different islands, or obtained as a result of the examination 

 of various specimens forwarded for this purpose to the Head 

 Office of the Imperial department of Agriculture. 



WHITE EU.STOF swKET POTATO. Some Specimens of leaves 

 of the sweet potato, showing peculiar blisters, were received 

 recently from the Curator of the Botanic Station in Mont- 

 serrat. These blisters were irregular in .shape, though usually 

 roughly hemispherical, and were either concave or convex, 

 when viewed with the upper surface of the leaf upwards. 

 When they were carefully examined with the naked eye, or 

 with a hand lens, it was seen that they were studded with 

 small, white, irregularly shaped pustules, occurring on both 

 surfaces of the leaf, and varying in number according to the 

 size of the blister. Some of the leaves also showed the 

 presence of these pustules without the blister-like malforma- 

 tion. In this case, each was well separated from the other, 

 was roughly circular in shape, and measured from ^-mm. to 

 |-mm. in diameter. These pustules were the fructifications 

 of one of the white rust fungi, Cystopus sp., probably 

 C. Jpomoeaepandnralac (Schw.) Stev. and Sw. This fungus 

 occurs on sweet potatoes in many parts of the world, includ- 

 ing the United States and Brazil, and is also found on various 

 other members of the Convolvulus family. It is closely related 

 to C'l/stopus candidus, the white rust found commonly on 

 cruciferous plants, such as the radish, cress, turnip, mustard, 

 watercress and several others found in temperate countries. 

 Hypertrophy of the host, owing to the presence of the mycel- 

 ium, is common, and takes several forms, according to the 

 species attacked. In this instance it appears as the blisters 

 mentioned above. 



The fungi of this genus belong to the Order Peronos- 

 porales, which includes the genus Phytophthora. They have 

 an intercellular mycelium, provided with haustoria which 

 penetrate the cells of the host; the mycelium is rarely septate. 

 When about to produce fruit, the hyphae form a small pad 

 beneath the epidermis, from which numerous short, erect, 

 basally branched sporangiophores are produced. These give 

 rise to chains of rectangular zoosporangia, or conidia, formed 

 in basipetal succession. As the pustules develop, they rupture 

 the epidermis, and the zoosporangia are thus set free. 

 These germinate, in the presence of moisture, and liberate 

 their contents tlirough a terminal or basal pore in the 

 form of a few free-swimming zoospores, each provided 

 with two whip-like cilia attached laterally. The zoospores 

 come to rest, and germinate by putting out a hypha which is 

 capable of causing fresh infection. The fungus may also 

 reproduce itself sexually, in an additional way, by means of 

 antheridia and oogonia. As a result of fertilization an 

 oospore is formed. This has a thick and warty outer 

 wall, and requires a period of rest before germinating. It 

 gives rise to freeswinuning zoospores, as does the zoosporan- 

 gium, and these are capable of producing a fresh infection. 

 The oospores .serve to carry over the fungus from one crop to 

 the next and help it to tide over unfavourable conditions. 

 Tliey are usually formed in the stems of the host plants 

 attacked, but in some instances they arc produced on the 

 leaves. 



The fungus on sweet potatoes does not appear to cause 

 any very serious damage, but its spread could probably be 



checked, if this is required, by spraying the infected plant.s, 

 and healthy plants in their neighbourhood, with Bordeaux 

 mixture. As the fungus is known on several members of the 

 Convolvulus family, which is well represented in the West 

 Indies, records of its occurrence on other hosts would be of 

 interest. 



ROOT DISEASE OF CASlIf.LOA AND OTHER PLANTS. Re- 



centl)', specimens of young Castilloa trees have been received 

 from Grenada, which had died from the effects of a root 

 fungus. This formed hard, lirownish-black masses of a stro- 

 matal nature on the surface of the bark, and black streaks 

 running into the wood, which was also turned grey in colour. 

 The bark and cambium of the main root were completely 

 destroyed as far as the ground level, and, as is stated above, 

 the mycelium had penetrated the wood. This fungus had 

 many points in common with that causing root disease of 

 cacao, described in the Agricultural Neim, Vol, IX, p. 366; 

 though the fan-shaped masses of mycelium between the wood 

 and the liark which characterize the latter were not present in 

 this instance. There were also many points of resemblance 

 between the fungus found on Castilloa and that on arrowroot 

 in St. Vincent ( see A;/ricultui-al li^en-s, Vol. X, p. 174). It 

 may be worthy of note that two other fungi were found on 

 some of the specimens. * )ne was a Nectria, probably 

 A^. I'ulgai-is, which is often found on decaying tissues killed 

 by root diseases in the West Indies; the other a species of 

 Lasiodiplodia — almost certainly L. theohromae. In addition, 

 numerous shot borers, probably Tomicus sp , were present. 

 These beetles were probably also saprophytes. In any case 

 they were not sufficiently numerous to have caused the 

 diseased condition. 



In connexion with this disease, interesting informa- 

 tion was recently received from the Hon. G. S. Hudson, in 

 St. Lucia. He stated that a tree of Castilloa elastica, w-hich 

 was growing among a group of cacao trees badly infected 

 with root disease^ died suddenly, and all the evidence pointed 

 to its having succumbed to the same fungus as that which 

 killed the cacao trees. Examination of specimens forwarded 

 to the Head Office by Mr. Hudson left little doubt that the 

 Castilloa had been killed b^' the disease found on cacao and 

 that Mr. Hud.son's conclusions were correct. Even more 

 recent information on the subject of this fungus and its host 

 plants has been received from Mr. J. C. Moore, Agricul- 

 ral Superintendent, St. Lucia. Tliis was communicated to 

 him by Mr. L. Mallet Paret, who observed that an orange 

 tree died suddenly, and that its roots were infected with 

 a fungus similar to one that had caused the death of cacao 

 trees on adjoining land. .Mr. Mallet Paret also stated that 

 a nutmeg tree died from the .same disease a short time ago. 

 In his description of the fungus causing root disease of cacao 

 in the West Indian Bulletin, Vol. II, p. 207, Howard records 

 its ])resence on nutmegs in Grenada, and expresses the opin- 

 ion that it is probably identical with one found by Barber on 

 cacao, mangoes, oranges, coffee and bread fruit in Dominica, 

 in 1892-3. The observations given above would appear to 

 confirm Howard's opinion. 



It is also interesting to note that the fungus found 

 on arrowroot in St. Vincent was stated by an observer 

 in that island to attack coffee bushes, and if this |)roves to be 

 the case, there is additional evidence for concluding that the 

 cacao root di.sease fungus is the cause of the ' burning' of 

 arrowroot. The full list of plants at present suspected of 

 being susceptible to the attacks of this fungus is: cacao, limes, 

 orange, Castilloa, nutmegs, mangoes, avocado pear, bread 

 fruit, bread nut, poinme rose, pois doux, immortel, pigeon pea 

 and coffee; while, if it is the same as that on arrowroot, 

 several other plants will have to be added to this list. 



