318 



THE AGRICULTURAL NEWS. 



September 27, 1913. 



FUNGUS NOTES. 



A DISEASE OF RICE. 



Lately the importaEce of the soil as a source of infection 

 of diseases of plant.s hiis become more and more recognized. 

 C. N. Jensen, in America, has directly i.solated from the soil, 

 facultative parasites belonging to the genera Colletotrichum, 

 Fusariuni, Thoma, etc., besides the iiornially occurring para- 

 sites, belonging to Pythiuiu and Phytophthora. Shaw in 

 India has demonstrated the innwrtance of the soil in regard 

 to Tihizoctonia, whilst in the same country .J. F. Dastur has 

 observed that P/it/to/'ht/im-a parasitica, which attacks the 

 castor oil plant, retains its vitality in the soil for two months. 

 In the West Indie.s, South has brought out the relation 

 between soil conditions and root diseases of cacao, lime and 

 sugar-cane. Not the lea.st significant aspect of the subject 

 is that which has arisen from the researches of Russell and 

 Petherbridge on soil-sickness and partial sterilization, namely, 

 that fungi in the soil maj' exert harmful influence on crops 

 through the secretion of toxic substances. 



Leaving these general considerations, attention will be 

 given in the following article to results obtained lately by 

 Shaw in India with regard to a new soil infecting disease of 

 rice described in the Memoirs of the Department of Agri- 

 eultwe of India, Botanical Series, Vol. VI, No. 2. 



^.\TURE OF THE PARASITE. 



The fungus which causes the disease in question is 

 known as Sclerolium Oryzne, Catt., and as the name implies, 

 it is a fuigus the mycelium of which produces hard masses 

 of resistant pseudoparenchyniatous tissue known as sclerolia. 

 It will be remembered that sclerotia are characteristic of 

 Rhizoctonia, too. In the case of the fungus under con.sider- 

 ation it is hardly necessary to add that no perfect fruiting 

 stage has been observed 



THE DISEASE "iN THE FIELD. 



Infected plants can usually be distingui.'^hed from 

 healthy ones by means of the phenomenon of 'tillering', that 

 is to say, the development of fresh green shoots from the 

 adventitious buds at the bare of the infected culm. The 

 diseased culm gradually turns yellow and dies; any grain 

 which it bears is light and poorly developed. It is the loss 

 of grain which constitutes the most serious damage due to 

 the fungus. 



If a diseased stem be split longitudinally, the basal 

 portion is found to be infected with the fungus. The 

 hyphae from a dark greyish waft within the hollow stem, 

 and small black sclerotia can be seen dotted all over the 

 inner surface. At first sight the sclerotia resemble that of 

 Elii'octoma Solani, Kiihn; they are, however, considerably 

 larger and have a smooth shiny surface. 



BEHAVIOUR OK THE FUNGUS IN CULTURE. 



No matter where the material came from, nor whether 

 the cultures were made from hyphae or sclerotia, the same 

 fungus was always obtained. Cultures were made on 

 several different medin, and in some cases the medium was 

 not without inthience on tlie character of the fungal growth. 

 For instance, infection upon nutrient agar containing extract 

 of paddy grains induced the formation of chlamydospores 

 in enormous numbers. Upon Lima beans, to give another 

 instance, the fungus gave rise to a curious red pigment; 

 ■whilst upon maize meal, the rate of growth was remarkably 

 rapid, a dense white mycelium being formed in so short 

 a time as twelve hours. 



INOCUL.VTIONS. 



One of the most interesting, and at the same time most 

 useful part of an investijiation of the present kind is that 

 which includes the artificial infection of the plants tc: see how 

 far the fungus is arv active parasite. Sclerofiwn Ori/zae was 

 found to be particularly vigorous as the following shows: — 

 'The young plants were infected when they were about 

 seven to ten days old and about 3-4 inches high. The first 

 .series of infections was made with small black sclerotia from 

 an agar culture about one month old. None of these inocula- 

 tions gave any result, the scleiotia failing to germinate. Sub- 

 sequent trials with aclerotia from old cultures showed that 

 they had, not iu.'reqaently, lost the power of germination. 

 Fresh inoculations were then made from a culture three days 

 old, in which the hyph\e were still growing vigorously, and 

 sclerotia were not jet formed. A small speck of agar was 

 removed from such a culture and placed upon a rice culm, 

 about I inch above the remnant of the seed; hyphae quickly 

 spread from this centre over the exterior of the culm, which 

 gradually lost its green colour and turned brown near the 

 seat of infection. As the outer leaf sheaih turns brown, the 

 lamina attached to jt also loses its green colour and wilts; 

 finally, the process extends to the central leaves, and the 

 whole plant dies. During the progress of the infection 

 a light weft of hyphae can be seen investing the culm; in 

 the later stages of the disease small, den^e, white aggrega- 

 tions of hyphae appear in this mycelium, and, ultimately, 

 become hard black sclerotia of the usual type. This superficial 

 production of sclerotia is a characteristic of the section Libera 

 of this genus. V\'e liave, however, seen that sclerotia may 

 arise in the more deep seated portions of the host, so too 

 much stress must not be laid on this character. The first 

 sclerotia usuall)' occur about the top of the first leaf sheath, 

 either on its inner or outer surface; in the former case thej' 

 appear as small dark swellings beneath the dry and withered 

 leaf base. The time taken from the first infection until the 

 death of the plant and the production of tlie sclerotia is about 

 two weeks.' 



On the whole, 70-80 per cent, of the inoculations proved 

 fatal. It may be added as an interesting fact that 

 a successful inoculation killed the infected plant completely, 

 but, in the field, the result of an attack seemed rither to be 

 a gradual weakening of the host, culminating in the failure 

 to produce good seed. 



KEMEDIES. 



Against a fungus like Sclerotiiim Ori/uc, which can 

 undoubtedly perenii^ate in the soil, it is difKcult to see what 

 remedial measures can be employed with success. The 

 application of sal-ammoniac to the soil has been suggested, 

 but more probably eflforts in the direclion of breeding 

 resistant varieties would answer a more useful purpose. 

 Fortunately the damage done in India at the present time 

 does not appear to justify any concern in regard to the 

 immediate necessity for selecting special strains of rice 

 resistant to the disease. 



THE STERILIZATION OF SEED. 



Several chemical reagents can be emploj'ed in the 

 sterilization of seeds. The following extract contains 

 a sun.raary of the results, with regard to the use of 

 hydrogen peroxide obtained by Iv}- Massee, and pub- 

 lished in the Kovi Bidlctiv of iMiscellancflus Iv forma- 

 tion No. 5, 1913.— 



