THE PESTS AND DISEASES OF THE CANE 171 



The Uba (Kavangire) cane in Porto Rico has been found to be quite 

 immune to yellow stripe. 



The reed-hke canes of British India, e.g., Chunnee, were found to be 

 immune to sereh, and were hence used as a parent in the earl}/ seedling 

 work of Kobus. 



2. Plant Hygiene. — Employ on the plantation only those methods of 

 agriculture that tend to give a healthy condition to the plant. It is highly 

 probable that many of the organisms connected with cane diseases are only 

 weakly parasitic, and only become active when the plant is weakened by 

 negligent agriculture. The remarks of Harrison^^^ on this point are well 

 worth quoting : — 



" I have personally never favoured the readiness so apparent of late years 

 to refer almost every instance of decreased yield in cultivated plants to the noxious 

 action of microbes or fungi. It appears to me for a long time back M^e have in the 

 tropics rather neglected what I may call the physical and chemical hygiene of our 

 cultivated soils, and have not paid sufficient attention to the soil conditions which 

 may have materially reduced the naturally resistant powers of plants to the attacks 

 of bacteria and fungi. And, further, I think that the susceptibility of certain 

 kinds of plants, for instance the Bourbon cane, to injury by drought and fungus 

 attacks is due in part at least to the defective conditions of soil hygiene, under 

 which in places they are now cultivated." 



3. Rotation of Crops. — It is highly probable that many of the cane 

 epidemics have been aided by the wide-spread custom of growing cane 

 continuously on the same soil. In this way the causal organism has a con- 

 tinuous habitat, and, being afforded opportunity for indefinite increase, 

 may in time develop a strain of intensified virulence. When other plants 

 are grown in rotation there is a period over which there is an absence of the 

 host plant, when the fungus must tend to disappear in quantity, and more 

 especially when it is an obligatory parasite of the host. It would 

 appear that it is the fungi causing the various " root " diseases that would 

 be most affected by this method of control, since in this case the soil itself 

 becomes infected. As bearing on this is the observation that in the West 

 Indies ratoon crops are known to be more liable to root disease than are plant 

 crops. Apart from a rotation of crops a rotation of varieties might be utilised 

 since the susceptibility of different varieties towards diseases varies widely. 



It may also be called to mind that wheat grown continuously at 

 Rothamsted has shown itself much more liable to disease than when rota- 

 tions were practised. 



4. Use of Healthy Seed. — This end can be obtained by careful selection, 

 by growing seed cane in nurseries remote from infected areas, or by using, 

 as seed, cane from parts of a plantation not affected. In Java, for example, 

 it was found that cane grown in mountain districts and used as seed gave 

 a certain degree of immunity to sereh, and an industry independent of the 

 plantations proper has developed. It has also been found that the yellow 

 stripe disease is hereditary, and may be controlled by the use of selected 

 disease-free cuttings. As this disease is more of the nature of a pathological 

 condition than a disease due to a specific organism, the method in this case 

 would amount more to the selection of an immune strain. It would also be 

 reasonable to hope that the continued selection of cuttings free from the 

 causal organism of any disease might give an immune strain, as the healthiness 

 of the particular cutting might in itself be due to immunity. 



