i6 DISEASES OF CROP-PLANTS 



their position. They are the result of some irritation of growing 

 tissue, the reaction often appearing very disproportionate to the 

 cause. The larger number of galls are due to insects, but there 

 are many which are of fungoid or bacterial origin. A woody 

 gall, once formed, has its own cambium layer, and consequently 

 continues to grow so long as this maintains connection with the 

 tree. Examples of fungus galls are afforded by the smut disease 

 of Indian corn, and Sphaeropsis disease of Citrus, while the 

 deformity produced in the so-called witch-broom disease of cacao 

 is more of the nature of a gall than of the structures after which 

 it was named. The crown gall disease, found on a wide range of 

 hosts, results from the presence in the tissues of Bacterium 

 tumefaciens. 



Witches-brooms are structures similar in their origin to galls, 

 in which an abnormal number of crowded shoots is formed at 

 some point on a branch and appears as a tuft or mass of inter- 

 lacing twigs. 



The affections of herbaceous stems may be surface spots akin 

 to leaf spots, invasions of the cortex alone, the vascular tissues 

 alone, or both together. Cortical invasion causes local rot or 

 canker, with yellowness of foliage or other sickly appearances as 

 general symptoms ; interference with the vascular bundles 

 produces progressive or sudden wilting. 



The type of disease known as bud-rot is peculiar to monocoty- 

 ledons in which the terminal bud from which growth proceeds 

 is deeply enclosed in the bases of the older leaves. Being thus 

 supported it remains longer in a soft and plastic condition and 

 offers a large mass of highly susceptible material to any parasitic 

 organism which gains access to it. Bud-rots occur in coconut 

 and sugar-cane, but conclusions as to their primary nature should 

 be made with caution, as experience has shown that in both 

 plants the failure of some other part may deprive the bud of the 

 required nutrition and bring about its decay. The closeness of 

 its covering, by excluding air, probably accounts for the pre- 

 dominant part which putrefactive bacteria take in the process. 



Root Diseases. 



Diseases of the roots are in general the least satisfactory of 

 any to investigate and to combat. Experiment is difficult, and 

 observation cannot always be depended on to distinguish cause 

 from effect. The consequence is that knowledge regarding root 

 diseases is not usually definite, except where very pronounced 

 parasites are concerned. 



The soil, with its accumulations of dead matter and its rela- 

 tively constant moisture, harbours many fungi, and among them 

 are found species possessing powers of parasitism in various 

 degrees. 



Sclerotium Rolfsii destroys herbaceous plants ])y attacking 

 the stem and crown roots near the surface of the soil . The forest 



