2i6 DISEASES OF CROP-PLANTS 



areas are produced, of an inch or more in diameter, irregular 

 in shape and marked by the dying of the epiderm and its 

 breaking up into silvery scales. Again it may take the form 

 of ridges, conical elevations, or other shapes. The corky 

 areas are dull brown in colour in some instances, but very com- 

 monly are a dull red with brown margins. This latter stage 

 occurs where the disease is especially virulent, the ridge and 

 plateau conditions being more often noted in instances where 

 but Httle disease is present. This latter is the type found on 

 oranges. Young fruits are frequently much distorted, assum- 

 ing triangular or other peculiar shapes. Those most dis- 

 torted fall to the ground soon after attack, It may be noted 

 that a larger percentage of scabbed fruits fall than of normal 

 ones. Infected fruits remaining tend to regain their normal 

 shape by subsequent growth, and as there is no increase in 

 size of the initial infections, the great increase of the surface 

 areas of the fruit gives all appearance of a partial recovery 

 or ' cleaning up ' from the disease. This, of course, is not 

 what takes place, but it merely means that there has been 

 no further spread of the scab areas. 



" Leaves are frequently distorted, the same corky areas 

 appearing as on the fruit. Where infection is not general, 

 very marked conical projections, often as much as a centimetre 

 in height, appear. 



" The disease areas are surface infections only, there being 

 practically no penetration in either leaves or fruit of the inner 

 host tissue. In fact, so effectively are the tissues protected 

 that scabbed fruits are not any more subject to decay than 

 normal ones ; less so, if anything. Many such have been 

 kept in the laboratory for long periods of time until they 

 finally become hard and mummified." 



Causation. 



There has been some dispute as to the causation of the disease, 

 but those mycologists most closely in touch with it are convinced 

 that it is due to infections, occurring only when external circum- 

 stances are favourable, with the fungus Cladosporium citri 

 Massee. The occurrence of the fungus in the scabbed tissue is 

 somewhat obscure and it is said to be masked, in Florida at least, 

 by a saprophytic species resembling the common Cladosporium 

 herbarum. The parasite is regarded as not typical of the genus 

 Cladosporium as ordinarily understood. The only reproductive 

 bodies obtained in pure cultures are thick-walled, dark-coloured 

 packet-like clusters or chains of cells, and these were also found 

 on old withered scabs. 



The description of C. Citri Mass. is as follows : — Conidiophores 

 tufted, erect, branched, septate, brown, 30-75 X 2-4 microns, 

 conidia fusiform, dusky, continuous, or 1-3 septate, 8-9X2.5-4 

 microns. 



