52 DISEASES OF CROP-PLANTS 



These affections resemble the typical mosaic diseases in that 

 they are systemic, affecting the whole plant, that recovery is 

 unknown, and that they find expression in general effects on the 

 foliage and in the eventual dwarfing of the shoots. Their 

 symptoms, however, are not nearly so uniform. 



In curly-top, leaf-roll, and sereh a degeneration (necrosis) 

 of the phloem strands in the vascular bundles has been claimed 

 to be characteristic, and has been variously regarded as the 

 cause or the consequence of the visible changes in the organs 

 of the affected plant. 



In the curly-top of beet the veins of the leaves are distorted, 

 and the leaf blade is either curled inwards, or retracted and 

 puffed out between the veins. The foliage is dull-coloured and 

 brittle, eventually turning yellow from below and dying. Dense 

 masses of rootlets are thrown out. None of the symptoms is in 

 itself specific, but the effect on the veins is regarded as the most 

 definitely characteristic. Where the stage of stem production 

 is reached the development is feeble and results in dwarfed 

 and stunted shoots which may or may not survive to the point of 

 producing seed. Plants may be infected and killed in the seedling 

 stage. 



In potato leaf-roll growth is slow, the shoots are dwarfed by 

 the reduction of the internodes, the leaves are discoloured 

 yellow and are rigid and brittle, the edges of the leaflets are 

 rolled inwards. In a later phase the leaf tissue dies locally, 

 producing black or brown spots. The symptoms may appear 

 from the outset of growth, or develop only at the top of the shoots. 



In sereh disease of sugar-cane there is great variation in the 

 development of symptoms. Typically there is a shortening 

 of the internodes, which in the leafy top of the shoot results in 

 a characteristic fan-like arrangement of the leaves ; the leaf- 

 blades are short and narrow and may die in an irregular manner ; 

 the stems are crowded with aerial roots ; the buds develop pre- 

 maturely, until in the later stages the stool is reduced to a dense 

 mass of grass-like shoots. Two or three years are occupied in 

 reaching the extreme condition. The vascular bundles of 

 diseased canes, especially in the rootstock and at the nodes, 

 are reddened by the deposit of gum in their elements. 



The most characteristic symptoms of peach yellows are 

 prematurely ripe, red-spotted fruits, and premature unfolding 

 of the leaf buds into slender pale shoots, or into branched,broom- 

 like growths. In successive seasons the trees become gradually 

 stunted and enfeebled, dying in 3 to 5 years. 



Peach rosette is a rapidly developed affection in which all 

 the leaf buds grow into compact tufts or rosettes, containing, 

 though seldom more than two or three incles long, usually 

 several hundred small leaves. A tree thus attacked dies during 

 the following autumn or winter. 



As already stated, symptoms of the types described above may 



