PLANT DISEASES 



583 



[Chap. XLV 



Plasmodium grows. If the cambium is invaded, further development of 

 the tissues of the root is very irregular ( Fig. 264A ) . The disease is most 

 prevalent in warm, wet, acid soils. Control methods recommended are: 

 use of disease-free plants, application of hydrated lime (from one to 

 two tons per acre), planting of less susceptible varieties, avoidance of 

 planting cabbage in infected fields. 





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Fig. 264. A, clubroot of cabbage; B, nematode galls on roots of tomato. Photos 

 from U. S. Department of Agriculture. 



Galls. The development of galls on leaves, stems, and other parts of 

 plants is induced by insects," bacteria, and fungi. The greater variety of 

 galls are caused by insects. Galls mav be formed on many kinds of 

 plants and are readily seen on oaks, hackberries, willows, goldenrods, 

 asters, and roses. In some way, perhaps primarily by means of hormones, 

 the organisms living in these plants initiate peculiar overgrowths of 

 certain local tissues, with the ultimate formation of a gall having a 

 specific form and pattern. Leaves and young twigs are the parts usually 

 affected. The different forms and patterns of galls are correlated with 

 the causal insects, rather than with the plants on which they occur. 

 In many instances the insects can be identified from the characteristics 

 of the galls (Fig. 265). 



Nematode galls. The nematodes, sometimes called roundworms, may 

 live either as parasites or free in moist soil containing organic matter. 



- These insects include gall-wasps, gnats, aphids, and mites. 



