[Chap. XLV PLANT DISEASES 573 



generations. These new races may remain constant or continue the vari- 

 ability of the species by further mutations. 



In those fungi where nuclear fusions occur, recombinations of heri- 

 table factors due to hybridization of different races may result in hybrids 

 varying in degree of virulence. For example, investigators at the Univer- 

 sity of Minnesota crossed two monoploid races of the corn smut species 

 and from the resulting hybrid obtained nearly 40 segregates, no two of 

 which were alike. 



How is an organism proved to be the cause of a disease? When lesions 

 are produced in a host plant by a parasite, other parasites may enter 

 and also become established there. The presence of more than one or- 

 ganism sometimes makes it difficult to decide which one caused the 

 initial lesion. When a new disease is discovered, it is important to deter- 

 mine the causal organism. These decisions are made by following a 

 standard procedure known as "Koch's rules of proof." 



1. The organism must be shown to be present wherever the disease 

 occurs. 



2. The organism must be isolated in pure culture from the lesions in 

 the host. 



3. Healthy tissues when inoculated with the pathogen from these cul- 

 tures must have the usual svmptoms of the disease. 



4. The organism should be reisolated from the second host and identi- 

 fied with the organism originallv isolated. 



Symptoms of plant diseases. The external symptoms of plant diseases 

 are of many kinds, and onlv a few of the commoner ones will be sum- 

 marized here. 



1. Pti.stiilcs are fruiting bodies or spore masses of parasitic fungi which 

 have ruptured the outer tissues of the host. Rusts and smuts are common 

 examples. 



2. Scabs and blotches are surface lesions caused by local erowth of 

 fungi. Apple scab is a common example. 



3. Mycelia may be prominent, such as the external mvcelia of powdery 

 mildew, the compact masses of mycelia of ergot of rve, and the black 

 "tar-spots" on leaves of willow and maple. 



4. Overgrowths of host tissues result in malformations of leaves, stems, 

 and roots. Crown gall of apple, raspberry, and grape; black wart of 

 potato; black knot of cherry and plum; clubroot of cabbage; root nodules 

 of legumes; nematode galls on roots of tomato and cotton; insect galls on 



