572 TEXTBOOK OF BOTANY 



Peach leaf curl is generally more widespread in years having cold wet 

 spring seasons. Potato scab has been found to be most severe at a tem- 

 perature of about 70^ F. The fusarium-wilt organism which infects 

 tomatoes growing in greenhouses makes little progress at soil tempera- 

 tures of 63° F. and 95° F., but develops rapidly at temperatures midway 

 between these extremes. When growing in soils of either low or high 

 water content, the tomato plant is not affected much bv the wilt organ- 

 ism. A medium water supply results in a vigorous and succulent plant 

 which is quite susceptible to wilt. Potato scab appears to be more severe 

 when the host plant grows in dry rather than in moist soil. 



Apple scab develops rapidly in cool, moist weather. Rainy periods be- 

 tween the time of opening buds and petal fall increase the prevalence of 

 the disease. Discharge of ascospores from the asci in fallen leaves occurs 

 only when they are thoroughly wet. The new leaves on the tree will 

 become infected only when they are continuously wet for several hours. 



Variations in hosts, parasites, and plant diseases. It has been known 

 for many years that the extent of infection and the destructiveness of 

 plant diseases vary from season to season, from host to host, and even 

 among individuals of the same species of host. Many of these phenomena 

 cannot be accounted for by the effects of the environmental factors 

 enumerated above. A better understanding is possible today because of 

 increased knowledge of the genetic complexitv both of host plants and 

 of plant pathogens. 



Varying degrees of infection and injury may occur even under ap- 

 parently similar field environments. A species of fungus causing disease 

 may be composed of several races, each differing from the others in its 

 virulence on specific hosts. Owing to population shifts, some of these 

 races may be more abundant or less abundant than others at various 

 times on different hosts. For example, it has been reported that the 

 species of fungus ( Puccinia graminis ) which causes stem rust of wheat 

 is a mixed population of at least 180 races, each of which can be dis- 

 tinguished by its relative virulence in different varieties of wheat. 

 Through a period of time the relative abundance of the different races 

 fluctuates, with corresponding changes in infection and destructiveness 

 of the disease. 



Both mutation and hybridization must be considered. They may occur 

 in the host, or in the pathogen when it is growing either in pure culture 

 or inside the host. From a highly mutating pathogen several new mutant 

 races, differing in virulence in various hosts, may appear within a few 



