BREEDING RUST RESISTANT TREES: MODERATOR'S SUMMARY 619 



THE EFFECT OF TEMPERATURE ON THE EVOLUTION OF 

 MAIZE AND ITS TWO RUST PARASITES 



Experimental and observational evidence indicates that the open- 

 pollinated maize varieties currently grown in Mexico consists of 3 rather 

 distinct groups of ecotypic races primarily differentiated by temperature 

 (elevations) . The 2 maize rust pathogens have evolved in an analogous 

 way. P. sorghi prospers better at low temperatures and predominates at 

 the higher elevations, where the maize varieties are resistant to this 

 disease, but not to P. polysora. The opposite is true in the lowland 

 tropics where P. polysora is predominant. At intermediate elevations 

 both P. sorghi and P. polysora occur and there the maize cultivars in 

 common use are resistant to both species (Schieber, Rodriguez, and Fuentes 

 1964) . 



Apparently through the long process of evolution and natural selec- 

 tion the maize varieties grown in each of the 3 elevational zones have 

 come into equilibrium with their rust parasites. So long as these 

 balanced host-parasite systems are left undisturbed by man - scientist 

 or farmer - the host exhibits a high degree of resistance to the rusts 

 and suffers little or no damage. 



Do not be misled into believing that the two species of Vuceinia 

 that parasitize maize are less pathogenic or less aggressive than those 

 that attack wheat, oats, flax, or pine trees (Borlaug, 1966). Their 

 pathogenicity becomes evident when the equilibrium between host and 

 parasite is disturbed. 



If the rust-resistant maize varieties, in either the temperate 

 uplands or tropical lowlands, are inbred and tested in their native 

 habitat, some lines will be developed which are killed by the rust while 

 others remain resistant like the parent variety. Moreover, if one moves 

 open-pollinated lowland maize varieties into the high elevations, they 

 will rust severely. Similarly, when high elevation maize varieties are 

 sown in the tropical lowlands, they are seriously infected. However, 

 if the tropical varieties that rust severely at high elevations are 

 inbred and the lines are evaluated at the high elevation sites, it will 

 be found that some inbred lines can be isolated which show good levels 

 of rust resistance. The same phenomenon is observed when inbred lines 

 are developed from high elevation varieties and tested in the lowlands. 

 The varieties from the intermediate elevational zone, however, can be 

 moved either into higher elevations or lower elevations and still retain 

 a high level of rust resistance. 



Such experiments indicate that polygenes for general resistance to 

 the 2 species of rusts exist in all varieties of maize in each of the 

 three elevational zones, but that the varieties differ in the frequency 

 of genes for resistance to the 2 species depending upon elevation. A 

 high frequency of polygenes for resistance to P. polysora exists in low- 

 land maizes whereas these same varieties possess only a low frequency of 

 genes for resistance to P. sorghi. The opposite is true for the high 

 elevation maize varieties. The maize varieties of the intermediate 

 elevational zone have a high frequency of genes for resistance to both 

 P. polysora and P. sorghi. 



