90 Prohlerns of Economic Biology in East Africa 



The third and perhaps most important observation is the effect of 

 altitude and, therefore, of temperature both on the tree and on Hemileia. 

 At the Mission Station of Bura near the coast, at an altitude of nearly 

 2000 feet, the annual rainfall is about 50 inches and the temperature 

 about 75-80° F. both day and night. The atmosphere is therefore warm 

 and moist, conditions favourable to the luxuriant growth of coffee, but 

 much more so to Hemileia vastafrix which has destroyed the coffee planta- 

 tion attached to the Mission. In the neighbourhood of Nairobi the general 

 altitude of the plantations lies between 5000 feet and 6000 feet, and the 

 rainfall of the district averages 30 inches. The atmosphere is therefore 

 dry, although it is to be noted that very heavy dews are precipitated at 

 night, and it is in this dew that the uredospores usually germinate. The 

 temperature is never very high, and rarely exceeds 75^^ F., dropping 

 again at night to the region of 50-40° F. and sometimes lower. The 

 atmospheric conditions are warm but not moist and the general balance of 

 conditions is less favourable to the spread of Hemileia than to the growth 

 of coffee. Ten miles to the north-west of Nairobi in the Limuru district 

 very different conditions prevail. The altitude is greater, between 6000 

 and 7000 feet, and hence the climate is colder on the whole. On the other 

 hand, the rainfall is much greater, averaging between 60 and 70 inches. 

 The atmosphere is saturated in the mornings and a " Scotch mist*' is the 

 normal experience. The climatic conditions at Limuru, therefore, are the 

 reverse of those 10 miles away, and a moist but comparatively colder 

 atmosphere prevails. Coffee under these conditions is not so luxuriant 

 in growth, is slower but more hardy. Hemileia is prevalent throughout 

 the district but is scarce; the first attack is the worst as is the case at 

 lower altitudes, but it is nothing like so severe, and in well-kept planta- 

 tions in a normal season the rust has to be searched for. 



The conditions then which prevail at altitudes of 6000 to 7000 feet 

 are still favourable to coffee but very much less so to Hemileia and 

 the limiting factor to the rapid spread of the disease is temperature. 

 At such altitudes the temperature is too low for the parasite to 

 flourish. 



It has been pointed out that subsequent attacks of the rust are less 

 severe than the first, which means that the coffee trees acquire a certain 

 power of resistance, or become less susceptible after the initial attack. 

 That this partial immunity is not due to a lessening of the virulence of 

 the parasite is demonstrated by the fact that a hitherto unattached 

 plantation in the vicinity of others which have been already visited is 

 much more severely infected when Hemileia is present on all at the same 



