98 Problems of Economic Biology in East Africa 



appearance generally in the third month after sowing, so that for the 

 last three months of growth " Bobs " has these two rusts to contend with. 

 On the other hand, "Florence" is nearly ready to be reaped when the 

 rust commences, and has reached a stage when the presence of rust can 

 do but little or no damage. Had "Florence" been sown two months 

 after "Bobs" so as to be ripe at the same time as the latter, it would 

 have been attacked and badly damaged. 



Another condition which plays an important part in the severity of 

 the attack is the amount of nitrogen in the soil. In England, wheat, 

 being an exhaustive crop, is usually grown on heavy land. In East 

 Africa the great majority of soils contain a larger proportion of nitrogen 

 than in England, and it has been shown by experiment that an excess 

 of nitrogen renders wheat more susceptible to the attacks of rust. A very 

 striking demonstration of this fact was provided unintentionally in a 

 certain large field of wheat grown in the Highlands (Njoro) of East Africa. 

 At a distance of a few hundred yards the wheat appeared brown in colour 

 with the exception of a small green triangular patch in one corner. The 

 brown colour was due to the presence of Puccinia graminis and Puccinia 

 triticina, while the green patch was nearly free from rust. The wheat was 

 the variety Kieti and had been sown all at the same time, but on the 

 triangular patch flax had been grown the previous season. Following 

 this observation, repeated trials were made at Nairobi with a susceptible 

 wheat such as " Bobs," one block of which was sown on land which had 

 borne a root crop the season before, and another block of "Bobs" on 

 land which had previously carried flax. The wheat on the old flax land 

 was rusted to a far less extent than that on land which had not borne 

 flax. The explanation offered is that flax, itself a very exhaustive crop, 

 removes from the soil that amount of nitrogen which would otherwise 

 render the wheat very susceptible to the attack of rust. That this ex- 

 planation is probably correct is borne out by the fact that wheat grown 

 upon land which has previously carried beans is so badly affected by 

 rust that in most cases the crop is almost destroyed. In this case the 

 already high nitrogen content of the soil has been augmented by the 

 activity of the nodule forming bacteria of the beans. It has been found 

 that the following rotation of crops is an excellent one and produces 

 satisfactory results: (1) Flax, (2) Wheat, (3) Beans, (4) Flax or Maize. 



The attempt to breed rust resisting hybrids of wheat on Mendelian 

 lines has met with a considerable measure of success, some of the selec- 

 tions so produced possessing such desirable characters as early maturity, 

 good milling grain and "strong" flour. Of two of these varieties, Cross 



