An annual average of 360 genotypes of durum wheat Triticum turgidum varo durum and 400 of bread wheat T. aestivum L. were studied under field conditions during 1979-1995.
In the first 7 years leaf rust was more severe on bread wheat than on durum wheat (t = 0.93 N.S.). The contrary happened in the next 10 years: durum wheats was significatively more attacked than bread wheat. During 1986 to 1995 many resistant durum varieties became susceptible in a relatively short time, except 6 samples of durum wheat included in the Crossing Block which mantained its resistance until 1995.
In Brazil, at EMBRAPA, Passo Fundo, 18 different rust races were identified in 38 isolations of urediospores sent from Chile. This could explain the great variability of leaf rust and the frecuent changes in the behaviour of durum varieties to the disease.
Among the genes effective to chilean rust races, Lr 9, located on genome B in bread wheat, could be easily transferred to durum wheat, because this species has the same genome B.
In order to solve this problem, each year hundreds of crosses are made to obtain lines which resistance are not dependable on a single gene but on the action of many genes.
Besides the gene Lr 9 could be easily transferred to durum wheat, located on genome B of bread wheat which showed to be effective iln the resistance to all Chilean races of P. recondita, as genome B is also carried by bread wheat.