A partial series of waxy reciprocal translocations involving chromosome 9 and one of each of the remaining chromosomes was used to identify linkage groups controlling slow rusting in the Puccinia polysora/maize interaction. The translocations were in a variable background and two lines of descent (LOD), one more resistant than the other, were evaluated for each inbred/translocation combination. The resistant LOD were analyzed separately from the more susceptible LOD. The inbreds B37R (resistant), Mo17 (slow rusting), B73 (moderately rusting), and Tx5855 (fast rusting) were evaluated in separate field tests in 1986 using a modification of Anderson's procedures (C.R. Burnham, Discussions in Cytogenetics, pp. 110-111). Area under the disease progress curve was calculated from 4 individual observations of pustule density on 10 plants per plot in a 5 block randomized complete block design test for each of the 2 LOD groups. Factors were translocation and endosperm character.
The complete resistance of B37R, which carries the Rpp9 gene, was linked to chromosome 10 in both LOD tests. It was previously known that the Rpp9 gene was linked to chromosome 10 (A.J. Ullstrup, Phytopathology 55:425-428), and B37R served as a positive check. The slow rusting character of B73 was almost entirely explained by linkage to chromosome 4 although chromosome 8 was also indicated important in the susceptible LOD test. The resistance of Mo17 was linked to chromosome 4 and 7 in the susceptible LOD test but not in the resistant LOD test. It was apparent that identification of linkage groups was impaired due to high background levels of resistance in the resistant LOD test. Chromosome 9 was indicated to be important in all the susceptible LOD tests except Mo17. The overall rate of epidemic progression varied between tests and seemed to affect the expression of resistance linked to chromosome 9. Hybrid vigor may also play a role in the resistance linked to chromosome 9 since in each case the normal kernel plots were held heterozygous for large sections of 2 chromosomes while the waxy kernels were forced toward homozygosity. In several cases resistance was linked to the translocation in the resistant LOD test, the most consistent of which were wx T5-9 and Wx T6-9.
It is notable that resistance to Puccinia sorghi (W.A. Russell and A.L. Hooker, Crop Science 2:477-480), and Helminthosporium turcicum (M.T. Jenkins et al., Crop Science 1:450-455) have been linked to chromosome 4. Chromosome 10 also carries a complex group of complete resistance genes to P. sorghi. The consistent association of resistance genes and linkage groups suggests similar gene groups may be controlling both P. polysora and P. sorghi.
B.A. Bailey, J.D. Smith and R.F. Fredericksen
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