AWN Vol 42

Wheat leaf rust virulence surveys, conducted annually throughout Brazilian wheat regions, now have identified up to 48 different virulence phenotypes of Puccinia recondita f. sp. tritici in 1995. Following Long and Kolmer (Phytopathology 79:525-529), the following host sets of resistance genes were used to differentiate the collections and to code the races: host set 1 = Lr1, 2a, 2c, 3; host set 2 = Lr9, 16, 24, 26; host set 3 = Lr3ka, 11, 17, 30; and the supplemental host sets 4 = Lr10, 18, 21, 23 and 5 = Lr14a, 14b, cultivar Alondra, Lr20. In 1995, the races LCG-RS (ineffective Lr genes: 1, 10, 11, 14a, 14b, 18, 23, 26) and LCH-RS (ineffective Lr genes: 1, 10, 11, 14a, 14b, 18, 23, 26, 30) were predominant in Brazil. Race TBK-RR (Lr ineffective genes: 1, 2a, 2c, 3, 10, 11, 14a, 14b, 17, 18, 20, 23, 30) was the third most important in occurrence. The races LCG-RS, LCH-RS, and TBK-RR comprised 25.1 %, 9.8 %, and 7.6 % of the population (183 isolates), respectively.

Six races were identified in 1995 for the first time in Brazil: SBJ-JR, LFB-RS, LFG-RS, LFH-RS, LFK-RS, and TBJ-RR.

The genes Lr19 or Lr32, as well as the gene combinations Lr2a + Lr26, Lr3 + Lr9, Lr3ka + Lr9, Lr9 + Lr10, Lr9 + Lr16, Lr9 + Lr20, Lr9 + Lr24, Lr9 + Lr26, Lr10 + Lr16, Lr16 + Lr24, and Lr16 + Lr26 continue to provide resistance to all known Brazilian pathotypes of Puccinia recondita.

The frequency of virulence to Lr genes was calculated for the states of Rio Grande do Sul, Parana, Sao Paulo, and Mato Grosso do Sul from leaf rust collections made in Brazil in 1995 (Table 1).

Wheat cultivars and lines from Brazilian breeding programs are evaluated annually for resistance to leaf rust. Among the 646 entries tested in 1995, 67 were resistant to each of the 13 races of P. recondita, which represents the pathogenic population in Brazil. Considering that durable resistance to leaf rust is associated with adult plant resistance, tests under greenhouse conditions were developed for flag leaf evaluation. In 1995, 94 entries, including old cultivars and present commercial ones, and new releases and breeding lines, confirmed susceptibility at the seedling stage and adult plant resistance to one isolate of the race MFT-HS (virulence formulae: Lr1, 3, 3ka, 11, 14a, 14b, 17, 18, 23, 24, 26, 30).

Table 1. Percentage of virulent leaf rust isolates in 1995 calculated for the

various wheat-producing states of Brazil.

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Lr Rio Grande Mato Grosso

Gene do Sul Parana Sao Paulo do Sul

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1 98.6 100.0 100.0 100.0

2a 30.1 15.7 46.1 92.9

2c 30.1 15.7 46.1 92.9

3 45.2 43.4 38.5 100.0

9 0.0 0.0 7.7 0.0

16 1.4 0.0 0.0 0.0

24 24.7 27.7 0.0 0.0

26 65.7 84.3 53.8 7.1

3ka 13.7 26.5 0.0 7.1

11 89.0 96.4 100.0 57.1

17 39.7 34.9 53.8 85.7

30 41.1 56.6 61.5 35.7

10 89.0 71.1 92.3 100.0

18 97.3 90.4 100.0 100.0

21 8.2 9.6 23.1 0.0

23 89.0 94.0 84.6 100.0

14a 100.0 100.0 100.0 100.0

14b 95.9 97.6 100.0 100.0

20 32.9 16.9 38.5 92.9

Alondra 67.1 83.1 53.8 7.1

No.of

isolates 73 83 13 14

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