This gene for adult plant resistance shows recessive inheritance.
Chromosome Location
3BS (Hare and McIntosh, 1979). Sr2 is closely associated with Lr27, one of two complementary genes involved in conferring resistance to leaf rust (Singh and McIntosh, 1984a) and with pseudo-black chaff which involves melanin pigmentation of the glumes and stem, particularly below the uppermost node.
Low Infection Type
Usually characterised by slow rust development and low terminal rust responses on field-grown adult plants. More severe rusting may occur above the nodal regions and on the spike.
Environmental Variability
May be less effective under wet and overcast conditions.
Origin
T. turgidum var. dicoccum cv. Yaroslav. Resistance was transferred to Hope and H44-24 by McFadden (1930).
Pathogenic Variability
Virulence is not common and would be difficult to detect on field-grown adult plants. Knott (1977) was unable to detect resistance in lines with Sr2 using North American race 15B-1.
Reference Stocks
s: Chinese Spring*6/Ciano 3B (Singh and McIntosh, 1984a); Chinese Spring*6/Ciano 5B (Singh and McIntosh, 1984a); Chinese Spring*6/Hope 3B (Hare and McIntosh, 1979).
v: Newthatch Sr5 Sr7b Sr12 Sr17. Hope Sr7b Sr9d Sr17; H44-24 Sr7b Sr9d Sr17 (Hare and McIntosh, 1979).
Source Stocks
Sr2 occurs in many wheats developed in areas where stem rust has been a problem. These cultivars were developed in Australia, Canada, Kenya, USA, Mexico and the Indian subcontinent (see Luig, 1983; Roelfs, 1988).
Australia: Songlen Sr5 Sr6 Sr8a Sr36 (Luig, 1983). Warigo Sr7b Sr17 (Hare and McIntosh, 1979). Suneca Sr8a Sr17 (Gyarfas, 1983). Hopps Sr9d (Hare and McIntosh, 1979). Hofed Sr17 (Luig, 1983).
Canada: Pembina Sr5 Sr6 Sr12. Rescue Sr5 Sr9g. Selkirk Sr6 Sr7b Sr9d Sr17 (Hare and McIntosh, 1979). Redman Sr7b Sr9d Sr17; Regent Sr7b Sr9d Sr17 (Hare and McIntosh, 1979); Renown Sr7b Sr9d Sr17 (Luig, 1983).
CIMMYT: Present in many CIMMYT selections. Bluebird series, for example Nuri 70 Sr5 Sr6 Sr8a. Lerma Rojo 64 Sr6 Sr7b Sr9e. Pavon Sr8a Sr9g Sr30 (McIntosh, 1988b).
Indian Subcontinent: Sonalika (McIntosh, 1988b).
Kenya: Kenya Plume Sr5 Sr6 Sr7a Sr8a Sr12 Sr17 (Singh and McIntosh, 1986b). Kenya Page Sr7b Sr17.
USA: Eagle (USA); Kaw. Arthur Sr5 Sr8a Sr36; Arthur 71 Sr5 Sr8a Sr36. Ottawa Sr9d. Lancer Sr9d Sr17; Scout Sr9d Sr17. Karl Sr9d Sr24. Scoutland Sr17.
| A. Stem rusted field-grown plants of (L to R): Chinese Spring/Hope 3B substitution line, the F1 of CS/Hope 3B and Chinese Spring, and Chinese Spring. CS/Hope 3B carries Sr2 which is recessive and shown by the susceptible F1 plant. Courtesy RA Hare. B. Stem rusted field-grown plants of (L to R): Renown, five homozygous F3 lines of the cross Renown/Line E and Line E. The first three F3 lines are resistant and the last two susceptible. Plants with Sr2 may develop large pustules, particularly just above the node. Cultivars differ in the amount of disease that develops, for example compare CS/Hope 3B in A. with Renown. Note the melanin pigmentation below the nodes of plants with Sr2. Blackening of the glumes and peduncles may also occur. Courtesy RA Hare. C. First and second seedling leaves, respectively, of (L to R): Sunstar, Hartog and two F2 seedlings of Sunstar/Hartog; infected with stem rust and incubated at 25°C. Hartog and the first F2 seedling possess Sr2 as indicated by the chlorotic leaf symptoms. Sunstar and the second F2 plant do not carry Sr2. The chlorosis seems enhanced by infection with either stem rust or leaf rust and is independent of pathotype. However, seedlings of plants with 5r2 frequently develop less sporulating uredinia than related lines lacking Sr2. Courtesy GN Brown. |
Use in Agriculture
Sr2 is arguably the most important gene for stem rust resistance and one of the most important disease resistance genes to be deployed in modern plant breeding (McIntosh, 1988b; Rajaram et al., 1988; Roelfs, 1988). With the possible exception of Canada, Sr2 has provided durable resistance since its introduction to hexaploid wheat in the 1920s (McFadden, 1930). In the North American epidemics of the 1950s cultivars such as Regent, Renown and Redman became moderately susceptible. Many wheats with Sr2, including Hope, possess additional genes for resistance such as Sr7b, S19d and Sr17. When the latter genes were effective, they conferred very high levels of resistance. However, the presence of virulence for Sr7b, Sr9d and Sr17 often revealed that the residual resistance conferred by Sr2 was much less effective.
In these situations, Sr2 was overlooked because of the apparent increased disease levels. Hare and McIntosh (1979) described phenotypic effects of Sr2 suggestive of attributes often associated with non-hypersensitive and/or non-specific resistances, thus supporting the conclusion that Sr2 was a source of durable resistance.
The degree of interaction involving Sr2 and other genes is unclear. The high degree of seedling resistance of Hope and several other wheats (IT ;) to North American race 56 was apparently caused by the interaction of Sr2 and Sr9d (Knott, 1968). When present alone, the latter gene confers IT 11+ (Knott, 1968; 1990). The association of Sr2 and distinctive stem and spike blackening (pseudo-black chaff) is well known (see Hare and McIntosh, 1979). Brown (1993) has shown a close association between Sr2 and a seedling leaf chlorosis that is evident at greenhouse temperatures above 22°C.