Agriculture Reference
In-Depth Information
Table 5.5. Popularity and resistance of cv. Triumph spring barley to powdery mildew
( from Parry, 1990)
Certified seed (t)
b
% total barley market
b
Harvest year
NIAB Recommended
List resistance rating
for mildew (1-9)
a
1980
8
4609
2
1981
8
41329
13
1982
8
58370
20
1983
8
65480
22
1984
7
49049
19
1985
2
45164
16
1986
2
36091
14
1987
2
30204
12
1988
2
28232
10
a
A high figure indicates a high degree of resistance
b
From MAFF Seed Certification Scheme statistics
The history of the
Mlg
gene in barley is mirrored in most of the major gene intro-
ductions in barley and other crops. For example, the wheat cultivar Eureka, released
as a rust-resistant variety in 1938, began to show signs of susceptibility in 1941; by
1942 it was becoming heavily rusted (Johnson, 1961). Arabica coffee is known to
have five resistance genes to leaf rust (
SH1, SH2, SH3, SH4, SH5
); however, these
have not provided durable resistance. Eskes (1983) observed that within three years
of rust presence in Brazil, the resistance genes had lost their effectiveness.
Not every plant-pathogen interaction that is characterized by single dominant
genes is liable to rapid breakdown of resistance. Examples of major gene resistance
that have a proven durability include the single gene for resistance in cabbage which
has not been overcome by the known races of
Fusarium oxysporum
f.sp.
conglutinans
(Fry, 1982) (Table 5.6).
Table 5.6. Examples of crop plants with single gene resistances against which pathogen races
have evolved slowly or not at all ( from Fry, 1982)
Crop
Pathogen
Cabbage
Fusarium oxysporum
Cucumber
Cladosporium cucumerinum
Corynespora melonis
Maize
Helminthosporium carbonum
Oats
Helminthosporium victoria
