Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 67

sunflower worldwide. In 1992, the second disease was Phomopsis
( Diaporthe helianthi ), followed by charcoal rot ( Macrophomina phaseolina ),
downy mildew ( Plasmopara halstedii ) and broomrape ( Orobanche cumana ).
However, in 1996, downy mildew overtook Phomopsis and in 2000 and
2004 there was a relative increase in papers on broomrape and black-stem
( Phoma macdonaldii ) (all of the last from France). The other main diseases
reported were Alternaria leaf blight ( Alternaria helianthi ), white rust ( Albugo
tragopogonis ) and rust ( Puccinia helianthi ).
Methods of breeding for resistance to disease are broadly of two types,
according to whether resistance is controlled by major genes (downy mildew,
broomrape, rust, Verticilium ), or follows quantitative inheritance (white rot,
Phomopsis, grey rot, Alternaria leaf blight). Below, two examples of each
type are described, but it should be noted that, because of problems of
pathogen changes resulting use of single or a few major genes, combinations
of the two types of inheritance are now envisaged, to obtain high levels of
durable resistance.
2.4.4.1 Diseases Controlled Mainly by Major Gene Resistance
2.4.4.1.1 Downy Mildew ( Plasmopara halstedii )
Of worldwide importance, the disease is maintained in the soil up to 10
years, and transported occasionally on seed. The hypocotyls and roots of
young plants are infected in humid conditions. The plants become dwarfed
and produce no seed.
Major gene resistance to downy mildew has been obtained from both
cultivated sunflowers and other Helianthus species. It is generally functionally
complete but some genes give resistance reactions which stop progress of
the pathogen more slowly than others, with the result that seedlings may
show some symptoms but the plants grow normally. This type of resistance
is determined by a test on germinated seeds which are soaked in
suspensions of zoosporangia and then pricked out and grown for two weeks
(Cohen and Sackston 1973). Then, after 48 h at 100% RH, susceptible plants
show sporulation on cotyledons and true leaves and resistant plants no
sporulation or only on cotyledons (Vear 1978). Major gene resistance is
inherited as single dominant genes which have been shown to be situated
in clusters (Vear et al. 1997). However, it is race-specific, many different P.
halstedii races being known in North America (10 races), and Europe
(14 races) for example (Gulya et al. 1991; Moinard et al. 2006).
For the pathogen, the center of origin appears to be North America, but
from about 1960, downy mildew was reported in Russia, with a race (100)
never reported in the USA, where the first race was 300. While new races
appeared quite regularly in the USA (Gulya et al. 1991), in Europe, there