Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 66

1994). However, this character is generally negatively correlated with oil
content, and often about 2% oil is lost in the hulling process, so it has not
been included in breeding programs.
2.4.3 Cycle Length
The earliness of a genotype can be defined in several ways. The date of
flowering is important in determining the period when the plant is most
susceptible to drought and to Sclerotinia capitulum attack. However, for the
farmer, the most important character is the date at which sunflowers can be
harvested. Physiological maturity (maximum yield and oil content) are
reached when the seed has about 35% water content (the capitulum is yellow
with brown bracts, and more than half of the leaves are dry). There is a
certain independence between drying of the seed and drying of the
capitulum. The most usual measure of earliness is therefore the moisture
content of seed at, or slightly before harvest, when the range between early
and late varieties is at least 10 points. Differences in maturity date depend
more on the length of the period flowering-maturity than on the pre-flowering
duration. Broad sense heritability of both flowering and seed water content
are moderate (0.4) but narrow sense heritability of flowering is higher (0.62)
than for seed maturity (0.29-0.43) (Chervet and Vear 1989). Only the water
content of female lines are correlated with those of their hybrids, restorer
lines, which carry apical branching, have small capitula which dry quickly.
Conventional breeding is generally by development of inbred lines with
acceptable flowering dates for the region and then choice among lines
according to the seed moisture content at harvest of their hybrids, compared
with their seed yield. In France since 1970, there has been little change in the
mean cycle length of widely grown varieties ( Table 2-1 ).
2.4.4 Disease Resistance
Resistance to diseases is one of the main factors determining the success of
a sunflower crop in different parts of the world. The problems are different
in various countries and in different environments. In addition, the situation
is not the same in North America, where sunflower and its main pests are
endemic and in countries where sunflower is a recent introduction. The
relative newness of the crop means also that new diseases can appear, as
parasites become adapted from other plant species.
Some diseases are of worldwide importance, others are more localized.
From recent publications and papers at International Sunflower Conferences
(I.S.C.), Vear (2004) defined the most important diseases according to the
number of papers published on each. At the last four I.S.C., from 1992 to
2004, white rot, Sclerotinia sclerotiorum, was the most important disease of