Page 6 - Volume 7, Issue 1, January 2014

Sakr

4

2012). Consequently, the

Pl

gene life period

seems to be very short from the important

use of

Pl

gene on a large cultivated zone.

The development of sunflower resistant cul-

tivars that retain the characteristics required

for the oil crop is a lengthy process and the

rate of new

P. halstedii

virulence emergence

is more rapid than the rate that newly resis-

tant hosts can be bred. Host and race evolu-

tion in downy mildew is therefore an issue of

key agricultural and commercial importance.

In the last decade, advanced tools of bio-

technology have enabled discernment of

groups of

P. halstedii

on the molecular lev-

el and led to the shift from a morphological

to a phylogenetic species concept (Spring

and Thines, 2004). Indeed, early studies of

P.

halstedii

typically found low levels of genet-

ic diversity using RAPD markers (Komjati

et

al.,

2004), ISSR (Intelmann and Spring, 2002)

and ITS sequences (Spring and Zipper, 2006).

A study of 77 samples from twelve differ-

ent countries of six virulence races using 21

RAPD primers found low levels of differentia-

tion within and between samples grouped by

race and country (Roeckel-Drevet

et al.,

2003).

With molecular markers based on partial se-

quence of the nuclear ITS regions, Spring

et

al.

(2006) detected polymorphism between

profiles of races 100, 310 and 330, as well as

between groups of populations representing

races 700, 701, 703, 710 and 730. Giress

et al.

(2007) found high genetic variability between

the isolates from France and Russia using SNP

markers. Also, Delmotte

et al.

(2008) identi-

fied three genetically differentiated groups of

isolates organized around the first three races

described in France: 100, 710 and 703. Recent-

ly, Ahmed

et al

. (2012) suggested that multi-

ple introductions have aided in the establish-

ment of

P. halstedii

in France, and noted that

recombination facilitated by these introduc-

tions is driving the emergence of new and en-

demic races in response to host resistance.

As with the neutral markers these SNPs sep-

arated races 100 and 304 from races 703 and

710 (As-sadi

et al.,

2011). Interestingly, new in-

sights from molecular phylogenetics often

verify a narrow species concept that was pro-

posed by Gäumann in 1918 (Voglmayr and

Constantinescu, 2008).

Variation of aggressiveness in Plasmo-

para halstedii

Decomposing the pathogen cycle into

elementary life traits allows the precise

identification and quantification of differ-

ences among pathogen or host phenotypes,

but relating these life traits to the pathogen

fitness under field conditions or to the ep-

idemic development rate is complicated.

However, to characterize aggressiveness of

sunflower downy mildew populations, sun-

flower inbred lines showing different levels

of quantitative resistance and carrying no

Pl

gene should be used (Tourvieille de La-

brouhe

et al.,

2008). Four aggressiveness cri-

teria were established for

P. halstedii

races:

percentage infection, latent period,

sporu-

lation density and reduction of hypocotyl

length (dwarfing). Here, the four criteria are

presented in brief: infection was considered

as successful when the seedlings showed

sporulation of the pathogen on the shoot

surface. Latent period was defined as the

number of days of incubation necessary to

obtain the pathogen sporulating on 80% of

the plants. Sporulation density was defined

as the number of zoosporangia of the path-

ogen produced on a cotyledon.

Reduction

of hypocotyl length (dwarfing) correspond-

ed to the distance from the stem base to

cotyledon insertion and was measured af-

ter 13 days of infection on diseased plants

showing sporulation of the pathogen on the

shoot.

Differences in disease development (per-

centage of infection) could be due to differ-

ences in the rate of maturation of zoospor-

angia or in the capacity to penetrate healthy

host tissue (Delanoe, 1972). Latent period

corresponds to the time interval between

infection and appearance of symptoms on

infected plants. The number of spores pro-

duced by a diseased plant will determine

the quantity of inoculum that can infect

neighbouring plants. But they are also in-

dicative of the interaction between the host

plant and the parasite, since the quantity

of spores produced will depend on the ag-