Page 8 - Volume 7, Issue 1, January 2014

Sakr

6

virulent races ranging from 1 to15. In 2001

and in 2003 three new races were encoun-

tered, within the same year of cultivation.

Recombination between races takes place

and probably contributes to the emergence

of new pathotypes in sunflower downy mil-

dew. This is in agreement with other Oomy-

cetes such as

Phytophthora ramorum

(Goss

et al.,

2009) and

Phytophthora

infestans

(Mill-

er

et al

., 1997) where sexual reproduction

occurs depending on geographical location

and environmental conditions. It was previ-

ously unknown whether field populations

of

P. halstedii

hybridise. For example, the ad-

mixed samples of race 707 first document-

ed in 2004 in France, are probably the prod-

uct of a hybrid between races 703 and 304

resulting in a new race able to overcome re-

sistance in sunflower differential lines D7, D8

and D9 as shown in Table 1. Novel patho-

gen types arising from interspecific hybrid-

ization in natural populations are becoming

increasingly documented due to molecular

studies (Joly

et al.,

2006). Although we were

Figure 1.

Plot of the principal component analysis for aggressiveness criteria (%INF=percentage infection, DL=latent

period, DS=sporulation density, TAILLE= dwarfing) on sunflower inbred line carrying a high level of quantitative resis-

tance for seven Plasmopara halstedii races (MIL001=race100, DU1842=race300, DU1767=race304, DU1943=race314,

MIL002=race710, DU1734=race704, DU1915=race714)

A single representative of each race was included in the Principal Component Analysis (PCA) based on data from Sakr (2013).

F1 x F2 plot explains more than 80% of the total variation for the four measured criteria of aggressiveness. The PCA revealed

strong differentiation in the spread of the data, with 64% of the observed variation on the first axis (latent period and spo-

rulation density criteria) and 20% on the second axis (percentage infection and dwarfing criteria). When plotted, these data

formed three distinct groups. Group 1 consisted of races 300, 304 and 314. Group 2 comprised of samples belonging to race

100. Group 3 was made up of races 710, 704 and 717. The first axis clearly separated the three groups, and the second axis

separated race 314 from the two races 300 and 304 in the first group.