Page 3 - Volume 7, Issue 1, January 2014

Department of Agriculture, Syrian Atomic Energy Com-

mission, Damascus, P.O. Box 6091, Syria

Previous address INRA-UBP, UMR 1095, 234 Avenue du

Brézet, 63100 Clermont-Ferrand, France

E-mail:

snachaat@hotmail.com

;

snachaat@gmail.com

Hellenic Plant Protection Journal

7:

1-13, 2014

REVIEW ARTICLE

Evolution of new

Plasmopara halstedii

races under the

selection pressure with resistant sunflower plants: A review

Nachaat Sakr

Summary

Downy mildew in sunflower is a disease caused by

Plasmopara halstedii

, an oomycete

with high virulence, aggressiveness and a great potential in developing new races. Understanding

the pathogenic characteristics underlying virulence and aggressiveness processes by which

P. halste-

dii

rapidly evolves is essential to developing effective long-term control measures for the disease. In

this review, new data are presented concerning all traits for the two components of pathogenicity (the

evolution of virulence and aggressiveness) under different categories of resistance selection pressure,

the relationships among several morphological, genetic and pathogenic traits and the intervention

between pathogenic variation and durable resistance in sunflower. By combining the data present-

ed in this study, comprehension of the complex interaction between the pathogen and its host plant

could be achieved.

Additional keywords

: aggressiveness,

Pl

genes, quantitative resistance, virulence

Introduction

Plant oomycete pathogens cause a vast ar-

ray of destructive diseases of plants impor-

tant to agriculture, forestry, ornamental and

recreational plantings, and natural ecosys-

tems. The most destructive pathogens oc-

cur within the class Peronosporomycetidae,

in the orders Peronosporales which include

Phytophthora

species and downy mildews

(Tourvieille de Labrouhe

et al.,

2000; Viranyi

and Spring, 2011). The biotrophic oomy-

cete

Plasmopara halstedii

(Farl.) Berl. & de

Toni is an invasive species where sunflow-

er (

Helianthus annuus

L.) is grown, which

can survive up to 10 years in the soil in the

form of oospores. Downy mildew is a com-

mon sunflower disease responsible for sig-

nificant yield loss and can be controlled by

fungicides and cultivation of resistant hy-

brids. Infection of up to 90% of sunflower

plants with typical symptoms of systemic in-

fection has been reported in the fields and

yield losses were estimated to be up to 50%

(Tourvieille de Labrouhe

et al.,

2000; Viranyi

and Spring, 2011).

Plasmopara halstedii

displays a gene-

for-gene interaction with its host plant

and

shows physiological races (pathotypes) ca-

pable of infecting a variable range of sun-

flower genotypes. The nomenclature of

these races is based on the reaction of a se-

ries of differential lines (Tourvieille de La-

brouhe

et al.,

2000). Disease resistance in

sunflowers to

P

.

halstedii

can be classified

into one of two categories. The first is qual-

itative resistance which is conferred by the

major

Pl

genes and tends to produce a dis-

ease-free plant (Tourvieille de Labrouhe

et

al.,

2000). Qualitative resistance is gener-

ally unstable, because

Pl

resistance genes

are quickly overcome by compatible races

in the pathogen population, and too short-

lived to be considered durable. The life peri-

od of

Pl

gene seems to be very short from its

important use on a large scale. In sunflower,