Taxonomy, distribution and biology of lettuce powdery mildew (Golovinomyces cichoracearum sensu stricto)

Abstract

This paper reviews the taxonomy, biology, importance, host-pathogen interactions and control of lettuce powdery mildew. The main causal agent of this disease, Golovinomyces cichoracearum s.s., is an important powdery mildew pathogen of many members of the family Asteraceae. The pathogen is distributed worldwide and occurs on Lactuca sativa as well as wild Lactuca spp. and related taxa (e.g. Cichorium spp.). Powdery mildew of lettuce can be a major problem in production areas with favourable environmental conditions for disease development (dry, hot weather). The fungus grows ectophytically and appears as white, powdery growth on both the upper and lower sides of leaves. There is rather limited information on the geographic distribution of powdery mildew on wild Lactuca spp. Most L. sativa cultivars have been found to be susceptible. Large variability in virulence was confirmed and existence of different races is supposed. Resistance in L. sativa and some related wild Lactuca spp. is characterized by race-specificity, but the genetic background of resistance is poorly understood. Sources of resistance are known in L. saligna and L. virosa. Lettuce powdery mildew can be effectively controlled by common fungicides (e.g. sulphur, myclobutanil, quinoline, strobilurins, etc.) and protective compounds (e.g. extract of neem oil, Reynoutria sachaliensis extracts). However, fungicide resistance may arise. Non-fungicidal activators of plant systemic acquired resistance (SAR) had no direct effect on the causal agent. Future issues regarding lettuce powdery mildew research are summarized.

Host specificity and virulence variation in populations of lettuce powdery mildew pathogen (Golovinomyces cichoracearum s. str.) from prickly lettuce (Lactuca serriola)

“…McDonald and Linde (2002) and Barrett et al (2008) stated that interactions between five main evolutionary forces (mutation, genetic drift, gene flow, reproduction/mating system and selection) ultimately determine variability in the genetic structure of pathogen populations and their evolutionary potential. Accordingly, G. cichoracearum is a pathogen with high evolutionary potential given its short cycle for asexual reproduction (5-7 days), rapid spread of conidia by wind (a high potential for migration and gene flow between , and a mixed reproduction system (alternation between asexual and sexual reproduction during the life cycle) (Lebeda and Mieslerová 2011). Combined, these factors (together with the pathogen responses to changes in host population structure) may generate rapid changes in pathogen population structure.…”

“…Since then, occurrence of powdery mildew on cultivated lettuce has been reported from various countries in Europe (e.g., Crute and Burns 1983;Klemm 1985;Lebeda 1985a, b, c) and elsewhere (Blancard et al 2006). It can be a serious pathogen of chicory and lettuce crops, as recently reported especially from the U.S.A. (Blancard et al 2006;Lebeda and Mieslerová 2011), but also occurs frequently on weedy growing Lactuca serriola (Lebeda et al 2012a).…”

“…Braun (1995) named the polyphagous races of this powdery mildew fungus as Golovinomyces orontii (Castagne) V.P. Heluta, while the original name (sensu stricto) was retained only for races that infect various representatives of the family Asteraceae and cause lettuce powdery mildew (Braun 1987;Lebeda and Mieslerová 2011). According to Ryder (1999), the first authenticated report of powdery mildew infection of cultivated lettuce was in Southern California (U.S.A), where an inter-specific hybrid between a susceptible Lactuca serriola L. (prickly lettuce) and the resistant crisphead lettuce 'Imperial 850' (Lactuca sativa L.) and its parents was inoculated with G. cichoracearum in a greenhouse test (Pryor 1941).…”

Incidence of lettuce downy mildew (Bremia lactucae) and powdery mildew (Golovinomyces cichoracearum) in natural populations of prickly lettuce (Lactuca serriola)

“…lactucae and G. cichoracearum is very complex and variable (Lebeda et al 2008;Lebeda & Mieslerová 2011). Pathogen incidence was found to be strongly influenced by ecological factors, such as climatic and microclimatic conditions as well as by the host population (mainly by its genetic variability).…”

“…G. cichoracearum s. str. has a world-wide distribution and its host range includes more than 100 genera and about 500 species of the family Asteraceae (Lebeda & Mieslerová 2011). The disease is favoured by moderate to warm temperatures and relatively dry weather conditions.…”