Abstract
The management of downy mildew (Plasmopara halstedii) in sunflower, is heavily dependent on genetic resistance, whilst entomopathogenic fungi (EF) can reduce other sunflower diseases. In this work, we characterized P. halstedii from Spain and other countries collected in the past few years. Twenty-three races were identified (the most frequent in Spain being 310, 304, 705 and 715), with an increasing proportion of highly virulent races. Five isolates from countries other than Spain overcame the resistance in RHA-340. In addition, we assessed the efficacy of five EF against downy mildew and their effects on sunflower growth in axenic conditions. None of the entomopathogens reduced disease severity, nor did they have any effect on plant growth when applied together with P. halstedii. In contrast, three EF reduced some of the plant growth variables in the absence of the pathogen. Microbiological and molecular diagnostics suggest that the axenic system and the short experimental time used in this study did not favor the successful establishment of EF in the plants or their potential biocontrol effect. Our results show a shift in P. halstedii racial patterns and suggest that soil as a growth substrate and long infection times are needed for EF effectiveness against downy mildew.
Highlights
Our study has focused on determining the effectiveness of both genetic resistance and biological control as sustainable methods for the management of sunflower downy mildew
We identified highly virulent races of P. halstedii in Spain throughout the duration of the study since, as previously mentioned, the isolates mostly came from samples collected in fields where unexpected downy mildew outbreaks had occurred
P. halstedii have beenfour identified around plants the world
Summary
Downy mildew of sunflower (Helianthus annuus L.), caused by the obligate biotrophic oomycete Plasmopara halstedii Farl. De Toni, is one of the most widespread diseases affecting this crop, and it is present in the five continents [1]. When P. halstedii zoospores emerge from dormant oospores present in the soil reaching seedling roots, primary, systemic infections of the host take place. Primary infections can cause dampingoff or severe stunting. Infected plants manifest a pronounced chlorosis, which is restricted to areas bordering the main veins in the lower leaves, they may cover the entire foliar surface in younger leaves [2]. When infected plants are exposed to high relative humidity and cool temperatures, the oomycete develops profuse cottony outgrowths from the underside of infected leaves, constituted by zoosporangia [3]. Sunflower downy mildew causes an average yield reduction of 3.5% worldwide.
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