Fungicide resistance and genetic variability in plant pathogenic strains of Guignardia citricarpa

Abstract

Citrus black spot (CBS) is a plant disease of worldwide occurrence, affecting crops in Africa, Oceania, and South America. In Brazil, climate provides favorable conditions and CBS has spread to the Southeast and South regions. CBS is caused by the fungus Guignardia citricarpa (anamorph: Phyllosticta citricarpa) and its control is based on the use of fungicides, such as benzimidazoles. In South Africa, the disease was kept under control for 10 years with benomyl, until cases of resistance to high concentrations of this fungicide were reported from all citrus-producing areas. Azoxystrobin (a strobilurin) has been found effective in controlling phytopathogens, including CBS, in a wide range of economically important crops. The present study investigated in vitro the effects of the fungicides benomyl and azoxystrobin on 10 strains of G. citricarpa isolated from lesions in citrus plants from Brazil and South Africa. Benomyl at 0.5 µg/mL inhibited mycelial growth in all strains except PC3C, of African origin, which exhibited resistance to concentrations of up to 100.0 µg/mL. The spontaneous mutation frequency for resistance to benomyl was 1. 25 × 10 -7 . Azoxystrobin, even at high concentrations, did not inhibit mycelial growth in any of the strains, but significantly reduced sporulation rates, by as much as 100%, at a concentration of 5.0 µg/mL. Variations in sensitivity across strains, particularly to the strobilurin azoxystrobin, are possibly related to genetic variability in G. citricarpa isolates.

Scientific Opinion on the risk of Phyllosticta citricarpa (Guignardia citricarpa) for the EU territory with identification and evaluation of risk reduction options

“…Strobilurin fungicides (QoI) and benzimidazoles, which are highly effective for CBS control (Goes, 2002;Goes et al, 2000;Schutte et al, 2003;Miles et al, 2004), are not currently labelled for citrus in the EU, and their future use will depend on private or public funding resources to cover the registration costs. Although the risk of resistance to QoIs is considered to be high (FRAC, 2013), molecular studies have suggested a low QoI resistance risk in P. citricarpa populations (Possiede et al, 2009;Stammler et al, 2013;Hincapie et al, 2014). Although the risk of resistance to QoIs is considered to be high (FRAC, 2013), molecular studies have suggested a low QoI resistance risk in P. citricarpa populations (Possiede et al, 2009;Stammler et al, 2013;Hincapie et al, 2014).…”

The Use of Plants to Protect Plants and Food Against Fungal Pathogens: A Review

“…Phytopathogens do not influence only food and floricultural production but in the medicinal plant industry fungi can also affect the production and the safety of the medicinal plant after harvesting. Existing control measures are not enough to deal with emergence or outbreaks of plant fungal pathogens (Ishii, 2006;Possiede et al, 2009). Therefore, continued research, including using plant based products, is required to provide effective biological products that are cheap, less toxic and effective (Martinez, 2012).…”

“…The introduction of various synthetic chemicals in agricultural productions over the years has reduced the impact of many plant fungal diseases and increased plant crop yield and led to financial gains (Gianessi and Reigner, 2006). Since the 1970s, however farmers have been struggling with the emergence of resistance against fungicides (Ishii, 2006;Possiede et al, 2009). This leads to yield financial losses.…”

Epidemiology, identification and disease management of grape black rot and potentially useful metabolites of black rot pathogens for industrial applications – a review

“…Furthermore, in vitro investigations of Possiede et al . () in G. citricarpa , the causal agent of citrus black spot, showed considerable variations in the sensitivity towards the strobilurine azoxystrobin across strains that can easily result in the selection of resistant strains.…”