Abstract:Onion (Allium cepa L.) is grown worldwide for its fleshy bulbs which are used as food and medicinal purposes. In Egypt, onion is the 2nd major export crop after cotton. Downy mildew of onion, caused by Peronospora destructor (Berk.)Casp. is considered one of the most destructive disease of onion and has a wide geographical distribution includes Egypt. P. destructor is a polycyclic pathogen: many infection cycles can follow one another during an onion-growing season. When weather conditions are favourable, the … Show more
“…Casp. ), occurs in most onion-producing regions worldwide, causing yield and quality losses [1][2][3][4]. In western Japan, onion seeds are primarily sown in nursery beds in autumn; seedlings are then transplanted in main fields in autumn-winter, and onions are finally harvested in spring.…”
In 2016, an onion downy mildew epidemic caused by Peronospora destructor severely damaged the commercial onion fields in Saga Prefecture, western Japan. To identify the factors underlying the outbreak, we investigated the symptoms of downy mildew caused by secondary infections and examined P. destructor’s sensitivity to metalaxyl, the most effective traditional fungicide used against this onion pathogen, in 2016–2018. Disease symptoms developed in late March 2016, which was earlier than symptom development in 2017 and 2018. Furthermore, there were synchronous repeated disease development and favourable meteorological conditions for infection in early and late April resulting in the development of polycyclic epidemics. In field trials from 2016 to 2018, the efficacy of chlorothalonil + metalaxyl-M application ranged 18–45%, as calculated by comparing disease severity at the final stage of each treatment to that in the untreated plots. On the basis of the metalaxyl sensitivity observed in 2016, the effective concentration, which reduced germ-tube elongation in P. destructor by 50%, exceeded 200 µg ai/mL for certain strains. Our observations indicate that these characteristic meteorological conditions were major factors contributing to the severe disease outbreak in 2016. The emergence of less-metalaxyl-sensitive P. destructor strains may be an additional predisposing factor.
“…Casp. ), occurs in most onion-producing regions worldwide, causing yield and quality losses [1][2][3][4]. In western Japan, onion seeds are primarily sown in nursery beds in autumn; seedlings are then transplanted in main fields in autumn-winter, and onions are finally harvested in spring.…”
In 2016, an onion downy mildew epidemic caused by Peronospora destructor severely damaged the commercial onion fields in Saga Prefecture, western Japan. To identify the factors underlying the outbreak, we investigated the symptoms of downy mildew caused by secondary infections and examined P. destructor’s sensitivity to metalaxyl, the most effective traditional fungicide used against this onion pathogen, in 2016–2018. Disease symptoms developed in late March 2016, which was earlier than symptom development in 2017 and 2018. Furthermore, there were synchronous repeated disease development and favourable meteorological conditions for infection in early and late April resulting in the development of polycyclic epidemics. In field trials from 2016 to 2018, the efficacy of chlorothalonil + metalaxyl-M application ranged 18–45%, as calculated by comparing disease severity at the final stage of each treatment to that in the untreated plots. On the basis of the metalaxyl sensitivity observed in 2016, the effective concentration, which reduced germ-tube elongation in P. destructor by 50%, exceeded 200 µg ai/mL for certain strains. Our observations indicate that these characteristic meteorological conditions were major factors contributing to the severe disease outbreak in 2016. The emergence of less-metalaxyl-sensitive P. destructor strains may be an additional predisposing factor.
“…Onions are an important vegetable crop worldwide, producing 46.7 million tonnes of bulbs from 2.7 million ha (Afifi & Zayan, 2009; FAOSTAT, 2001; Survilienė et al, 2008). However, onion downy mildew caused by Peronospora destructor is a major cause of production losses (Lorbeer & Andaloro, 1984; Pelter & Sorensen, 2003), affecting seed production and subsequent yield (Colnago & Galvan, 2010).…”
Section: Introductionmentioning
confidence: 99%
“…Proper fertilization management is important to control this disease (Develash & Sugha, 1997). Because the weather also affects disease occurrence, a disease prediction model has been proposed (Afifi & Zayan, 2009). Control measures include the application of pesticides with active ingredients such as manzeb, dimethomorph, azoxystrobin, fluopicolide, propamocarb and pyraclostrobin (Survilienė et al, 2008),…”
Section: Introductionmentioning
confidence: 99%
“…
dew caused by Peronospora destructor is a major cause of production losses (Lorbeer & Andaloro, 1984;Pelter & Sorensen, 2003), affecting seed production and subsequent yield (Colnago & Galvan, 2010).Proper fertilization management is important to control this disease (Develash & Sugha, 1997). Because the weather also affects disease occurrence, a disease prediction model has been proposed (Afifi & Zayan, 2009). Control measures include the application of pesticides with active ingredients such as manzeb, dimethomorph,
Onion downy mildew caused by Peronospora destructor has been widespread in Japan since 2016. Soil disinfection and use of fungicides have been implemented as control measures against oospores in the soil, which are the primary source of infection. Measurements of oospore density are needed to clarify the risk of disease development and inform disease management. In this study, an experimental system capable of detecting P. destructor DNA from field samples was developed. A TaqMan probe‐type primer was used to target the coxII (cytochrome c oxidase subunit II) region of the mitochondrial genome of P. destructor and no false positives were observed. Using this method, the detection limit was equivalent to 5 oospores of P. destructor/g soil in grey lowland soil and andosol. The correlation coefficient between oospore density and the primary infection rate was investigated by testing soil samples from 115 fields. Although no correlations between oospore density and the primary infection were observed in pesticide‐treated fields, significant correlations were observed in untreated fields. Continuous cropping of onions increased the oospore density. The correlation between primary and secondary infection was relatively weak, and the negative predictive value of primary infection was relatively high (89.0). These results suggest that the disease‐risk potential of onion downy mildew is very high, and thus it is necessary to set a low pathogen detection threshold. The system presented here provides a highly sensitive method for supporting decision‐making aimed at disease control.
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