REDUCING THE INCIDENCE OF ONION DOWNY MILDEW DISEASE USING BIO-FORMULATION OF Pseudomonas fluorescens, LIMONENE AND ACETYL SALICYLIC ACID

Formulated Pseudomonas fluorescens isolate JY8, limonene (L) and acetyl salicylic acid (ASA) were evaluated individually or in combinations for their ability to decrease onion downy mildew disease severity under greenhouse and open field conditions. The foliar applications of formulated P. fluorescens JY8, limonene and acetyl salicylic acid were used at the rate of 0.25%, 0.5% and 4 mM, respectively. On the other hand, the recommended fungicide, Acrobat MZ 69% WP was utilized at the rate of 0.25%. In addition, two genes-encoding enzymes, chalcone synthase and chalcone isomarease (CHS and CHI), in flavonoids biosynthesis pathway in Allium cepa were detected at the functional and transcriptional level (mRNA level) in onion tissues under different treatments. The obtained results showed that the foliar spray with recommended fungicide was the most effective treatment to decrease onion downy mildew disease severity under greenhouse and open field conditions followed by combination of limonene and acetyl salicylic acid (L+ASA), and limonene (L) alone. All treatments containing P. fluorescens JY8 gave highly yield of onion bulbs except treatment with combination (JY8+L+ASA), which was the least effective treatment. Electrophoretic pattern of PCR products for detection of CHS and CHI encoding-genes demonstrated an induction of CHS gene expression at a main transcriptional band of ≈ 420 bp but detection of CHI gene expression was observed at main two transcriptional bands of ≈ 340 and 970 bp. The higher expression of CHS gene was found in treatment of ASA but in case of CHI gene the higher expression was found in treatments of JY8, ASA, JY8+ASA and L+ASA in comparison with other treatments and untreated control. In conclusion, treatments with JY8 and ASA induced encoded polyphenol synthetic genes in onion tissues and these genes might play an important role in defense reaction towards infection of downy mildew disease. But treatment with limonene (L) reduced disease severity of onion downy mildew through its effect as antifungal material.