MANAGEMENT OF SUGAR BEET POWDERY MILDEW BY FOLIAR SPRAYING OF POTASSIUM PHOSPHATE SALTS

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Mosa¹, A.A.

ABSTRACT

          The efficacy of three potassium phosphate salts (KH₂PO₄, K₂HPO₄ and K₃PO₄), as foliar spray treatments, were tested against the powdery mildew fungus Erysiphe betae on sugar beet plants, either 3 days before or after inoculation. All potassium phosphate  salts  reduced  powdery  mildew development on sugar beet. The most  effective  treatments were  KH₂PO₄  and  K₂HPO₄ at 25 mM which showed both protective and curative effects against E. betae. A single foliar spray of 25 or 50 mM KH₂PO₄ or K₂HPO₄ on the lower leaves of sugar beet induced systemic resistance in the upper leaves to powdery mildew.   However, powdery mildew infection  was  significantly reduced by 83.4% on   plants treated by 25 mM KH₂PO₄, 3 days after inoculation. Production of conidia was greatly reduced on phosphate treated leaves. The higher dose (50 mM)  of  the  tested  phosphate  salts  didn’t  cause  greater  protection than the lower dose (25 mM), however, no observable phytotoxic effects were found. Only, K₃PO₄ at 50 mM  caused slight chlorotic stippling or restricted necrosis on treated leaves. A further application of 25 mM KH₂PO₄ or K₂HPO₄ to the same plants inhibited disease development compared with water sprayed plants. Meanwhile, a remarkable enhancement of peroxidase activity was occurred in treated plants compared with that of untreated plants.   The  development  of  infection structures, density of surface hyphae and conidiophores and conidial production was significantly  inhibited by KH₂PO₄ treatment. The inhibitory effectiveness   of  mono-and di-potassium phosphate against powdery mildew on sugar beet makes it a potential major component of an integrated pest management program.