Effect of acetochlor treatment on Fusarium wilt and sugar content in melon seedlings

Pretreatment of soil with the herbicide acetochlor at 0.1-1 mu g g(-1) significantly decreased incidence of wilt due to Fusarium oxysporum f. sp. melonis in melon seedlings. Glucose, fructose and sucrose increased in leaves of inoculated and uninoculated melon plants following acetochlor treatment. The increase in sugar levels in stems and roots was less pronounced. Light intensity affected sugar content and disease incidence. The percentage of diseased plants was significantly higher in untreated plants grown under 165 mu E m(-2) sec(-1) compared to plants grown under 300 mu E m(-2) sec(-1). Lowering light intensity resulted in reduction of levels of total sugars on the third and sixth day after inoculation. Acetochlor had little or no effect on growth rate or sporulation of the pathogen in culture. The colonization rate of diseased plant stems by the pathogen was similar in herbicide-treated and untreated plants, thus excluding the possibility that disease reduction by the herbicide is related to direct fungitoxicity.