Effects of Water Stress on Symptomatology and Growth of Parthenocissus quinquefolia Infected by Xylella fastidiosa

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Abstract
A greenhouse study was conducted to test the hypothesis that bacterial leaf scorch symptoms, caused by Xylella fastidiosa, are more severe during periods of drought stress. A two-by-two complete factorial design with two pathogen treatments (control and infected) and two soil moisture levels (high and low) was used with Parthenocissus quinquefolia vines in 1999 and 2000. In each year, a high percentage of P. quinquefolia plants inoculated with X. fastidiosa expressed typical bacterial leaf scorch symptoms, with the outer scorched portion of the leaf separated from green tissue by a chlorotic halo. X. fastidiosa was detected in all symptomatic plants using an immunomagnetic capture and nested polymerase chain reaction technique, and was reisolated and cultured in modified periwinkle wilt liquid media. In both years, symptoms progressed further along the stem and were more severe at corresponding leaf positions in low-water-infected plants compared to high-water-infected plants. Total leaf area, shoot length, and number of nodes on the longest shoot per plant were all reduced due to drought and X. fastidiosa infection. This study is the first to verify the hypothesis that bacterial leaf scorch symptoms are enhanced during drought stress. Maintaining plant vigor with regular watering can be used to sustain plants infected by X. fastidiosa, particularly during periods of water stress.