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Broad resistance to U.S. powdery mildew isolates in newly developed watermelon germplasm lines

Chandrasekar Kousik: U.S. Vegetable Laboratory, USDA, ARS

<div>Powdery mildew (PM) of watermelon caused by <em>Podosphaera xanthii </em>has been occurring more frequently in commercial fields in recent years and is known to result in significant yield reduction. Resistance to PM is available in non-edible pollenizers and in U.S. Plant Introductions (PI), whereas, commercial watermelon cultivars with resistance are lacking. We developed PM resistant red and pink-fleshed watermelon germplasm lines for use in breeding programs after six generations (S<sub>6</sub>) of phenotyping and selection from PI. The lines USVL608-PMR, USVL313-PMR, USVL255-PMR, USVL585-PMR, and USVL278-PMR exhibit high levels of resistance in hypocotyls, cotyledons and true leaves to a local isolate of <em>P. xanthii </em>compared to watermelon cultivars Mickey Lee and a susceptible watermelon line USVL677-PMS on which abundant development of conidia was observed. In two field trials in 2017, resistant lines had <2% of leaf area infected compared to >70% for USVL677-PMS. Detached true leaves of these lines placed in petri-plates were sprayed with a conidial suspension (10<sup>5 </sup>conidia<sup>-ml</sup> in 0.02% tween 20) of <em>P. xanthii</em> isolates collected from different states (SC, FL, GA, NY and CA). All PM resistant watermelon germplasm lines were resistant to isolates from different states compared to USVL677-PMS on which severe PM (>70%) was observed. Watermelon lines with broad resistance to PM will be good sources for incorporating resistance in commercially acceptable cultivars for use across the U.S.</div>