Flowering Stage Resistance to Bacterial Fruit Blotch in the Watermelon Germplasm Collection

Cucurbit bacterial fruit blotch caused by Acidovorax avenae subsp. citrulli is a significant threat to watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] production worldwide. In the United States, seedless cultivars are primarily used in watermelon production, which now relies largely on transplant production in greenhouses to ensure a high germination rate. Unfortunately, the warm and humid greenhouse environment provides ideal conditions for the spread of bacterial fruit blotch. Treatments designed to remove bacteria from the surface of the seed coat were investigated previously, but none eliminated the bacteria despite significant reductions reported in research studies. resistant cultivars offer a solution to the problem if genetic resistance can be identified. The objectives of this study were to (i) identify germplasm resistant to bacterial fruit blotch using the available pI accessions in the USDA germplasm collection and (ii) improve the methods for screening in the field. Field evaluations on the basis of foliar disease symptoms at the flowering stage were conducted at Clinton, NC in 2011 to 2013. The field experiment was a randomized complete block with 1699 cultigens, 3 yr, and two replications of single-plant plots. Disease rating was on a 0 to 9 scale when the disease was uniformly distributed throughout the field (0 = no symptoms, 1–2 = trace, 3–4 = slight, 5–6 = moderate, 7–8 = severe, and 9 = dead). plots were rated multiple times each year. Significant differences were found for disease resistance among accessions (P = 0.05). rating dates having the greatest F ratio for differences among accessions were identified as best ratings for each year–block combination. resistant accessions have the best ratings (<4.5), low standard deviation across replications, and multiple replication (4). The 23 most resistant cultigens originated from Africa, mainly Zimbabwe, Zambia, republic of South Africa, and Nigeria, and they were either cultivated watermelon (C. lanatus var. lanatus) or citron (C. lanatus var. citroides). Dep. of Horticultural Science, North Carolina State Univ., Raleigh, NC 27695. Received 27 Jan. 2014. *Corresponding author (tcwehner@ gmail.com). Published in Crop Sci. 55:727–736 (2015). doi: 10.2135/cropsci2014.01.0071 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Published March 20, 2015