Field evaluation of muskmelon cultivars for resistance to Fusarium wilt of watermelon, 2006

摘要

In previous greenhouse studies, some isolates of F. oxysporum f. sp. niveum (F. o. niveum), the causal agent of Fusarium wilt of watermelon, were cross pathogenic to some cultivars of muskmelon. The current experiment was conducted toevaluate whether F. o. niveum can colonize stems of muskmelon and cause symptoms of Fusarium wilt under field conditions. Plots were established in a field of Norfolk “A” loamy sand soil at the University of Maryland’s Lower EasternShore Research and Education Center, Salisbury. The field had a history of severe losses to Fusarium wilt caused by a mixture of races 0, 1, and 2 of F. o. niveum. The experiment was conducted as a randomized complete block design withthree replications. The treatments were 27 cultivars of muskmelon: Ananas, Aphrodite, Athena, Classic, Eclipse, Golden Muskmelon, Hale’s Best No. 36, Honeydew Greenflesh, Honeydew Orangeflesh, Imperial 4-50, Israeli (Old Original),Jumbo Hale’s Best, Minerva, Odyssey, Oriental Muskmelon, Primo, Rocky Ford Green Flesh, Superstar, Tam Dew (Honey Dew), Tam Uvalde, Vienna, Yellow Canary, and the five cultivars listed in the table. Three watermelon genotypes (seeTable) also were included as the controls. Plots consisted of single rows 30 ft long, 5.9 in. high and spaced 6-ft apart. Fertilizer, 700 lb/A of 16-03-18 plus S (5%) and B (0.2%), was applied to plots on 15 May. Four-week old plants weretransplanted into black plastic 30 in. apart within rows on 26 May. Weeds, insects and foliar diseases were managed according to local extension recommendations. The percent of plants showing symptoms of Fusarium wilt (stunted, yellowedor wilted) was assessed on 13 Jul. Marketable fruit were harvested on 26 Jul and 3 Aug. At the end of the season, three plants were collected from the remaining living plants of each plot and were assayed for lower stem colonization by F. o. niveum, orF. oxysporum f. sp. melonis (F. o. melonis), the causal agent of Fusarium wilt of muskmelon. Lower stems (5 cm above the crown node) were surface disinfected in 0.6% NaOCl, cut into pieces, and blended in sterile water. The resulting suspensionswere filtered through four layers of cheesecloth and 1-ml aliquots of undiluted or diluted tissue homogenates were plated on Komada’s medium. A portion (10 to 100% of total colonies) of Fusarium-type colonies on the medium were randomlyselected for greenhouse pathogenicity tests. A 10-ml sample (104 to 105microconidia/ml) of each fungal culture was pipetted onto potting medium around two emerging watermelons (cv. Sugar Baby) and two muskmelons (cv. Topmark) in a pot (360cm3). A fungal isolate was considered as pathogenic to watermelon, muskmelon, or both if at least one plant of watermelon, muskmelon, or both watermelon and muskmelon respectively developed symptoms of Fusarium wilt 3 weeks afterinoculation. The average number (adjusted for dilution) of fungal colonies that grew on six plates was used for stem colonization by F. o. niveum or F. o. melonis for each cultivar.