Greenhouse and in vitro experiments for evaluation of fungicides for control of Fusarium wilt of watermelon, 2008

摘要

The experiment was conducted in a greenhouse located at the University of Maryland’s Lower Eastern Shore Research and Education Center, Salisbury. The experiment was conducted as a randomized complete block design with four replications (pots). Sixteen fungicide treatments were evaluated in the experiment. The watermelon cv. ‘Sugar Baby’ wasseeded into plastic pots filled with 1.5-liter potting mixture on 2 Apr and thinned to 3 plants per pot on 8 Apr. Approximately 150 ml of a diluted fungicide solution or water control were drenched to each pot on 15 Apr (at the first true-leaf-stage).Three days later 150 ml of a solution (1 x 106 microconidia/ml) of a race-1 isolate of Fusarium oxysporum f. sp. niveum (FON) was drenched into each pot. The inoculum was produced in a mineral salts liquid medium grown in a shaker at 130rpm at room temperature for 14 days, filtered through four layers of cheese cloth and adjusted to the conidial concentration with the assistance of a Spencer hemacytometer. Inoculation with water served as the control. Plants were watered andfertilized as needed. Wilt incidence, the percentage of plants showing symptoms of Fusarium wilt, was assessed at 33 days after inoculation. Wilt severity, the percentage of the foliage area of each plant showing symptoms of Fusarium wilt, wasrated starting 1 week after inoculation at 2-to-3-day intervals for 12 times with the Horsfall-Barratt scale. The area under wilt severity progress curve (AUWSPC) was calculated from these wilt severity data using trapezoidal integration. An in vitroassay was also performed on potato dextrose agar (PDA) plates to evaluate the effects of vapor activity of the fungicides on growth of FON. The in vitro experiment was conducted as a completely random design with three replications (plates). Soilfumigant K-PAM HL was included as a positive control. Each of 5-mm discs of Whatman Filter paper no. 1 was pipetted with 0.8 ml of a diluted fungicide solution or water control, or with 0.4 ml of K-PAM. The treated discs were individuallyplaced in the top of an inverted PDA plate (25-mm deep Petri dish), which had been inoculated with FON on a 3-mm agar disc. The agar pieces were cut from the periphery of 1-week old cultures grown on PDA under 12-hr fluorescent light at roomtemperature. The plates were kept inverted, double sealed with parafilm, and incubated in the dark at room temperature. Radial growth was measured at 72 hrs after incubation. Relative inhibition of FON growth was based on growth of the watercontrol.