Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: January 30, 2011
Publication Date: March 28, 2011
Citation: Kousik, C.S. 2011. Evaluation of Watermelon Plant Introductions (PI) for Tolerance to Phytophthora Fruit Rot, 2010. Plant Disease Management Reports. 5:V059. Technical Abstract: This experiment was conducted at the US Vegetable Laboratory farm in Charleston, SC. The soil was Yonges loamy fine sand. In 2009, we evaluated about 180 plant introductions (PI) belonging to the watermelon core collection with the USDA, ARS, germplasm resources information network (GRIN). Based on the results of the 2009 study, we selected both susceptible and resistant PI for further evaluation in 2010. The details of the various PI evaluated in this study can be obtained from the GRIN website (http://www.ars-grin.gov/cgi-bin/npgs/html/desc.pl?151021). The experimental design was a randomized complete block with two replications for each PI. Four-week-old seedlings of various PI were grown in 50-cell jiffy trays and transplanted on 4 May onto raised beds with 40-in centers. Beds were spaced 21-ft apart. Beds were covered with white plastic mulch. Each PI plots was a single row of 5 plants spaced 18-in. apart with 9-ft spacing between plots. Only 5 plants per plot were used because of limited availability and poor germination of seeds of the various PI. Vines of the watermelon plants were regularly turned every week so as to keep the plants from growing into the neighboring plots. Plants of susceptible commercial cultivars Mickey Lee, Black Diamond and Sugar Baby were used as controls. Plants were irrigated as needed using drip irrigation. After bedding, but before planting, the row middles were sprayed with Roundup Pro (1 pt/A) and Strategy (2 pt/A) for weed management. Weeds between beds were controlled during the season with spot application of Roundup. On 9 Jul, mature fruits were harvested from all the plots and placed on wire shelves in a sealed room. At least five fruits (mean=5.4 fruits/plot) were harvested from each plot. Each fruit was inoculated in the center by placing a 7-mm agar plug from a 3- day-old actively growing isolate of P. capsici. The agar plug was placed on the surface of the fruit without injuring the fruit. The isolate used was insensitive to mefenoxam. After inoculation, high relative humidity (>95%) was maintained in the room using a humidifier. The temperature in the room was maintained at 26.7 ºC (80 ºF). Four days after inoculation the lesion diameter on each fruit was measured in two directions perpendicular to each other. The agar plug was considered the center of the lesion. Similarly, the diameter of area within the lesion with sporangia was measured and called the sporulation length. The intensity of sporulation within the lesion was recorded on a 0-5 scale, where 0=no visible sporulation, 1 = sparse sporulation, few seen next to the agar plug, 2= some sporulation and covering less than ½ the lesion area, 3=medium sporulation covering ½ the lesion area, 4=heavy sporulation covering ¾ of the lesion area and 5 = abundant sporangia covering >80% of the entire lesion area. The length and width of the each fruit was also recorded to determine the area of each fruit covered by lesion. All the data were analyzed using SAS and means were separated using Fisher’s protected LSD (a=0.05). Significant differences (P=0.0001) in the response of the various PI to Phytophthora fruit rot was observed with respect to all the disease parameters measured (lesion length, percent fruit area covered by lesion, sporulation length and sporulation intensity). Significant disease development was observed on the susceptible commercial cultivars; Sugar Baby, Black Diamond and Mickey Lee indicating that the test had worked effectively. PI 435991 was the most susceptible with respect to fruit area covered by the lesion and sporulation intensity. The Citrullus colocynthis, PI 388770 was one of the most resistant PI in this study. Similarly the C. lanatus var. citroides PI 189225 was significantly more resistant to fruit rot compared to the susceptible cultivars. Several C. lanatus var. lanatus lines such as USVL-901 to USVL-904 being developed in Charleston were significantly more resistant than the susceptible cultivars. Variability within the PI for their response to fruit rot was also observed. This indicates the need for single plant selections followed by screening within each PI to develop resistant germplasm.