Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: January 30, 2012
Publication Date: June 1, 2012
Citation: Kousik, C.S., Ikerd, J.L., Hassell, R. 2012. Effect of grafting on resistant rootstocks on the development of Phytophthora fruit rot on susceptible scion, 2011.. Plant Disease Management Reports. 6:V012. Technical Abstract: This experiment was conducted at the U.S. Vegetable Laboratory, Charleston, SC, where the soil was Yonges loamy fine sand. The objective of the experiment was to determine if grafting a Phytophthora fruit rot susceptible variety (scion) on watermelon rootstocks with resistance to fruit rot would provide resistance to fruit of the susceptible variety. The fruit rot susceptible cultivar Sugar Baby was used as the scion. Plants of four different fruit rot resistant selections from wild watermelon plant introductions (PI) and a fruit rot susceptible PI were used as rootstocks. Sugar Baby plants grafted on Sugar Baby as a rootstock and non-grafted Sugar Baby plants were used as the susceptible controls. The experimental design was a randomized complete block with 5 replications for each graft or the non grafted control. Grafting of Sugar Baby on the fruit rot resistant or susceptible rootstocks was done using previously described methods (Hassell et al., 2008. HortScience 43:1677-1679). Two weeks after grafting all the grafted plants were transplanted on 6 July 2011 onto raised beds with 40-in. centers. The beds were spaced 21-ft apart and covered with white plastic mulch. Plots were on a single row of 5 grafted plants spaced 18-in. apart with 15-ft spacing between plots. 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 in the row middles were controlled during the season with spot application of Roundup and cultivation. Five fruits were harvested from each plot and placed on wire shelves in an enclosed room. Each fruit was inoculated in the middle by placing a 7-mm agar plug from an 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 and the temperature in the room was maintained at 80 °F. Four days after inoculation, the lesion diameter on each fruit was measured. The agar plug was considered the center of the lesion. Similarly, the diameter of area within the lesion with sporangia was measured. The intensity of sporulation 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 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 each fruit was also recorded to determine the area of each fruit covered by lesion. Data on the percent fruit area covered by lesion was arcsine transformed and analyzed. All data were analyzed using SAS and means were separated using Fisher’s protected LSD (a=0.05). The size of watermelon fruit from Sugar Baby grafted on various rootstocks was not significantly different. Varying levels of Phytophthora fruit rot was observed on all fruit. Severe fruit rot was observed on the non grafted Sugar Baby and Sugar Baby grafted onto itself. Similarly, severe fruit rot was observed on Sugar Baby grafted on the susceptible PI 536464. The percent fruit area covered by rot, lesion and sporulation diameter were significantly less on Sugar Baby fruit harvested from grafts on USVL-1101, USVL-1102 and USVL-1103. However, no significant differences were observed for sporulation intensity among the fruit from grafted plants.