Location: Vegetable ResearchTitle: Effect of grafting on resistant rootstocks on development of powdery mildew on susceptible watermelon scion, 2012
Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 2/1/2013
Publication Date: 5/1/2013
Citation: Kousik, C.S., Ikerd, J.L., Hassell, R. 2013. Effect of grafting on resistant rootstocks on development of powdery mildew on susceptible watermelon scion, 2012. Plant Disease Management Reports. 6:V012.
Interpretive Summary: N/A
Technical Abstract: An experiment was conducted in a greenhouse at the U.S. Vegetable Laboratory in Charleston, SC, to determine if grafting a powdery mildew susceptible variety (scion) onto watermelon rootstocks with resistance to powdery mildew would provide resistance to foliage of the susceptible scion. Plants of the powdery mildew susceptible cultivar Mickey Lee (obtained from Willhite Seeds) was used as the scion. Plants of different powdery mildew resistant selections from wild watermelon plant introductions (PI), bottle gourd (Lagenaria seceraria) and a susceptible watermelon PI 269677 were used as rootstocks. Original seeds of the plant introductions were obtained from the Plant Genetic Resources and Conservation Unit, Griffin, GA (www.ars-grin.gov). Each resistant PI line used in the study was obtained by self-pollinating (by hand) the most resistant single plant selections after screening plants from the original PI seed. Mickey Lee plants grafted onto Mickey Lee as a rootstock and non-grafted Mickey Lee plants were used as susceptible controls. Mickey Lee seedlings were grafted onto the powdery mildew resistant or susceptible rootstocks using previously described methods (Hassell et al., 2008. HortScience 43:1677-1679). Two weeks after grafting all the grafted plants and controls were transplanted into 3-in. square pots on 1 Jun and placed on greenhouse benches. The experimental design was a randomized complete block with 5 replications and each replication consisted of four plants. Ten days after transplanting the plants were inoculated by spraying with a powdery mildew conidial suspension (2 x 104 conidia/mL) prepared in sterile water containing 0.02% Tween 20. Conidia of powdery mildew race 1 (classified based on Cucumis melo differentials) naturally occurring in the greenhouse were collected from severely infected watermelon leaves to prepare the inoculum suspension. Powdery mildew naturally occurs in the greenhouse however, plants were inoculated for uniformity. The pots were carefully watered everyday to prevent splashing onto the leaves. To prevent whitefly and aphid infestation, the plants were treated two days after transplanting with insecticide Merit 0.5 G (a.i. imidacloprid). The greenhouse was maintained at 26±2 ºC without supplemental lighting. Powdery mildew severity ratings were recorded on 19 Jun using a 0-10 scale, where 0 = no visible powdery mildew, 1 = very sparse mycelial growth on leaves or cotyledons with very few to no visible conidia (<10%), 2 = 11-20%, 3=21-30%, 4=31-40%, 5=41-50%, 6=51-60%, 7=61-70%, 8=71-80%, 9=81-90% and 10=91-100% of leaf or cotyledon area covered with abundant conidia. Data was recorded using the 0-10 scale for the rootstock cotyledons, the second true leaf of the Mickey Lee scion, and the leaves at the top of the plant (upper leaves). All data were analyzed using the PROC GLM procedure of SAS and means were separated using Fisher’s Protected LSD test (a=0.05). Severe powdery mildew was observed on the cotyledons of the highly susceptible check PI 269677 and Mickey Lee that were being used as rootstock controls. On the lines that were used as powdery mildew resistant rootstocks, significantly (P<0.001) less powdery mildew colonization was observed on the cotyledons. Severe powdery mildew was observed on the leaves of the susceptible Mickey Lee scion grafted onto Mickey Lee or PI 269677. Significantly (P=0.001)less powdery mildew was observed on the second true leaf of the Mickey Lee plant grafted on several of the powdery mildew resistant selections that were used as rootstocks. Powdery mildew on the second true leaf and the upper leaves was significantly less on the Mickey Lee scion grafted on to the two powdery mildew resistant bottle gourd rootstocks (USVL351-PMR and USVL482-PMR). Similarly several of the powdery mildew resistant watermelon selections made from PI