2010 Annual Report
1a.Objectives (from AD-416)
Objective 1. Develop improved resistance to root-knot nematodes in pepper and watermelon.
Objective 2. Identify and develop improved resistance to diseases in cucurbits, e.g. powdery mildew, downy mildew, Phytophthora blight, and watermelon vine decline.
Objective 3. Elucidate virulence parameters of selected plant pathogen populations and develop integrated methodologies for managing cucurbit diseases.
1b.Approach (from AD-416)
Develop root-knot nematode-resistant sweet banana, sweet cherry, pimiento, and Cubanelle type peppers using backcross breeding procedures.
Determine inheritance of resistance to root-knot nematodes (Meloidogyne incognita) in watermelon in greenhouse studies, and use techniques such as single sequence repeat (SSR) and sequence-related amplified polymorphism (SRAP) to develop molecular markers linked to root-knot nematode resistance genes in watermelon.
Screen watermelon and melon germplasm for resistance to Phytophthora capsici and develop resistant germplasm.
Screen bottle gourd (Lagenaria siceraria) plant introductions for resistance to powdery mildew in the greenhouse and develop resistant germplasm for use in rootstock breeding program.
Screen watermelon plant introductions for resistance to watermelon vine decline, and evaluate strategies and develop integrated methodologies to manage watermelon vine decline in field tests.
Identify prevalent races of powdery mildew using melon host differentials and determine their virulence on other selected cucurbit species in greenhouse and field studies.
Determine genetic variability in the downy mildew pathogen (Pseudoperenospora cubensis) isolated from watermelon and cucumber using host differentials and molecular markers.
Field studies were conducted to determine usefulness of wild watermelon, squash hybrid, and bottle gourd rootstocks for managing root-knot nematodes in grafted watermelon. Wild watermelon rootstocks were moderately resistant to southern root-knot nematode and one rootstock produced significantly greater fruit yields than the other rootstocks. Studies on inheritance of resistance to southern root-knot nematode in watermelon were conducted. Two greenhouse studies to assess whether root-knot nematode predisposes pepper plants to Phytophthora blight, caused by Phytophthora capsici, were completed. Pepper germplasm lines were screened and selected for resistance to northern root-knot nematode (Meloidogyne hapla) in greenhouse studies. Field studies were conducted to determine usefulness of several cucurbit species as rootstocks for managing southern root-knot nematode in grafted melon (cantaloupe). Sweetpotato and southernpea (cowpea) germplasm lines were evaluated for resistance to southern root-knot nematodes. Single plant selections from Squash Vein Yellowing Virus (SqVYV) resistant wild watermelon germplasm lines were made to develop resistant watermelon germplasm. Wild melon (cantaloupe) germplasm lines were evaluated for resistance to crown rot caused by Phytophthora capsici and several resistant lines were identified. Over 1,800 wild watermelon germplasm lines were re-evaluated for resistance to Phytophthora capsici and several lines resistant to Phytophthora foliar blight and fruit rot were identified. Over 200 plant introductions of watermelon were evaluated for resistance to Phytophthora fruit rot and several resistant lines were identified. Single plant selections from the most resistant lines were made to develop resistant watermelon germplasm. Wild bottle gourd germplasm lines were re-evaluated for resistance to Phytophthora crown rot and foliar blight and several resistant lines were identified. Single plant selections were made to develop crown rot and foliar blight resistant bottle gourd germplasm. Isolates of Phytophthora capsici from the southeast U.S. (Florida, Georgia, and South Carolina) were collected and will be used to study genetic variability of this fungus within specific fields and within regions of the southeast U.S. Strategies to manage white-fly transmitted viral watermelon vine decline including insecticide treatments and resistant pollenizers were developed and tested.
Grafting watermelon on resistant rootstocks for managing root-knot nematodes. Root-knot nematodes (RKN, Meloidogyne spp.) are microscopic roundworms that cause extensive damage to root systems of watermelon and many other vegetable crops, resulting in significant yield losses. ARS scientists (Charleston, SC) have identified wild watermelon (Citrullus lanatus var. citroides) lines resistant to RKN. These wild watermelon lines were evaluated as rootstocks for grafted watermelon in field tests in South Carolina and Florida. The RKN resistant lines performed well as rootstocks, and produced high yields for the grafted watermelon plants. These wild watermelon rootstocks could be a useful alternative to soil fumigation with pesticides such as methyl bromide for managing RKN in watermelon fields. Also, they can be useful for seed companies interested in developing rootstock varieties for grafted watermelon.
Thies, J.A., Ariss, J.J. 2009. Comparison between the N and Me3 gene conferring resistance to the root-knot nematode (Meloidogyne incognita) in genetically different pepper lines (Capsicum annuun). European Journal of Plant Pathology. 125(4):545-550.
Turechek, W., Kousik, C.S., Adkins, S.T. 2010. Patterns of Virus Distribution in Single and Mixed Infections of Florida Watermelons. Phytopathology. doi:10.1094/PHYTO-01-10-0018.