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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Research

2008 Annual Report

1a. Objectives (from AD-416)
Diseases and nematodes are major limitations in vegetable crop production in the U.S. There are many tools available to manage plant diseases. However, host-plant resistance is probably the easiest, most economical and environmentally friendly method for managing plant diseases and is readily acceptable to growers. Identification of host plant resistance to existing, new, and emerging pathogens in vegetable crops is critical for the development of efficient and effective disease management strategies that will reduce crop loss and pesticide use. Therefore, research under this project will identify new sources of resistance, determine inheritance of previously identified resistance, and develop molecular markers for important resistance genes that will facilitate their rapid incorporation into commercial cultivars. An understanding of variability in pathogen populations also is necessary to develop new vegetable cultivars with durable resistance. Thus, the proposed research will be accomplished through the following objectives: 1) Develop improved resistance to root-knot nematodes in pepper and watermelon, and 2) Identify and develop improved resistance to diseases in cucurbits and elucidate genetic variability in the cucurbit downy mildew pathogen. Successful attainment of these objectives will result in basic knowledge that facilitates development of new resistant germplasm, identification of molecular markers that will aid selection for resistance genes in breeding programs, and development of knowledge of pathogen variability.

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 and identify molecular markers for root-knot nematode resistance genes in watermelon. Identify and develop improved resistance to Phytophthora capsici in watermelon and melon. Identify sources of resistance in watermelon plant introductions and develop management strategies for mature watermelon vine decline. Elucidate genetic variability in the downy mildew pathogen (Pseudoperenospora cubensis) isolated from watermelon and cucumber using conventional and molecular approaches.

3. Progress Report
Over 1,500 watermelon germplasm lines were evaluated for resistance to Phytophthora capsici. Several germplasm lines were identified as sources of resistance. Over 200 wild bottle gourd germplasm lines were evaluated for resistance to Phytophthora crown rot and foliar blight. Several moderately resistant germplasm lines were identified. Sweet pepper breeding lines (sweet cherry, Cubanelle, sweet banana, and pimiento) were evaluated for resistance to root-knot nematodes. Homozygous resistant lines were selected for further evaluation for horticultural characteristics and nematode resistance. Five watermelon populations (F1, F2, and F1 backcrosses to parents) were developed for use in studying inheritance of resistance to southern root-knot nematode and one inheritance study was conducted. Field studies to determine the usefulness of grafting watermelon on different squash hybrid, bottle gourd, and wild watermelon rootstocks for managing root-knot nematodes are in progress. Wild watermelon rootstocks developed by USDA scientists in Charleston, SC appear to be useful in managing root-knot nematodes in watermelon. However, squash hybrid and bottle gourd rootstocks were susceptible to root-knot nematodes. Greenhouse studies to determine usefulness of pepper genotypes differing in resistance to root-knot nematode and Phytophthora capsici for managing root-knot nematode and Phythophthora blight in pepper are underway. Strategies to manage watermelon vine decline were developed. A combination of silver mulch and insecticide treatments helped manage viral watermelon vine decline in Florida. This research contributes to National Program 303 Plant Diseases, Component 3: Plant Disease Resistance, Problem Statement 3B: Disease resistance in new germplasm and varieties; and Component 4: Biological and Cultural Strategies for Sustainable Disease Management, Problem Statement 4C: Application of sustainable disease management tools.

4. Accomplishments
1. Discovery of powdery mildew resistant bottle gourd. Powdery mildew is an important foliar disease that can cause severe damage to plants in the pumpkin and watermelon family grown in both open fields and greenhouses. During spring and summer of 2006, we observed that commercial bottle gourd cultivars used as rootstocks for grafted watermelon were severely affected by powdery mildew, reducing their value as a rootstock. ARS scientists in Charleston, SC tested many wild bottle gourd plants collected from different regions of the world for their reactions to powdery mildew in the greenhouse and laboratory. Although none of the plants from the wild bottle gourd collection were completely immune to powdery mildew, several accessions were moderately resistant. These resistant wild bottle gourds should be useful sources of resistance for developing bottle gourd rootstocks with resistance to powdery mildew. This accomplishment contributes to National Program 303 Plant Diseases, Component 3: Plant Disease Resistance, Problem Statement 3B: Disease resistance in new germplasm and varieties.

2. Discovery of broad mites infesting watermelon and identification of resistance. Watermelon is an important crop grown in 44 states in the U.S. Broad mites are a serious pest of many cucurbit crops such as melon and cucumber that belong to the same family as watermelon. ARS scientists at Charleston, SC were the first to recognize and report this important pest in watermelon. Broad mites have the potential to become a serious pest of watermelon in the U.S. Therefore, we tested many wild watermelons from different regions of the world and identified several accessions that had no visible damage and few broad mites. These wild watermelons may be useful as a source of resistance for developing watermelon varieties that are resistant to broad mites. This accomplishment contributes to National Program 301 Plant Genetic Resources, Genomics and Genetic Improvement, Component 3: Genetic Improvement of Crops, Problem Statement 3C: Germplasm Enhancement/Release of Improved Genetic Resources and Varieties.

5. Significant Activities that Support Special Target Populations

Review Publications
Kousik, C.S., Shepard, B.M., Hassell, R., Levi, A., Simmons, A.M. 2007. Potential Sources of Resistance to Broad Mites (Polyphagotarsonemus latus) in Watermelon Germplasm. HortScience. 42(7):1539-1544.

Kousik, C.S., Keinath, A.P. 2008. First Report of Insensitivity to Cyazofamid among Isolates of Phytophthora capsici from the Southeastern United States. Plant Disease. 92:979.

Thies, J.A., Dickson, D.W., Fery, R.L. 2008. Stability of Resistance to Root-Knot Nematodes in 'Charleston Belle' and "Carolina Wonder' Bell Peppers in a Sub-Tropical Environment. HortScience. 43:188-190.

Thies, J.A., Levi, A. 2007. Characterization of watermelon (citrullus lanatus var. citroides) germplasm for resistance to root-knot nematodes. Journal of Nematology. 42:1530-1533.

Adkins, S.T., Webb, S., Baker, C., Kousik, C.S. 2008. Squash vein yellowing virus detection using nested polymerase chain reaction demonstrates Momordica charantia is a reservoir host. Plant Disease. 92:1119-1123.

Kousik, C.S., Levi, A., Ling, K., Wechter, W.P. 2008. Potential Sources of Resistance to Cucurbit Powdery Mildew in US Plant Introductions (PI) of Lagenaria Siceraria (bottle gourd). HortScience. 43(5):1359-1364

Last Modified: 04/23/2017
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