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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Research

2012 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.

3. Progress Report:
For fiscal year 2012, the watermelon lines were selected for resistance to root-knot nematodes and evaluated as rootstocks for managing root-knot nematode in grafted seedless watermelon in fields that were highly infested with southern root-knot nematodes. Several of the selected watermelon rootstocks exhibited significantly higher resistance to root-knot nematodes than rootstocks, such as bottle gourd and squash hybrid rootstocks, which are commonly used for grafted watermelon. Selected lines of root-knot nematode resistant wild watermelon were evaluated for fruit type, flesh color, and fruit yields in field tests. Sweetpotato and southernpea (cowpea) germplasm lines were evaluated for resistance to southern root-knot nematodes. Homozygous lines of sweet banana, sweet cherry, and Cubanelle selected for resistance to southern root-knot nematode, Meloidogyne incognita, and back-crossed to susceptible parents in order to recover fruit types of the original parents. Watermelon germplasm was screened for resistance to Phytophthora fruit rot and 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 powdery mildew, single plant selections were made to develop powdery mildew resistant bottle gourd germplasm, and crosses between resistant and susceptible bottle gourd lines were made to develop populations for inheritance studies. Isolates of powdery mildew and Phytophthora capsici from Florida, Georgia, and South Carolina, were collected for use in studies of genetic variability for each of these fungi within specific fields and within regions of the southeastern U.S. Strategies to manage white-fly transmitted viral watermelon vine decline, including insecticide treatments and resistant pollenizers, were developed and experiments have been completed. ARS scientists detected and reported a severe outbreak of powdery mildew on watermelon fruit in Florida. The disease appeared on the fruit surface and reduced fruit quality, thus reducing yield. Accurate identification of the pathogen is essential to the development of appropriate management practices. This work is being continued under 6659-22000-026-00D, Development of Disease and Nematode Resistance in Vegetable Crops.

4. Accomplishments
1. Root-knot nematodes are an important and re-emerging pest of pepper in the southern U.S. Root-knot nematodes are microscopic roundworms that attack plant roots causing “galls” or swellings on the roots, which divert nutrients from the fruit and foliage of the plants, resulting in fewer and smaller pepper fruit yields in infected plants. Although root-knot nematode-resistant pepper varieties have been developed, an ARS scientist in Charleston, SC, recently discovered a new strain of southern root-knot nematode that can infect many of the previously released resistant bell and hot pepper varieties. This discovery is important because it may become necessary to screen wild pepper collections for resistance to this and other new strains of root-knot nematodes in order to identify new sources of nematode resistance genes that can be incorporated into existing pepper varieties.

2. Powdery mildew is a common disease that generally occurs on watermelon leaves in commercial farms and is known to reduce fruit yield. However, the disease is not known to occur on fruit surface in the field. ARS scientists in Charleston, SC, detected and reported a severe outbreak of powdery mildew on watermelon fruit in commercial watermelon fields in Florida for the first time. The disease appeared on the fruit surface and reduced fruit quality, thus directly reducing yield. ARS scientists helped in accurate identification of the pathogen. Accurate identification of the pathogen will allow commercial watermelon growers to use appropriate management practices to manage powdery mildew on the fruit.

Review Publications
Thies, J.A. 2012. Virulence of Meloidogyne incognita to expression of N gene in pepper. Journal of Nematology. 43(2):90-94.

Guan, W., Zhao, X., Hassell, R., Thies, J.A. 2012. Defense mechanisms involved in disease resistance of grafted vegetables. HortScience. 47(2):164-170.

Kousik, C.S., Donahoo, R.S., Webster, C.G., Turechek, W., Adkins, S.T., Roberts, P.D. 2011. Outbreak of powdery mildew on watermelon fruit caused by podosphera xanthii in southwest Florida. Plant Disease. 95:1586.

Keinath, A.P., Kousik, C.S. 2011. Sensitivity of isolates of phytophthora capsici from the eastern United States to fluopicolide. Plant Disease. 95:1414-1419.

Adkins, S.T., Webster, C.G., Kousik, C.S., Webb, S.E., Roberts, P.D., Stansly, P.A., Turechek, W. 2011. Ecology and management of whitefly-transmitted vegetable viruses in Florida. Virus Research. 159:110-114.

Last Modified: 2/23/2016
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