Phytophthora fruit rot, caused by Phytophthora capsici, is prevalent in most watermelon-producing regions of southeastern United States and is known to cause pre- and post-harvest yield losses. A non-wound inoculation technique was developed to evaluate detached mature fruit belonging to U.S. watermelon PIs for resistance to fruit rot caused by P. capsici. Mature fruit were harvested and placed on wire shelves in a walk-in humid chamber [greater than 95% relative humidity (RH), temperature 26 ± 2 °C] and inoculated with a 7-mm agar plug from an actively growing colony of P. capsici. Twenty-four PIs that exhibited resistance in a preliminary evaluation of 205 PIs belonging to the watermelon core collection in 2009 were grown in the field and greenhouse in 2010 and 2011 and evaluated in the walk-in humid chamber. Fruit rot development was rapid on fruit of susceptible controls ‘Black Diamond’, ‘Sugar Baby’, and PI 536464. Several accessions including PI 560020, PI 306782, PI 186489, and PI 595203 (all Citrullus lanatus var. lanatus) were highly resistant to fruit rot. One C. colocynthis (PI 388770) and a C. lanatus var. citroides PI (PI 189225) also showed fruit rot resistance. Fruit from PIs that were resistant also had significantly lower amounts of P. capsici DNA/gram of fruit tissue compared with the susceptible commercial cultivars Sugar Baby and Black Diamond. The sources of resistance to Phytophthora fruit rot identified in this study may prove useful in watermelon breeding programs aimed at enhancing disease resistance.
Phytophthora capsici is an aggressive pathogen that is distributed worldwide with a broad host range infecting solanaceous, fabaceous, and cucurbitaceous crops. Over the past two decades, increased incidence of Phytophthora blight, particularly in eastern states, has threatened production of many vegetable crops. Cucumis melo L. (honeydew and muskmelon), although especially susceptible to fruit rot, is also highly susceptible to crown rot. Currently, little is known about host resistance to P. capsici in C. melo. To assess crown rot resistance in C. melo seedlings, 308 U.S. PIs, and two commercial cultivars (Athena and Dinero) were grown under greenhouse conditions. Seedlings with three to four true leaves were inoculated with a five-isolate zoospore suspension (1 × 104 zoospores per seedling) at the crown and monitored for 6 weeks. All the susceptible control plants of Athena died within 7 days post-inoculation. The majority of the PIs (281 of 308) were highly susceptible to crown rot and succumbed to the disease rapidly and had less than 20% of the plants survive. Several PIs (PI 181748, PI 182964, and PI 273438) succumbed to crown rot earlier than the susceptible melon cultivars. Eighty-seven PIs selected on the basis of the first screen were re-evaluated and of these PIs, 44 were less susceptible than cultivars Athena and Dinero. Twenty-five of the 87 PIs were evaluated again and of these six PI, greater than 80% of the plants survived in the two evaluations. Disease development was significantly slower on these PIs compared with the susceptible checks. High levels of resistance in S1 plants of PI 420180, PI 176936, and PI 176940 were observed, which suggests that development of resistant germplasm for use in breeding programs can be accomplished. Further screening and careful selection within each of these PIs can provide a framework for the development of resistant germplasm for use in breeding programs.
The pathogen Phytophthora capsici Leon. is known to be a limiting factor of chile pepper (Capsicum L.) production around the world. The genetics of the resistance is becoming better understood due to the specific nature of the host-pathogen interaction, i.e., all plant organs are subject to infection. It has been shown that phytophthora root rot resistance and phytophthora foliar blight resistance are under different genetic mechanisms. This study aimed at understanding the inheritance of resistance of phytophthora stem blight and to determine whether phytophthora stem blight was the same disease syndrome as phytophthora root rot and phytophthora foliar blight. Stem cuttings of a segregating F2 population and testcross progeny facilitated the ability to screen for two disease syndromes concurrently. When the three disease syndromes were compared separately, the F2 populations fit a 3 resistant (R): 1 susceptible (S) ratio and the testcross progenies fit a 1R:1S ratio. When comparative studies were performed (stem vs. foliar and stem vs. root), the F2 populations fit a 9R/R:3R/S:3S/R:1S/S ratio and the testcross fit a 1R/R:1R/S:1S/R:1S/S ratio. These ratios are consistent of a single gene controlling the resistance of each system. Therefore, phytophthora stem blight, root rot, and foliar blight are three separate disease syndromes.
are considered stress factors affecting the incidence of many crop diseases ( Schoeneweiss, 1975 ). Heavy rainfall and severe weather conditions are known to provide a window of opportunity for diseases caused by oomycetes such as Pythium and
and vitamin C ( Parkinson, 1984 ). Yields of taro have been declining in many of the taro-growing countries as a result of diseases caused by a host of pathogens, which include fungi, oomycetes, bacteria, viruses, mycoplasmas, and nematodes ( Brooks
stored in a cooler with ice and submitted for testing 2 h after sample collection. In the tests, membrane-type “fungi and oomycetes” was used to analyze irrigation water samples collected on Day 6 and membrane type “G” on Day 71. Both membrane types
and processing cucumber market was valued at $29 million ( USDA, 2012 ). Downy mildew, caused by the oomycete pathogen P. cubensis Rostov, is a major foliar disease of cucumber ( Palti and Cohen, 1980 ). The disease symptoms on cucumber are
Phytophthora blight caused by the oomycete Phytophthora capsici Leon. is a very destructive disease and was first described attacking chile pepper plants in New Mexico ( Leonian, 1922 ). The pathogen produces several disease syndromes: foliar
-Blondon, A.-F. Thomas, M.R. Dry, I.B. 2013 Genetic dissection of a TIR-NB-LRR locus from the wild North American grapevine species Muscadina rotunidifolia identifies paralogous genes conferring resistance to major fungal and oomycete pathogens in cultivated