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- Author or Editor: Angela R. Davis x
The primary purpose of grafting vegetables worldwide has been to provide resistance to soilborne diseases. The potential loss of methyl bromide as a soil fumigant combined with pathogen resistance to commonly used pesticides will make resistance to soilborne pathogens even more important in the future. The major disease problems addressed by grafting include fusarium wilt, bacterial wilt, verticillium wilt, monosporascus root rot, and nematodes. Grafting has also been shown in some instances to increase tolerance to foliar fungal diseases, viruses, and insects. If the area devoted to grafting increases in the future, there will likely be a shift in the soil microbial environment that could lead to the development of new diseases or changes in the pathogen population of current diseases. This shift in pathogen populations could lead to the development of new diseases or the re-emergence of previously controlled diseases. Although grafting has been demonstrated to control many common diseases, the ultimate success will likely depend on how well we monitor for changes in pathogen populations and other unexpected consequences.
Information on the mode of inheritance of powdery mildew resistance in watermelon is important for designing a breeding strategy for the development of new cultivars. Resistance in the watermelon accession PI 270545 was investigated by generation means analysis by crossing it with susceptible PI 267677. The analyses showed involvement of two genes, a recessive resistance gene, pmr-1, and a dominant gene for moderate resistance, Pmr-2. Resistance to powdery mildew in the leaf had a large dominance effect and a heritability of 71%. The additive-dominance model was inadequate in explaining variation in leaf resistance as revealed by the joint scaling test. However, nonallelic interactions could not be detected by the nonweighted six-parameter scaling test. For stem resistance, the additive-dominance model was adequate, and inheritance was controlled mainly by additive effects. A high narrow-sense heritability of 79% suggested that selection for stem resistance in early generations would be effective.
Genetic diversity among 95 watermelon (Citrullus lanatus) ecotypes was evaluated and compared with representative Chinese, American, Japanese, and Russian watermelon cultigens, landraces, and a wild watermelon relative (Trichosanthes kirilowii). Open-pollinated, hybrid, and inbred lines were included for most of the ecotypes and are hereafter collectively referred to as cultigens unless an ecotype group is being discussed. Morphological characteristics (including days to flower, female to male flower ratio, branch number, fruit length and diameter ratio, fruit soluble solid content, fruit yield, and simple sequence repeat (SSR) markers were used to estimate genetic diversity. Of 398 watermelon primer pairs tested, 9.5% (38) produced polymerase chain reaction amplicons in watermelon. Of these 38 primer pairs, the average number of polymorphic bands among the 96 cultigens was 2.4, even with 12 primer pairs demonstrating monomorphic banding patterns. Based on the SSR data, the genetic similarity coefficients were calculated and a dendrogram constructed. All cultigens were clustered to six groups. The wild species and landraces formed distant clusters from the cultivated watermelon. The genetic similarity coefficients within the Chinese cultigens ranged from 0.37 to 0.99, but except for a wild relative to watermelon, most cultigens were closely related. The genetic distance among non-Chinese cultigens ranged from 0.67 to 0.91 with an average of 0.88. When combined morphological traits and molecular traits were assessed, Russian and U.S. fruit were more genetically similar to each other than to Chinese and Japanese cultigens. Crossing Russian and/or U.S. cultigens with Chinese or Japanese cultigens should thus improve genetic diversity and introduce new traits for the resulting watermelon cultigens.
Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past 9 years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.
Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past nine years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.
Vegetable grafting began in the 1920s using resistant rootstock to control soilborne diseases. This process is now common in Asia, parts of Europe, and the Middle East. In Japan and Korea, most of the cucurbits and tomatoes (Lycopersicon esculentum Mill.) grown are grafted. This practice is rare in the United States, and there have been few experiments to determine optimal grafting production practices for different geographical and climatic regions in America. This is beginning to change as a result of the phase out of methyl bromide. The U.S. cucurbit and tomato industries are evaluating grafting as a viable option for disease control. Because reports indicate that type of rootstock alters yield and quality attributes of the scion fruit, some seed companies are investigating grafting as a means to improve quality. It has been reported that pH, flavor, sugar, color, carotenoid content, and texture can be affected by grafting and the type of rootstock used. Reports vary on whether grafting effects are advantageous or deleterious, but it is usually agreed that the rootstock/scion combination must be carefully chosen for optimal fruit quality. Additionally, it is important to study rootstock/scion combinations under multiple climatic and geographic conditions because many rootstocks have optimal temperature and moisture ranges. This report gives an overview of the effect of grafting on vegetable quality.
RNA isolation from ripe fruit can be complicated by high concentrations of sugar and water. These sugars interfere with RNA extraction often resulting in low RNA quality and quantities, and high water concentrations dilute the RNA, making isolation difficult. We report a simple but novel method by which the majority of the excess sugar and water in mature fruit of tomato (Lycopersicon esculentum Mill.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], and muskmelon (Cucumis melo L.) can be easily removed from tissue before RNA extraction. This method produced quality RNA in a shorter time than the currently accepted method for fruit tissue RNA isolation and does not require liquid nitrogen or a freeze dryer.