Genetic, Phylogenetic And Molecular Studies Of Melon (Cucumis Melo L.) Resistance To Gummy Stem Blight (Didymella Bryoniae)

Melon (Cucumis meIo L) is an important horticultural crop the world over. Originally from Africa, melon is now cultivated throughout the tropical, subtropical and temperate climates. Asia and Europe are the leading melon producers worldwide. Currently China is the leading producer and consumer of melons in the world. The usage of melons is extremely diversified depending on the type of melon. The sweet type melons are consumed as dessert while the non-sweet types have diverse uses e.g. used as pickle in the case of pickling or snake melon or cooked and used as a vegetable as for winter melon. The demand for melon will increase in tandem with population increase and inverse to reducing farmlands. Melon research is expected to meet this challenge. Also scientific evidence regarding the health benefits of melon would imply a potential shift in consumption towards melon even in countries that do not traditionally consume melon.However, the production of melon is severely constrained by soil-borne disease pathogens; of these DidymetIa bryoniae (Auersw), Rehm the fungus that causes gummy stem blight is the most serious accounting for up to 100% losses in yield, quality and monetary terms. Moreover, all cucurbits are susceptible to the fungus. Gummy stem blight is cosmopolite, occurring in all melon growing areas in tropical, subtropical and temperate climates. Gummy stem blight is conventionally managed by the use us chemicals e.g. benzimidazoles. However, the use of chemicals is complicated by a number of factors: in wet weather, chemicals are ineffective; pathogen resistance to benzimidazoles and related compounds is increasingly being reported. Besides, chemicals are not environmentally friendly; consequently, the use of benzimidazoles and related compounds is not strategically sustainable in the management ofD. bryoniae.Use of host resistance is the most strategic, environmentally friendly, economical and universally accepted approach to gummy stem blight management. Suitable tools towards accomplishing host resistance to D. bryoniae include germplasm collection and evaluation, studying the genetics of host resistance, understanding their genetic relationships and the use of molecular techniques to tag host resistance. The horticultural importance of melon and the economic importance of D. bryoniae make them both key candidates for continued host-plant resistance studies. In the present study: 1) We critically evaluated a large collection of germplasm sources, 2) Studied the genetics of resistance in resistant germplasm, 3) Analyzed the genetic relationships among germplasm sources that were resistant and susceptible to D. bryoniae and 4) We screened for AFLP makers associated with resistance to the D. bryoniae locus in the highly resistant PI 420145. Additionally we analyzed the genetic relationships among a selected group of Chinese F1 melon hybrids.The main findings are summarised below.1. Screening germplasm sources for resistance to Didymella bryoniaeIn order to identify host resistance, a large number of germplasm sources were screened against the fungus. Our collection included novel germplasm, germplasm from previous screens (critical re-evaluation), materials not previously screened but in public domain, wild relatives (mainly Cucurais species) and related cucurbits (mainly Citrullus lanatus). This collection was screened against a single highly virulent isolate (IS25) of D. bryoniae. The major findings were: most of the C. meIo germplasm sources were susceptible to D. bryoniae; usable resistance was found in only a few accessions (e.g. PI436534, 157076, 420145, 505599). Fruits of these accessions were, however, not of much economic value. No resistance was found in C. melo var. flexuosus or inodorus. Most of the other cucurbits (e.g. Citrullus spp.) and all he wild relatives were resistant to D. bryoniae. An important observation made in the present study that impacts on future decisions for introgressing D. bryoniae into commercial cultivars was that some accessions reported resistant in previous studies were susceptible under our conditions.2. Genetics of resistanceIn order to enhance the introgression of resistance genes into desirable cultivars, knowledge of the mode of resistance is essential i.e. it is imperative to define the mode of inheritance of the target gene(s), whether mono- or polygenes are involved and whether these genes are dominant, recessive, incompletely dominant etc. In this study we characterized the mode of inheritance in 3 genotypes, PI 323498, PI 157076, and P1420145. We found that these genotypes have monogenic dominant resistance to D. bryoniae. We however did not determine their allelic relationships.3. Genetic relationships3.1 Genetic relationships among PIs resistant and susceptible to Ddidymella bryoniaeTo investigate the genetic relationships among melon germplasm resistant and susceptible to D. bryoniae we analyzed 58 accessions that were either highly susceptible or highly resistant to D. bryoniae using RAPD markers. RAPD analysis revealed that some of the accessions which were closely related had similar reactions to D. bryoniae. Contrastingly, some of accessions from the 2 different groups were more closely related than some from within the groups, this observation is especially important if heterosis is a consideration besides the introgression of disease resistance. The resistant accessions were more distantly related than the susceptible accessions.3.2 Genetic relationships among Chinese F melon hybridsWe also analyzed the population structure of selected Chinese F1 hybrids using RAPD markers. RAPD analysis revealed a fairly broad genetic background among the hybrids. Further analysis on a large scale would reveal the exact extent of the genetic diversity observed in the present study. No association was observed for fruit pericarp thickness. 4. Molecular markers linked to the Didymalla bryoniae resistance locus We constructed an F2 population from a cross of PI 420145 x 136170, resistant and susceptible parents respectively and using amplified fragment length polymorphism technology and bulked segregant analysis we screened AFLP primers to identify markers associated with the D. bryoniae resistance locus. Four putative markers, E-TG/M-CTC₂₀₀, E-AT/M-CTG₉₀, E-TC/M-CAG₆₀, and E-TG/M-CTA₇₀ were found to be closely associated with the D. bryoniae resistance locus. If shown to be detectable in non-mapping populations with PI 420145 as the resistant parent, these markers could have potential use in marker assisted selection (MAS) breeding of melons resistant to D. bryoniae.