| Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. & Curt) Rostow, is a major foliar disease of cucumber. Today, 10 years after the emergence of the new race(s), downy mildew continues to be a major threat to cucumber production in the United States. Most cultivars currently grown have some resistance; however, the observed level of resistance is significantly lower than that observed during the latter half of the 20th century. Prior to 2004, host resistance was sufficient to control disease without fungicide application. Modern fungicide programs required to produce a marketable crop of cucumber under disease pressure represent a significant cost to growers. A return to a host-based resistance system is necessary to protect the profits of growers and stabilize the cucumber market.;High resistance sources were identified in a screening of cucumber germplasm. Experiments were conducted to (i) evaluate the effect of plant age on the expression of resistance; (ii) determine the type of gene action, estimate genetic variance and heritabilities of resistance in the highly resistant Plant Introduction (PI) 197088; (iii) assess the availability of non-allelic variants among the PI accessions identified as highly resistant; (iv) identify quantitative trait loci (QTL) associated with the resistance in PI 197088.;We tested a diverse set of cucumber cultigens in order to evaluate the effect developmental stage has on resistance. Among those cultigens identified as highly resistant, some appear to respond differently to disease as varying developmental stages. The resistance present in PI 330628 and PI 605996 appears capable of fighting off downy mildew later into life. Their indeterminate growth habit may allow these accessions to outgrow the disease and appear resistance for a longer duration. Regardless of the mechanism, plant breeders may wish to utilize the resistance present in PI 330628 and PI 605996 over other resistant accessions due to their ability to maintain a high level of resistance after multiple harvests.;Resistance in PI 197088 failed to fit the single gene model, indicating that resistance should be considered quantitative and under the control of multiple genetic factors. Genetic effects were greater than environmental effects in most cases. Our estimates of additive variance and broad- and narrow-sense heritability were consistently large; suggesting gain from selection should be relatively effective. Furthermore, we estimated that resistance is under the control of only a few loci and estimated gain from selection indicates that a gain of one to two points (on a 10 point scale) per cycle under high intensity should be possible.;In general, our test of resistant lines PI 197088, PI 330628, and PI 605996 suggest that minor genetic variation may exist among the lines. We observed a significant number of susceptible offspring in the segregating F2 generation, suggesting genetic differences are probable. However, further testing must be performed to confirm these results, as environmental variance was as high as genetic variance in many cases.;Finally, we have identified multiple QTL associated with resistance in PI 197088 utilizing a 157 F6:7 recombinant inbred line population. In brief, three putative QTL were identified and were tightly linked to SSR markers: SSR17911 (Chr 4), SSR15321 (Chr 5), and SSR18489 (Chr 5). Overall, the total variation among RILs explained by the putative three-QTL model was 52.29%, with a model LOD score of 21.70. All three QTL were identified in separate analysis across two locations and two years, suggesting they are environmentally stable. |
