From Clemson Plant Pathologist Tony Keinath
This blog is an update on the preliminary cucumber cultivar reactions to downy mildew posted on June 16, 2023. Use the info in this blog to choose cultivars. Clearly, a few cultivars had temporarily “escaped” infection, although I don’t know how that was possible.
In addition, the resistances originally listed have been updated based on the current reactions. Unfortunately, until recently, breeders (and pathologists) had no easy way to tell the difference between the cucumber and the squash strains of downy mildew (think fraternal twins). Genes for downy mildew resistance are still currently being mapped. So, it’s not always clear how much resistance certain cultivars have to the two strains until they are tested in different regions and years.
| Cucumber cultivar | Cucumber type | Resistance | % Leaf area with disease | P=0.05 |
| Straight 8 | Slicer, heirloom | None | 90 | a |
| Silver Slicer | Slicer | None? | 90 | a |
| Poinsette | Slicer | Squash strain | 75 | ab |
| Gershwin | Pickling, parthenocarpic | None? | 75 | ab |
| Bristol | Slicer | Squash strain | 70 | b |
| Peacemaker | Pickling | Squash strain | 70 | b |
| Citadel | Pickling | Squash strain | 67.5 | b |
| DMR-NY401 | Slicer | Squash and cucumber strains | 12.5 | c |
On June 9, cultivars had no symptoms, and 2½ weeks later, the two most susceptible cultivars had 90% leaf area diseased, the best way to measure the effects of downy mildew on cucurbits. We don’t need the statistics shown in the last column to see that only one cultivar, DMR-NY401 (also called DMR 401 in some seed catalogs), has enough resistance to rely only on resistance to manage downy mildew. Nevertheless, the partial resistance in Bristol, Peacemaker, and Citadel can be useful to get an extra week or two of harvests from these cultivars compared to completely susceptible cultivars.
Cultivars were planted twice at Coastal REC: originally direct seeded on March 22 and transplanted in a second field on April 21. Five of the six less susceptible cultivars had more disease when transplanted in April than seeded in March. This effect was verified statistically (P = 0.005). The following photos illustrate the benefits of early planting, as reported previously.
| Cultivar | March seeding | April transplanting |
| Straight 8 | 90 | 90 |
| Silver Slicer | 90 | 90 |
| Poinsette | 70 | 80 |
| Gershwin | 60 | 90 |
| Bristol | 60 | 80 |
| Peacemaker | 70 | 70 |
| Citadel | 65 | 70 |
| DMR-NY401 | 10 | 15 |
The following table summarizes management options for cucumber downy mildew for organic and conventional growers. For more information see Land-Grant Press 1002.
| Management Option | Organic Production | Conventional Production |
| Plant DMR-NY401 | X | |
| Plant partially resistant cultivars | X | X |
| Plant early | X | X |
| Spray copper preventatively* | X | |
| Spray fungicides as needed | X |
*NOP certified organic growers should check with their certifier to make sure preventative applications of copper are allowed.
The South Carolina Grower 