Five commercial cultivars and one species of rose were evaluated in a field trial for resistance to blackspot caused by Marssonina rosae. The trial was set up as a split-plot with two treatments. Each subplot was either protected on a weekly basis with a fungicide application to control blackspot or left to progress naturally with the disease. The trial was evaluated for blackspot resistance and for growth characteristics. Only the one species, Rosa roxburghii, was disease-free in both situations. Two cultivars, Peace and Sunflare, were the least resistant. They averaged 65% defoliation and a 50% infection rating. This corresponded to a 50% reduction in height and a 90% reduction in fresh weight of the plant. `Red Radiance' had ≈20% less defoliation than the two above cultivars, which was reflected by a 20% increase in growth. `Old Blush' had a higher resistance rating, but its propensity to quickly drop its foliage upon infection left it with a similar growth reduction as `Red Radiance'. `Carefree Wonder' was the most-resistant commercial cultivar. It had only a 43% decrease in fresh weight. `Red Radiance' and `Carefree Wonder' both displayed much greater defoliation during December, whereas the other susceptible cultivars showed the same degree of defoliation as earlier in the season.
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