I agree about the different methods of disease resistance being diluted in crosses. This is a very good reason to backcross to one or the other resistant parent, or cross among seedlings of different crosses involving one parent, in order to reinforce that species’ particular method of disease resistance.
For example, one might cross the various Wichuraiana/Luciae hybrids with dwarf polyanthas to recover rebloom, but it might be better to choose one, e.g., ‘Yvonne Rabier’, that has Wichuraiana as a recent ancestor.
Even ‘New Dawn’ can come down with a nasty case of mildew in some climates (e.g., Santa Clara, CA). To build a better ‘New Dawn’ one might cross one of the diploid Wichuraiana/Luciae hybrids (take your pick) with one of the Brownell “Sub-zero Roses”. Maybe ‘Francois Foucard’ x ‘Lily Pons’, or ‘Francois Juranville’ x ‘Shades of Autumn’. Wichuraiana/Luciae ancestry on both sides of the family tree should reinforce the ancestral resistance.
BTW, the rough and glaucous leaves of ‘Asta von Parpart’ seem to be healthier than many glossy types, at least in San Jose.
As for long-lived and short-lived leaves, heres an article that sheds some light on the subject.
HortScience [38(1):71-74. 2003]
Changes in Mineral Nutrient Concentrations in Petunia Corollas during Development and Senescence
Sven Verlinden
Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV 26505
“Abstract: To observe changes in the nutritional status of corollas during development and senescence, Petunia x_hybrida_ cv. Mitchell corollas were analyzed for macronutrient and micronutrient content, dry weight, fresh weight, and ethylene production. Carbon content decreased at slightly lower rates than dry weight during corolla development between anthesis and senescence, while fresh weight and ethylene production followed patterns expected of climacteric flowers. Nitrogen, phosphorus, and potassium content declined during development. Both phosphorus and potassium content gradually declined throughout development with overall losses of about 75% and 40%, respectively. Nitrogen content declined 50% during development but losses occurred only during the final stages of senescence. No significant changes were observed in sulfur, calcium, magnesium, and micronutrient content of the corollas during development. Most elements were present in much lower concentrations in corollas than in leaves. The concentrations of calcium, magnesium, and manganese were about 1-, 5- and 15-fold lower in corollas than in leaves, respectively. Results indicate that remobilization of selected macronutrients from corollas occurred before and during senescence. Taken together with the presence of low concentrations of macronutrients, my data support the contention that petunia corollas are nutritionally inexpensive and therefore easily disposable organs.”
The leaves of Rosa foetida are built to last 3 or 4 months. I assume that they are then dropped in time to avoid the summer drought. Other species have leaves that are built to last several months or even more than a year. Hybrids between an evergreen (or nearly) species with Foetida would be neither here nor there. They would probably receive a half-measure of nourishment combined with an unwillingness to shed leaves when the nutrients are used up. The result would be an inability to combat fungal infections late in the season.
I once saw a demonstration of this: I had an African Violet blooming in a terrarium on my desk at work. As soon as the petals passed their prime, they were attacked by a fungus. I considered removing the plant to protect it from the assault, but the experimenter in me left it to fend for itself. Which it did. The fungus did not spread onto the leaves or any other part of the plant.
Karl
). So your questions piqued my interest.