Hi Judy,
" I never knew there were two kinds of zinnias, one with resistance to fungus. "
The Zaharas, Profusions, and Pinwheels are resistant (essentially immune) to Powdery Mildew, but only to that fungus disease. There are other fungus diseases. Powdery Mildew is a special case, because there are different species of it, that are unique to their "host" species. The species of Powdery Mildew that can grow on Zinnia elegans (violacea) cannot grow on other species of plants.
Powdery Mildew also affects Lilacs and Phlox, for example. Each "has" its own unique species of Powdery Mildew. Zinnias cannot "catch" PM from Lilacs or Phlox, and vice versa.
A few decades ago, plant breeders experimented with crossing two different species of zinnias, Zinnia elegans (violacea) and Zinnia angustifolia. Since Z. elegans has 24 chromosomes and Z. angustifolia has 22 chromosomes, the resulting interspecies hybrids had 23 chromosomes. Plants with an odd number of chromosomes cannot produce viable seeds, so those hybrids were sterile.
The plant breeders (at the University of Maryland) solved that problem by using the drug Colchicine to double the number of chromosomes of their sterile interspecies hybrids. That produced zinnia plants with 46 chromosomes, an even number, so they could produce viable seeds. Since their 46 chromosome count distinguished them from both Z. elegans and Z. angustifollia, they were actually a new species of zinnia. The plant breeders named the new species Zinnia marylandica, in honor of the University of Maryland where the experimental work was done.
Since Zinnia marylandica was a different species from Zinnia elegans, it was "immune" from the Zinnia elegans species of Powdery Mildew. Perhaps some day Powdery Mildew will "mutate" to form a species of Powdery Mildew that can grow on Zinnia marylandica, but it could be a very long time before that occurs. In the meantime, the Zaharas, Profusions, and Pinwheels don't have a species of Powdery Mildew that can grow on their species of zinnia, which gives them a significant advantage.
As an amateur plant breeder, I choose not to work with Zinnia marylandicas (Zahara, Profusion and Pinwheel) because their flowers and plants are significantly smaller than the larger strains of Zinnia elegans. And I can make my breeder Zinnia elegans plants temporarily "immune" to their species of Powdery Mildew by treating them with a systemic fungicide, like Tebuconazole.
But I do understand the popularity of the Zahara and Profusion zinnias because of their freedom from Powdery Mildew. And because of that popularity, you can expect new improved commercial varieties of them in the future.
ZM
