Cercospora is a new disease for me. It seems to love my small collection of explorers. Can anyone elaborate what conditions promote this disease and more how it it can be bred out? It seems to be a real problem on the prairies and appear to be getting worse. I can’t keep a ‘John David’ foliated past Aug 1. J
Should read ‘John Davis’ - love autocorrect!
It seems to be an emerging disease in this area.
A few years ago William Baffin was totally healthy for me, but I knew that it defoliated in the Minneapolis area. Sure enough the disease showed up and now Baffin is pretty ratty later in the season. John Davis also gets it badly.
Julie Overom of northern WI has extensive experience with cercospora, calling it her biggest breeding challenge. She says it seems like 99% of roses eventually get it. They can go for years in perfect health before breaking down.
I noticed a Sunrise Sunset, which is normally quite healthy, break down with cercospora. It was growing near my huge William Baffin. Carefree Beauty seems to get it badly.
I remember someone mentioning that it seemed like bad blackspot years and bad cercospora years alternated.
Yes, if blackspot wasn’t bad enough, cercospora can have one feeling like throwing in the towel altogether! Of all my roses, only the rugosas have yet remained spotless along with an arkansana hybrid that had only displayed a few suspicious spots very late in the season. ‘Campfire’ has been relatively clean along with just a couple seedlings of mine. Even ‘Frontenac’ was rather hard hit the last few years and thus I’ve dropped it from my breeding work. Oh, and yes, ‘John Davis’ can be left totally leafless! So, this has thrown a curveball and redirected focus upon my few selections that do display a higher degree of immunity.
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Last year I planted several John Davis in my seedling area, so serve both as indicators and inoculants for cercospora. When I see the John Davis defoliating and other established seedlings near them untouched, perhaps I can be optimistic about those roses. However, Julie’s voice of experience would caution that the onset of cercospora can be frustratingly delayed.
I used to grow William Baffin and John Cabot. WB would defoliate completely by the end of the summer whereas JC had better resistance, but not immune. I used WB in a number of crosses mostly because it is quite hardy where I only used JC a few times. The seedlings of WB turned out to be quite susceptible to cercospora as well and most were culled. In hind sight maybe I should have used JC in more crosses instead. But I did use several WB seedlings in crosses and a number of the second generation seedlings are showing more resistance to cercospora than their parents. WB can pass on sparse repeat bloom and that appears to be bread out in the second generation as well.