Dealing with blackspot fungus, enemy of roses

by Stan Cutler
Posted 9/18/20

Years ago, after struggling to keep a couple of dozen hybrid tea and floribunda rose bushes healthy in our Germantown garden, we decided that the effort was not worth the reward. For two decades, the …

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Dealing with blackspot fungus, enemy of roses

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Years ago, after struggling to keep a couple of dozen hybrid tea and floribunda rose bushes healthy in our Germantown garden, we decided that the effort was not worth the reward. For two decades, the memory of our first rose garden dissuaded Valerie, my wife, from trying again in Chestnut Hill. But memories fade. Valerie finds it difficult to resist the flowers she sees in the catalogs. She consulted with her enabler (me) and ordered a few more rose bushes than were sensible. 

In May, halfway through Covid Phase One, I cleared daylilies from a 10’ by 10’ patch in front of  the toolshed at the back of the property, planted four new bushes and transplanted three specimens that had not thrived elsewhere in the garden. Valerie was not pleased when she noticed the first signs of blackspot in late June but had studied the disease and believed she had the cure.

Blackspot is a fungus, Diplocarpon rosae, that starts at the bottom of the plant and gradually rises. By the time the weather starts to cool, two-thirds of a bush’s leaves have dropped after turning yellow with black spots. Amazingly, after blackspot has done its damage, the roses find the strength to set new buds. When they flower in late summer, the roses are considerably smaller than the show-quality June blooms.  

The books will tell you that you can eradicate blackspot if you catch it early, spray with copper sulfate and clear the ground of fallen leaves. We started spraying even before the first signs. We sprayed from early June through July, spacing applications ten days apart, as advised by the “experts.” The advertising hype for the chemical fungicide claims it will do no harm. In late July, watching the blackspot appear on the leaves of every one of the plants despite spraying, Valerie looked online for an alternative and discovered that copper sulfate is harmful to insects. We stopped using it at the end of July. It’s now early September and every one of the plants is quite sick.  

There are no other blackspot killers that she was able to discover. One experienced grower told her she had two choices: use chemicals and have roses, or not have roses. We risked puncture by rose thorns, crawling between the bushes to collect fallen leaves. Mother Nature laughed. Our hot humid summers are perfect for Diplocarpon rosae.  

Each spot is a cluster of speck-size fruiting structures ( acervuli ) that produce hundreds of microscopic spores. The hotter and more humid, the more spots and spores. The spores are carried to the ground by splashing rainfall, dripping dew, overhead sprinkling and gardeners working among wet plants. The next day, if the weather is right, after it matures on the ground surface, a spore floats upward to a leaf, germinates and penetrates rose tissue in 9 to 18 hours. It starts producing new spores the next day. Spots become visible in about a week, too late to save the leaf. As far as the fungus is concerned, the more humid the atmosphere the better. Welcome to the Delaware Valley.

For addicted gardeners, there are few things more intoxicating than a clear patch of soil. After I cleared out the daylilies and turned over the ground, Valerie’s craving intensified. When the bare-root plants arrived from Oregon, she just had to put them in the ground, regardless of whether there was enough room. We planted the bushes about three feet apart, violating the recommended four-foot spacing. And the area is too close to the shed that blocks the breeze. There were many overcast, humid days in August when the leaves did not dry.

If you are thinking about planting a rose bush, put it in an open area at least four feet away from others. Go for shrub roses that are more blackspot resistant than teas or floribundas. And consider the beach rose (Rosa rugosa) that produces dozens of deep-pink flowers and is almost immune to blackspot. The flowers are not as showy as hybrids but have a wonderful fragrance. Other than occasional watering, they require no care at all. The fat red rose hips, the seed pods that remain after the petals fall off, are rich in Vitamin C and used to make medicinal tea.   

We had learned all the lessons about spacing and the persistence of blackspot the hard way, years ago, but chose to forget. Addicts make bad choices.  

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