Black spot in soil
As we just had an extensive article put forth by one of our plant disease experts at OSU, I thought it would be most helpful for you to have more info not only on that disease, but some of the others that you may encounter. You can remove one rose and plant another, but it is likely that black spot may still infect the new roses. Probably the main consideration you will want to make is to find more resistant roses--and there are many, susceptible to black spot, and not so much so--to choose from; also be sure to use the cultural considerations mentioned below:
CORVALLIS, Ore. – Charmingly mild weather this winter will most likely coax roses into bloom early, which means dealing with the usual diseases and pests earlier, too.
When it comes to one of the country’s most popular perennial plants, the “big four” vexations are powdery mildew, black spot, rust and aphids, according to Jay W. Pscheidt, a plant pathology specialist with Oregon State University Extension Service. More recently, downy mildew, which looks strikingly similar to black spot, has become an increasing problem.
“People are confusing them,” he said. “It’s difficult to tell the two apart.”
Black spot shows up as ragged black spots on leaves that often turn yellow and eventually drop off the shrub. The black spots made by downy mildew spores are not as dark and can have an angular appearance. Under moist conditions, you might see a gray, downy-looking growth on the undersides of the leaves.
As the name suggests, powdery mildew shows up as a white powder covering the leaves, but without the black blotches typical of downy mildew and black spot, Pscheidt said. It usually hits roses during the transition into summer’s driest time, particularly when dry days are followed by nights with high humidity.
Rust, which begins in spring and peaks in early summer, appears as orange pustules that can blanket the foliage in worst-case scenarios.
Because of the relatively warm winter, black spot may be the biggest issue this year, Pscheidt said. Rose plants held onto some of their leaves so there are plenty of disease spores that overwintered and are ready to attack newly emerging shoots.
Insects are less of an issue with roses. Aphids are about all gardeners have to worry about in Oregon and are fairly easy to control, according to Gail Langellotto, Master Gardener coordinator for OSU Extension. Her first line of defense would be to squish some with her fingers to release a chemical signal that attracts natural enemies like lacewings, ladybird beetles and parasitoid wasps. Follow that with a strong spray of water from the hose to wash the remaining aphids to the ground.
“Aphids are poor climbers,” she said, “and are less likely to reestablish because they run a high risk of getting eaten by ground-roving predators, such as spiders and beetles.”
Since aphid numbers can explode on nitrogen-rich plants, it’s a good idea to use an organic fertilizer, or a slow-release synthetic fertilizer that will give them less readily available nitrogen.
As for diseases, the best strategy is to start with disease-resistant cultivars.
“Buy fisherman roses,” Pscheidt said. “The types that you can plant and then go fishing. Breeders are always coming up with new plants that resist pests. Local retail nurseries will generally have a good list for you.”
But just because a rose is an older variety doesn’t mean they don’t fight off disease. OSU Extension’s Pacific Northwest Plant Disease Control Handbook offers a list of classics that fight the good fight against black spot, rust and powdery mildew. Longtime favorite hybrid teas ‘Just Joey,’ ‘Chicago Peace’ and ‘Mr. Lincoln’ are on the roster. ‘Gold Medal’ and ‘Queen Elizabeth’ grandifloras made the cut, as did the well-known floribunda roses ‘Playboy,’ ‘Sexy Rexy’ and ‘Iceberg.’
In addition to careful selection, prevention is the next best strategy and Pscheidt has some recommendations:
- Don’t crowd plants. Space adequately so sun will penetrate and air will circulate to dry foliage quickly after rain or watering.
- When pruning roses in late winter, clean up foliage thoroughly with a rake or leaf blower and remove diseased stems.
- Use soaker hoses to water when possible. If using an overhead sprinkler, irrigate so that foliage has time to dry out before evening.
- If desired, spray early with an organic or chemical fungicide or pesticide. Always follow label instructions.
Thanks -- I did see that article -- in the Oregonian maybe? Very helpful. I was in England several years ago at the Royal Botanic Gardens and they had installed a gigantic fairly new rose bed. The information said they had dug up the old rose bed and "sterilized the soil" then replanted a year later. I haven't been able to find any information on how they sterilized the soil (clear plastic maybe?). I think I emailed them but didn't get a reply. I wouldn't want to use clear plastic for fear of killing all the good organisms in the soil.
In reading your response, it sounds like a lot of work to eliminate your rose issues. It's not that you can't do so, but depending on such factors as your neighbors gardening practices and the number of roses in your vicinity, the "Sterilization" method might be more trouble than it is worth, especially if digging up of soil is involved. Now, if you were to grow seeds or start cuttings, then your efforts to provide a sterile environment might be helpful, to prevent soil pathogens to stop issues like damping off of new plants shoots. Also, how big an area are you considering?
University of California provides a good guide for using solarization to assist you in such a pursuit if you are to use clear plastic, which would also be effective in weed control. the biggest issue will probably be how big an area you decide to cover, the length of time the plastic would need to be in place to be effective, as well as the cooler temps on the coast which would make this a challenge to accomplish. The link at http://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html will give you more information on this process; copy that in your browser to get to the UC site.