Anyone who works outside for a living, maintains a lawn or garden or who even attended the recent Hancock County fair will probably agree that the weather report could be summed up in two words: warm and wet.
While these conditions might be hard on humans, pets and livestock, they can be exactly what some plant-feeding fungi thrive on, including those that feed on roses.
Roses are one of the most versatile ornamentals for landscaping, with cultivars adapted for any garden site and landscape purpose. They offer many positive attributes to a landscape including flower color, form, texture, winter color and interest.
One of the greatest challenges, however, to successfully growing garden roses is disease management.
And one such disease — a fungus called Black Spot — may be one of the greatest single challenges rose-growers face each year — especially when conditions are warm and wet.
Once infected with black spot spores, leaves become spotted, turn yellow and drop prematurely from the plant.
Premature defoliation decreases plant energy reserves and results in reduced flowering of roses and ultimately to increased susceptibility to winter injury or dieback due to other causes.
As the name implies, this disease results in black, nearly circular spots up to about one-half inch in diameter on the upper leaf surfaces. These lesions have characteristic feathery margins.
Once infected, leaves turn yellow and drop from the plant with lower leaves leading the way, followed by middle and finally upper leaves.
The good news is that in resistant cultivars or during dry weather, only small spots may form without defoliation. The bad news is that this year is anything but dry.
The pathogen survives the winter in lesions on older canes and other above-ground plant parts.
Rose leaves are most susceptible to infection when they are young and actively expanding.
At least seven hours of continuous wetness is required for spores to cause infection, especially when temperatures hover between 72 and 86 degrees. Sound familiar? The environmental conditions we currently have are the equivalent of a petri dish — not only for rose black spot but also for a variety of other landscape and crop fungi.
What to do?
Regardless of the variety of rose, before reaching for the “magic in a bottle “ fungicide, first, let’s do all we can to create a plant and growing environment that is less favorable to the disease organism. It sure doesn’t need any help.
Roses planted in full sun and pruned to an open canopy and with lots of room between plants are better able to dry off between rains and tend to have lower humidity within their canopy. These are good things.
Removing and destroying infected canes and fallen leaves in the fall will reduce the amount of over-wintering inoculum, which could cause primary infections during the following growing season.
Drip irrigation or watering the soil is preferred to the use of sprinklers. If sprinkler irrigation must be used, water in the morning and not in the evening. A couple of inches of mulch around the drip line of plants will reduce splashing of spores from fallen leaves.
As a rule, rose cultivars vary widely in their resistance to black spot. Typically hybrid teas, grandifloras and miniature roses tend to be more susceptible with floribundas, shrub roses and climbers demonstrating more resistance.
When planting new roses, it is a good idea to select disease- resistant cultivars. Growing resistant cultivars will save time and money spent on buying and spraying fungicides.
Despite their disease resistance and beauty, rose gardeners should not feel compelled to limit their choice to just the popular Knock Out series. While they are fine roses, there are other varieties that are worth considering as well. A new list of roses and their resistance to disease is available upon request. Older lists and descriptions of rose disease resistance might not be reliable for a variety of reasons.
Even once we have taken all of these good management practices, in weather like we are currently “enjoying,” it might be important to combine with these a well-selected and maintained fungicide treatment regimen.
Several fungicide products are available for control of black spot.
Repeated applications of these at intervals specified on the label might be necessary to protect newly emerging foliage.
Thorough coverage of the foliage with a couple of fungicides of differing modes of action applied in alternating applications will reduce the chances of fungicide resistance build-up in the black spot fungus. Adding a commercial spreader sticker (surfactant) to the fungicide spray mixture will improve coverage on the waxy rose leaves.
Since fungal spores move during rain events, spray in anticipation of rain (provided there is time for the spray to dry), to offer the leaves the best protection.
For those reading this whose passion is tomatoes or fruit rather than roses, many of the above practices hold true for protecting those plants from fungi as well.
Always read and follow label instructions for mixing and applying fungicides.
Remember, we have free chemical-resistant gloves in the extension office while supplies last.
Roy Ballard is an agriculture and natural resources educator with the Hancock County office of Purdue Extension (extension.purdue.edu/ hancock). Contact him at 317-462-1113 or [email protected].
