Although this fungal blight can infect many types of plants, including apple trees, tomatoes, and more, it is most often associated with roses. It’s the rose gardener’s most dreaded foe.
Host Plants
Any type of rose can become infected with black spot. Some species have been bred to be resistant to this fungus, but most of the thousands of rose species have not. Other plants related to roses, such as apples, may also be hosts – some may not show much of an infestation, in fact, and can act mainly as carriers instead.
Symptoms on Roses
The symptoms are straight-forward and well-known amongst rose gardeners. It generally manifests first on the leaves of the rose bush. It will then quickly spread, usually via water dripping from leaf-to-leaf or from leaf shaking from a breeze or animal. It quickly infests the plant’s leaves and their stems, reaching eventually for the flowers themselves. The first sign is usually a dark gray or black spot on the leaf’s underside wherever moisture might collect. It will then spread in patches across the leaf, eventually eating through it.
Life Cycle
Like most fungi, black spot needs warmth, moisture, and shade from the sun. This is usually found on the underside of leaves and inside the flowers. The fungus spreads most often through soil contamination and falling leaves and petals that have been infected. Spores will lie dormant in the ground until spring.
How This Fungal Disease Impacts the Plant
Black spot will kill leaves and, worse, destroy the roses themselves. At the very least, it will make the plant and flowers ugly and non-presentable. It can eventually kill the rose plant if it begins early enough in the season.
How to Prevent
As with most fungal infections, prevention is worth a pound of cure with black spot. Planting roses in a well-aerated spot where they receive light breezes and good air flow is a must. Proper care of the soil around them, including choosing mulches that are unable to carry black spot, is also important. Watering from the root rather than with a sprinkler system is a good idea as well. Keeping infected plants quarantined from others is vital. Many gardeners will destroy one rose bush that’s been infected in order to save the rest from possible infection. Destruction should be through bagging, removing, and burning the plant. Never compost an infected plant of any type.
Treatment of Black Spot
Less extreme treatments include fungicides, which have a high rate of success in most areas. Many of these must be hand-applied as a spray or powder. Apply before roses bud, if at all possible, but anytime the black spot has appeared is a good time. The disease’s treatment is more important than the aesthetics of the plant. Heat lamps, large fans, and other options have also been tried with limited success. The idea is to destroy the climate that the fungus thrives in.
Resources:
http://web.extension.illinois.edu/cfiv/homeowners/000805.html
http://www.aaes.auburn.edu/comm/pubs/highlightsonline/summer96/blackspot.htm
I’ve been using plain old corn meal for my black dot fungus because it’s all over my yard, tree’s and other plants and flowers. Believe it or not it actually has worked! I simply mix it in my soul when planting or sprinkle over infected area. It takes about a week or so and a few applications but it’s cheap and not harmful to pets or children.