There are new weapons for the gardener’s fight against the black spot. Of all the maladies roses are subject to, blackspot is universally dreaded.
The fungus Diplocarpon rosae causes this destructive disease. It cripples or kills its victims mainly by attacking the leaves and causing them to die.
Depending on the fungus’s virulence and the roses’ resistance, they may stop growth slowly or quickly.
Dead canes and dieback result. Even the most inexperienced eye can then see that the bushes are in a dangerous state.
Blackspot stunts bushes and is responsible for puny flowers. These usually lack perfume and are of poor color.
Victims Of Blackspots
Because of the drain on their vitality, victims of blackspots are subject to winter killing because they have little or no resistance to severe weather.
Defoliation of roses due to blackspots is especially pronounced during periods of frequent rainfall in warm summer months, according to Dr. E. W. Lyle, plant pathologist of the Texas Rose Research Foundation.
Such loss of leaves has an adverse effect on the quality of the affected roses. As a result, development is retarded, and the plants may finally die.
Experiments On Using Dust And Spray Fungicides
During the past few years, the Texas Rose Research Foundation in Tyler, Texas, carried out experiments involving dust and spray fungicides.
In these tests, sprays gave better results than titan dust compounds in halting or preventing blackspot.
Once it was critical to start fungicide treatments early, maintain a close schedule of subsequent treatments to check for any infection.
Knowing that the disease may be halted even after it is evident and roses may be kept healthy is now cheering.
This does not mean the rose grower may sit back and permit the disease to attack. However, it is wise to continue preventive measures that will keep in advance of a possible infection.
Five Effective Fungicides
Dr. Lyle lists at this time five fungicides that effectively control black spots. They are the following:
- Dithane M-22
- Manzate
- Phaltan
- Captan
- Cyprex
He comments that Cyprex is so new that further trials should be made before employing it extensively in rose gardens.
A rose highly susceptible to blackspot was chosen for the control experiment. It was ‘Floradora.’ Out of 14 fungicides used, three spray types proved superior to the others.
They were maneb (Dithane M-22), Phaltan, and Cyprex. Blackspot and its effects were held to a harmless degree by these three sprays.
It was noted that Cyprex did cause a very slight yellow spotting of some of the leaves. More dilution, it is felt, might be the answer to this.
Using Du Pont’s Manzate, a maneb fungicide, Dr. Lyle found that the weight of bushes sprayed with it increased up to 83% percent over the weight of untreated check bushes.
Not only did Manzate spraying increase weight, but it also increased the starch content so necessary for extra vitality.
In garden roses, this represents the reserve food needed to harbor the bushes over winter to a healthy start in the spring.
Lanzate-sprayed bushes also showed more fibrous roots and canes of better color than the untreated checks.
In the Texas experiments, only 2% percent of blackspot infestation occurred where Manzate was used.
There was a 60% percent infestation on the unsprayed check bushes. Manzate gave good control where spraying was belated.
Interesting Blackspot Control Data
In six years of testing, Dr. L. M. Massey of Cornell University came up with some interesting blackspot control data.
His New York tests included plantings of over 1,500 roses representing seven named varieties.
Manzate ranked among the top fungicides in the trials, comparing as many as 35 different treatments in a single season.
In Dr. Massey’s tests, overhead irrigation provided water needed for inoculation, incubation, and infection of the test plants.
Even in seasons favoring blackspot disease, plants were inoculated once or twice each season to ensure enough spotting and uniformity for reliable data.
Treatments were applied two days before injection so that the fungicides were already on plant surfaces to provide protection when the blackspot spores landed.
Applications were made weekly. Counts were made as soon as spotting was visible. Records were taken of injury to foliage, the severity of objectionable disfigurement, and defoliation.
Studies at Beltsville, Maryland, found that early blackspot infections were correlated with heavy infections of the previous year.
Maneb and Zineb
Inadequate control in one year resulted in unhealthy, smaller plants, which bore fewer flowers early the succeeding year.
In the Beltsville tests, maneb and zineb were outstanding in controlling black-spot. However, through the season, zineb was more effective as a spray.
Zineb tested on the ‘Helen Traubel’ rose and had no significant effect on flower yield until September.
Then it was noted that test plots showed greater amounts and weights of flowers than the untreated check plots.
This was true also of maneb. Plots treated with captan were somewhat better than those protected with ferbam.
Malathion-Captan Combination
A malathion-captan combination used on check plots proved superior to cap-tan used alone or in combination with Aramite. In addition, the malathion-captan combination resulted in heavier flowering.
Dr. George and Nora Jorgenson, writing in the American Rose Magazine, have this advice to offer from their wealth of blackspot research.
They urge rose growers to begin fungicidal treatment as early as possible, repeat it regularly through the summer, and into fall until dormancy begins.
The roses may be pruned clean of their infected leaves and stems when dormant.
Jorgenson’s Experience In Blackspot
It has been Jorgenson’s experience that blackspot may gradually develop a resistance to one kind of fungicide used repeatedly.
To prevent this, they shifted to other chemicals. For example, when for a given period captan was used, they switched to Phaltan, a chemical analog of captain.
Finally, maneb might be the third fungicide used. Again, they repeated this sequence over and over with excellent results throughout the summer.
For those rose growers who abhor chemicals and their applications or those who, for one reason or another, find it impossible to give treatment to their roses, it is suggested that only resistant varieties be planted.
Species Immune in Blackspots
Three species are practically immune to blackspots:
- Rosa bracteata,
- Rosa rugasa
- Rosa wichuraiana
Hybrids developed from these species have a high resistance to black spots.
Here are three such hybrid tea roses:
- ‘Pink Princess’
- ‘Orange Ruffels’
- ‘Red Duchess’
Then we have two resistant floribundas:
- ‘Pinocchio’
- ‘Pink Bouquet’
- ‘Mermaid’ (resistant climber)
44659 by John Krill