by Claude Thomas
In my experience of advising not only rose growers on disease control over the past five years, but also in advising vegetable crop growers on disease control over a 38-year period, I have found that they all seem to experience more successful control when they have a better understanding of why the recommended measures work against the disease that they are combating. So before your say to yourself, “Oh no, not another article on black spot control,” and turn the page to the next article, read on a bit more. Chances are you may learn something that will help you do a more effective job in controlling the most important disease of roses in South Carolina and most of the rose growing areas of the world. You should at least gain a better understanding of why certain control measures work, which often helps us justify to ourselves the efforts and costs required to exercise these measures.
Blackspot is usually the most prevalent disease problem facing rose growers throughout the USA, except perhaps in the arid areas of the southwest. It is also found wherever roses are grown throughout the world and it has been taken along as cultivated roses have been spread to new geographic areas. As plant diseases incited by fungi go, it is not well-equipped to spread over long distances without the help of man or other means.
Even though those who grow roses are all too familiar with the symptoms of blackspot, this is just a brief review of those symptoms to provide something of a framework for what follows. Symptoms of the disease first appear as 1/8- to ½-inch circular blackspots (lesions) that develop on the surfaces of rose leaves. These spots have feathery margins and may merge into irregular shapes. Leaf tissue around these spots turns yellow and infected leaves eventually drop off. The fungus is restricted to the black lesions, and the yellow areas and leaf drop are the result of biochemical reactions that occur because of the infection. Symptoms of the disease can also appear on the immature wood of young canes as irregular purple-red blotches that also turn black. Initially only a few leaves may show symptoms, but with favorable environmental conditions the disease spreads to more and more leaves and young stems resulting in a severe outbreak that, if left unchecked, will eventually defoliate the entire plant. Likewise, as new leaves emerge, they also become infected and drop off. This wholesale loss of foliage results in a continuous weakening of the plant along with the accompanying unsightly appearance it presents.
Blackspot on roses is incited by a plant parasitic fungus that has over the eons of time become highly specialized in infecting and inciting disease in roses. As far as is known, this fungus only infects roses, so even though you may see black spots or other symptoms similar to those of rose blackspot on other plants in your garden, they are not caused by the same fungus and thus do not represent a source of disease that threatens your roses. Blackspot of rose can only spread from rose plants to other rose plants.
The disease is spread by spores of the fungus that are produced in the black spots. When these spores are dispersed and land on healthy rose tissue they germinate and infect these new sites. The black spot fungus produces two types of spores. One type breaks loose and can be blown in the air for long distances. Fortunately for rose growers, the production of this type of spore by the black spot fungus is so extremely rare that you could count on your fingers the number of times that scientists have reported finding them. Therefore, they are considered of no significant importance in the spread of the disease. However, in the black spot lesions the fungus produces another type of spore in abundance. This is the spore that must be dealt with to control the disease. Thankfully, it is not an airborne spore and is not naturally spread over long distances. These spores are produced in sticky masses in the black lesions that are spread primarily in water droplets such as from splashing rain, overhead watering and dew. They can also be spread by adhering to gardening tools, to a gardener’s clothing, to insects and to other animals; or on diseased fallen leaves that are dispersed locally by the wind.
The fungus can survive cold winters in infected fallen rose leaves or in infected stems; and forms new spores in the spring to initiate the disease cycle. In coastal South Carolina where winters are often mild enough that roses do not drop their leaves, it can survive quite easily in the living leaves, even though its development and spread is interrupted until warmer temperatures return.
The spores of the pathogen must be wetted before they will germinate and remain wet at least 7-8 hours for infection to occur on either the upper or lower surface of the leaf. Infection is most efficient from 66-70oF, but the fungus is active over a temperature range of 59-81oF. Above and below these temperatures, its development is progressively arrested so that very high summer and very low winter temperatures markedly limit or stop its development. Blackspot spores are killed at temperatures above 910F. Depending on environmental conditions after infection, visible symptoms of the disease may take from a few days to a couple of weeks to appear.
“Well,” you ask, “what does all this have to do with controlling blackspot?” The answer is that to control black spot you must take advantage of this fungus pathogen’s vulnerabilities by attacking the weak spots in the disease cycle with the methods available to you. These methods are basically sanitation, cultivation practices, and fungicide application. Sanitation can be broken down into exclusion and eradication; and fungicide application includes the use of a protectant material and a systemic material. You can also avoid black spot by only growing roses that are highly resistant to the disease, such as the ‘Knock Out’ varieties and some of the older rose types. However, this option is not available for hybrid teas, grandifloras, floribundas, minifloras, miniatures and other highly popular types of roses.
To exclude blackspot from your rose garden do not bring it in yourself. Man is by far the primary culprit responsible for the long distance spread of this plant disease. When you acquire new rose plants, no matter what the source, they should be kept isolated in pots and not planted in your established garden until you have a high degree of certainty that they are free of blackspot (as well as other diseases and pests). Inspect them carefully and remove, and keep removing, any obviously diseased leaf or stem tissue until nothing remains except disease and pest free material, keeping in mind that it may take several days or even a couple of weeks for infected tissue to show disease symptoms. For these new plants you should also remove, bag and discard any plant debris on the surface of the soil in the pots. While the plants are in isolation, make at seven day intervals three or more applications of both a protectant fungicide, such as Dithane M-45, Maneb, Manzate, or Mancozeb combined with a systemic fungicide, such as Banner Maxx, Cleary’s 3336-F; Compass or Bayer Disease Control for Roses, Flowers and Shrubs. These combined fungicide applications are also effective against other foliar fungus diseases that you do not want to introduce into your garden, and including at least one application of a miticide and an insecticide during this period will help keep those pests from being introduced.
There are several sanitation practices that will help eradicate blackspot that is already present in your garden. The winter removal of old mulch and other plant debris from your rosebeds is a first step to prevent the carry-over of diseased material from the previous growing season so that spores from this material do not infect the coming season’s rose crop. This material can be raked off and then the surface of the bed further cleaned with a leaf blower. For growers who use relatively heavy mulch material such as wood or bark chips so that raking is impractical, a leaf blower can be helpful in removing small, loose surface material and rose debris.
After cleaning the surface of your rosebeds and completing your spring pruning, the next eradication measure is to spray lime-sulfur on the surface of the beds and the newly pruned rose plants before they begin to bud out. You should also spray the soil surface in the pots of any plants that you are holding in isolation. The lime-sulfur acts as a disinfectant killing any blackspot fungus that remains in debris on the soil surface or on pruned plants, further decreasing the possibility that agents of the disease are carried over from the previous season.
Your eradication efforts should continue throughout the growing season through the routine removal and destruction of any leaf or stem material with symptoms of blackspot just as part of your normal rose care routine. The sooner you spot and remove this material the better. It is also worthwhile to mark areas on bushes where you have removed diseased material and keep checking these places over the next couple of weeks and removing any additional plant material on which black spot symptoms appear. This practice is a simple, but effective way to interfere with the disease cycle by eliminating sources of spores that can incite more disease.
Rose cultivation practices that work against the development and spread of blackspot are wide-spacing between bushes and keeping bushes pruned openly enough to enhance air circulation that speeds the drying of water deposited from rain or dew. It is also helpful to remove leaves and stems near the ground to both improve air circulation and to help reduce the probability that black spot spores from diseased rose debris on the ground will be splashed onto these low-lying leaves and stems. On hybrid teas, I like to keep the lower leaves removed for at least a foot, often more, above the ground. The most important cultivation practice against black spot is to avoid overhead watering, because this is a great way to both spread the spores and to provide the wetting necessary for infection to occur. The only exception is the occasional use of a water wand to help control spider mites.
The application of fungicides as a control measure against blackspot should be employed when leaves begin to emerge and continued throughout the year while leaves are present on the plants. Fungicide applications should include the simultaneous use of a protectant fungicide along with a systemic fungicide at 7-14 day intervals. The shorter interval should be used when your plants have a lot of blackspot or when environmental conditions are favorable for its development, such as in the spring and fall in South Carolina when both mild temperatures and frequent wetting of the plants from rain or dew occur simultaneously. Some of the effective protectant and systemic fungicides that can be used against black spot are mentioned above. The protectant materials act on the surface of the plant by killing germinating spores of the fungus, thus preventing infection. Systemic materials may have some fungicidal action on the plant’s surface, but they are most effective because they are absorbed into the plant, where they arrest the growth of the blackspot fungus by interfering with its life processes. The use of both a protectant fungicide combined with a systemic serves two purposes. First, the protectant prevents infection and the systemic stops development of the disease once infection has taken place. Second, the protectant greatly reduces the chances that the fungus may develop resistance to the systemic fungicide. Though not an absolute necessity, frequently rotating the systemics that you use is an additional practice that prevents development of resistance by the fungus. It is not necessary to rotate the protectant you use in your combined fungicide applications. Always read and follow the label instructions when using any pesticide.
Using the techniques described above you can control, eliminate, and keep black spot out of your rose garden. Remember that you have the advantage when it comes to fighting black spot. You are a highly mobile, intelligent, decision making organism, and the fungus that incites blackspot has none of these characteristics. Therefore, if you exert the efforts described, both you and your roses will win out over blackspot.
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