Preventing and Curing Rose DiseasesBy Sandy LundbergIn our climate, there are two major diseases that affect our roses: blackspot and mildew. The two types of mildew, powdery mildew and downy mildew can occur in this area, but powdery mildew occurs more frequently than downy mildew. Other rose diseases that can occur include botrytis, anthracnose, canker, crown gall, and virus diseases.In this article I will deal with the two major diseases that can affect roses in the Lowcountry of South Carolina. Blackspot is caused by a fungus and can be a major problem because unchecked, it can defoliate an entire bush. Without foliage to nourish the plant, the bush will gradually decline and eventually die. The goal should be to prevent blackspot by beginning a regular spray program as soon as spring pruning is finished and to continue the spray regimen throughout the season. No variety is immune to the fungus, even the socalled "blackspot resistant" varieties. They take a longer time to become infected, but will become infected under favorable conditions. Blackspot can be prevented with a combination of the two types of fungicides. These two types of fungicides are multi-site surface protectant (contact) and single site systemic fungicides (systemic). The rationale for using the two simultaneously is that the contact fungicide is on the surface and will hopefully prevent a blackspot spore from penetrating into the tissue. If the spore penetrates, then the systemic becomes the second line of defense. Multi-site fungicides include Daconil, Dithane M45, Mancozeb, Manzate 200 DF, or Fore. The single site systemic fungicides include Compass, Banner Max, Cleary's 3336 or Ortho Rose Pride. Banner Maxx is now available in a less expensive version called Honor Guard. Immunox is also a systemic fungicide, but is used for mildew prevention. It is a much less concentrated version of Eagle or Rally 40X. Whatever you use, be sure that the plants have been thoroughly watered to prevent spray burn. Make sure that the spray coverage is thorough saturating the undersides and upper parts of the leaves. It is a good idea to rotate the systemic fungicides, choosing perhaps two from the list above and adding a multi-site fungicide to the mixture each time. For example, one week you could use Funginex to which you have added Manzate and the next time you might use Clearys' 3336 to which you would still add the Manzate. Follow the directions on the bottle and never use more than the recommended amount. Manzate should be added at the rate of 1 tablespoon per gallon unless the weather is hot. During 90-degree temperatures, you should cut back on the Manzate. For example, if mixing ten gallons, you would still use the same amount of the systemic fungicide, but cut back the Manzate to perhaps 6 tablespoons in order to prevent spray burn. Curing Pre-Existing BlackspotWhat should be done to cure blackspot once an outbreak is present? Don't spray more than the recommended amounts, instead spray more frequently. As an example, spray every four or five days for a total of three or four times. Then go back to your weekly, or bi-weekly program. Powdery Mildew &BotrytisAn infestation of powdery mildew will cause crinkling and twisting of the leaves and a white powdery mildew will be seen. Mildew must be prevented because it cannot be cured. It is necessary to begin applications of Immunox immediately after the spring pruning. Weekly spraying with Immunox should continue as long as there is a 20-degree or more differential between day and night time temperatures. This means that in our area, we should use Immunox through most of the spring and then begin using it again in late August or early September. Botrytis is a fungus that attacks the flowers and canes of rose plants. It generally occurs during wet, cool weather. It may appear as red "freckles" on the bloom or in its more severe form the buds will fail to open and the bud will have a grayish-brown mold on them. Cut and destroy infected buds. Keeping the garden clean is most important for preventing botrytis. Preventative spraying with Daconil should help control botrytis.
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