Fungicides for Newbies!
Now that you have all these new roses and you want them to look great for the season, or you’ve had roses for a while and had issues with fungal diseases and want to start a spray program, what should you do? In our area, Northeast Georgia, the pressure for fungal diseases is very high. High humidity and the possibility of rain at any time create a perfect breeding ground for blackspot and other fungal diseases.
It can be daunting for the new rose grower to understand why roses have disease and what can be done about it. How you deal with disease is totally up to you. No one needs to use fungicides or any pesticide if they don’t want to. However, if left unabated in susceptible roses, blackspot and other fungal diseases can cause failure to thrive. Blackspot will not kill a rose outright, but if the rose is constantly defoliated, it will not be able to supply energy from the leaves to the rest of the plant and will be more susceptible to other diseases or weakened to cold damage. A few facts:
Fungal disease are always around. They live in the air. They will bloom when the conditions are right. A rule of thumb is that the spores will activate when they are wet for a number of hours. If you have rain, especially at night, and the leaves don’t have a chance to dry off, you will have blackspot on susceptible roses.
Since fungal spores can over winter in rose canes and leaves, it is important to remove all old leaves from the rose when doing your spring pruning. Older leaves will be more susceptible to disease and will spread it to the rest of bush. Also, any leaves that fell to the ground should be removed and fresh mulch spread on top to stop the fungal spores from splashing up onto the bush with watering or rain. Air circulation is important, so spacing of roses apart and pruning to allow air circulation is key. Planting roses in full sun, while important for growth and flowering, will help the roses dry off before blackspot can take hold.
Some roses are more susceptible to fungal diseases. Hybrid Teas, the fancy roses with the big blooms are usually more susceptible. They are bred for bloom form, color, size and not disease resistance. David Austin’s, very popular English type roses with lots of petals, dramatic colors and fragrance are also not very disease resistant, at least not in our climate. The Rose Industry has created some resistant varieties, Knockouts, now available in different colors, are extremely resistant to black spot (notice I did not say immune!). Newer rose varieties are coming out every year touting disease resistance. However, some of the testing is done in places like California and Arizona, which certainly don’t have the same conditions that we have. So, your mileage may vary.
If you want some of the less resistant roses, you will likely need a spray program. It can be tailored to your yard, your roses and your tolerance to disease.
I Don’t Want Yucky Chemicals in my Yard! (or the organic approach)
There are a many new options for the organic gardener for fighting fungal diseases. First, the rose selection, spacing and pruning/bed cleaning process mentioned above are more important in the organic garden. These practices will cut down on the diseases you will need to deal with.
The Organic Spray Program:
Dormant Spray (also part of the chemical spray program): as we noted above, fungal spores can over winter on roses and in rose beds. When roses are dormant (anytime after a hard frost to beginning to bud out in the late winter/early spring) you can apply dormant sprays. Lime Sulphur and Horticulture Oil are two of the types of dormant sprays you can use. These are sprayed on the roses and the rose beds.
Lime-sulfur: is a form of sulfur mixed with lime (calcium hydroxide), and is mostly used as a dormant spray, meaning it should not be applied to plant foliage. Lime-sulfur is more effective than elemental sulfur at lower concentrations; however, its strong, rotten-egg odor usually discourages its use over extensive plantings.
Horticultural oil: can be beneficial in smothering insects that also harbor in the soil over winter. Oils effectively manage powdery mildew on many plants, but are significantly less effective against other leaf spot diseases.
During the growing season:
Neem oil: is pressed from the fruit and seeds of the neem tree (Azadirachta indica). At a 70 percent concentration, neem oil kills powdery mildew spores, virus vectors (such as aphids and white fly), and the eggs of numerous insect pests. It is less effective against rose black spot (caused by Diplocarpon roseae) and other fungal diseases.
Copper: Liquid Copper Fungicide helps control powdery mildew, downy mildew, black spot, peach leaf curl, rust, and many other listed diseases. Copper is a naturally occurring chemical which makes it a great option for use on all of your plants. It is approved for Organic Gardens.
Sulphur: Sulfur is the oldest recorded fungicide and has been used for more than 2,000 years. Early in agricultural history, the Greeks recognized its efficacy against rust diseases on wheat.
Although few homeowners grow their own wheat, sulfur can be a preventive fungicide against powdery mildew, rose black spot, rusts, and other diseases. Sulfur prevents fungal spores from germinating, so it must be applied before the disease develops for effective results. Sulfur can be purchased as a dust, wettable powder, or liquid. Do not use sulfur if you have applied an oil spray within the last month — the combination is phytotoxic (plant-killing). Likewise, do not use sulfur when temperatures are expected to exceed 80°F to reduce the risk of plant damage.
Bicarbonates: Baking soda (sodium bicarbonate) has been used as a fungicide since 1933. Recent research has demonstrated that although baking soda can be effective against plant diseases when used with oil, its sodium component can build up and become toxic to plants.
I don’t care, I want pretty roses and no disease (or the Chemical approach)
There are many different types of fungicides available if you want to go the chemical route. You don’t need to be a chemist to understand fungicides, but it’s important to know a few things:
Mode of Action is very important. Mode of Action is how the chemical works on the fungus. Many commercially available fungicides are actually the same chemical or have the same MOA , so using just one or the same MOA may lead to resistance by the fungal disease or not getting complete coverage.
Systemic or Contact: Most rose growers who use chemicals use a combination of a systemic and a contact fungicide. A systemic is what it says, it will enter the plant system and protect from the inside. A contact fungicide will kill spores on the surface for a period of time. Both will need to be re-applied every 7-14 days to be totally effective.
You will have to base your spray program on the number and type of roses you have. For the small garden under a dozen roses, you can probably get by with a RTU (ready to use ) spray that is already mixed, you just buy in the store and spray on your roses bushes according to the directions. For larger gardens, it’s recommended to use concentrates that you then mix with water in a pump sprayer. You can also combine two different fungicides in the mix, one systemic and one contact to get a double whammy on the blackspot (check product labelling on mixing with other chemicals).
For Roses, we have a few MOA groups that we use. Most use MOA 3, the chemicals known as either Propiconazole or Tebuconazole. These are systemic fungicides that are available in big box stores like Home Depot and online from Rosemania and other suppliers. I’ve listed some of the brand names in the table.