Fungal Diseases and Control

 Blackspot on Rose Leaves


Black spot is the number one nemesis of rose growers. Black spot, (bs), is a fungal disease characterized by black spots on the upper sides of the leaves. Once infected the leaves then turn yellow and fall off. This weakens the plant and makes it susceptible to other diseases and opens it up for injury during the winter. Our warm humid climate provides perfect conditions for bs to thrive.

I along with the other members of the Savannah Rose Society have always been asked the question of how to combat and control black spot. The answer is simple and complex, let me explain. The first thing to understand is that like in most things, a good offense is easier than implementing a defense after the problem has already developed. This means that when you are thinking about starting a bs spraying program, you should think of it as a preventative one.

Why is bs such a big deal?To have beautiful roses you have to have a strong healthy plant. A plant that is being attacked by bs is greatly weakened and left unchecked will put the plant in a downward spiral and eventual death. Your plants bloom cycle is between five and six weeks on average. Think of this, this cycle of rose blooms are a direct result of the plant’s health it’s last cycle of growth. This is especially true at the end of the year. If they finish the year strong and healthy they’ll reward you the coming spring.

Before talking about a spray program, there are some very important cultural practices that will aid you in prevention.

1 – Roses need 6-8 hours of direct sunlight, the morning sun is best as it drys the dew off the leaves. BS needs water to live, reproduce and spread.

2- Your plants need good air circulation, again to aid in drying off the leaves. Common mistakes are to plant your plants too close together, to close to structures or other plants, or in enclosed areas.

3- Irrigate your plants in a fashion that doesn’t wet the leaves. Overhead irrigation is a no-no.

*** Rain and or irrigation water splashes and spreads fungal spores from leaf to leaf, plant to plant.

4- Plant “disease resistant” plants, the key word is resistant. Even Knock out roses will get bs if the conditions warrant.

5- Prune your bushes at the end of winter to open up the interior cutting out any inward growing canes. An example of this is if you hold your hand out with fingers extended. The center is open with your fingers, (rose canes), spreading out. You do this also for air circulation.

The best time to start spraying for the year is when the plants first break winter dormancy and start leafing out. Of course there is no obvious signs of bs on the new leaves, but again I stress, we are wanting to prevent. When spraying be sure to coat the top and bottom of the leaves. The best time of day to spray the roses is when the leaves are dry and the temperature is moderate. The mornings are cool but the problem there is the dew on the leaves. When spraying, when dew is present, your spray is instantly diluted and won’t adhere to the leaves thus leaving your spray ineffective. This narrows the best time in the summer months to the late afternoon when the sun is not bearing down. You don’t want to wait too late as you want your spray to dry on the leaves before the sun goes down.

Rain? If it rains before your spray dries, you’ll need to do it again tomorrow. If your spray dries and then it rains, your plants ARE protected.

You should spray your roses weekly. This is just a habit you’ll need to get into, again you’re wanting to spray to prevent. Once it invades your garden, it is much harder to get rid of!

Now, we’ll get into the meat of the spray program; the chemicals to use and the schedule to do so. Our program is simple and highly effective in the prevention of bs. The complex part comes in with the chemical products that are available on the shelves. Of the five products we use in rotation, you could probably only find one readily available. That’s where we had a problem; we couldn’t send someone down to the store with a list of what to buy that would help them keep their roses clean and healthy.

What if you already have bs? Once a leaf has been infected by bs it cannot be cured. The infected leaves should be picked off and disposed of. The same spray program can be used but would suggest spraying twice a week until new signs of outbreaks disappear.

Savannah Rose Society black spot spray program:

You have to treat your roses with two modes of chemicals; a single site and a multi site. A preventative and a spore killer. The multi site you can use every week while the single site you have to rotate weekly. The reason for the rotation is that the fungus will mutate and render your spray ineffective if you use the same one week after week.

We also use a spreader sticker water ph adjuster called Scarlet. This is added to your spray tank before adding any other spray material. Basically, what this product does is make your water wetter. Instead of your spray beading up and dripping off the leaves it will adhere like if you were spray painting the leaves. This gives your plant the best protection.

Multi site fungicides; Dithane or a manzate product. Again, use this weekly. I find the best results with using Dithane.

Single site fungicides; Honor Guard (propaconizol), Clearys 3336, & Compass are what the majority of our rose society members use in their rotation.

Mix concentrations;Scarlet- Approximately 1 tsp. Gal. (measurement will differ depending on the starting ph of your water. You want to stir as you add and get your water to a yellowish/ orange color, this signifies that your water will be in the 6 ph range.

Dithane- 1 Tablespoon per gallon. When the temps go up into the 90’s decrease to ½ Tbs.

Clearys 3336- 2/3 to 1 tsp. Per gal.

Compass- 1/8 tsp. Per gallon

Honor Guard- 2/3 tsp. Per gal. (Propaconizol is the active ingredient. This can be found in other products, usually more diluted, follow label mixing instructions.)

Ebenezer Rose and Garden keeps all of these products available and in stock.

So in review;

* Add the Scarlet to your water 1st, then add and stir in the remaining chemicals.

week 1- Scarlet, Dithane, Honor Guard

week 2- Scarlet, Dithane, Clearys 3336

week 3- Scarlet, Dithane, Compass

week 4- back to start of rotation

When do I stop spraying? Good question. The nursery is here in Rincon, Ga. and we get a good bit colder with a lot more frosts than down the road in Savannah. It differs even more near the intercoastal areas where they hardly ever get a frost or freeze. I stop spraying after we get that first frost. I think most in the Savannah area stop when it cools down and the roses go semi- dormant. (no new growth) Problem with both of these cases is that we get those warm winter spells and guess what crops up? Yep, good ol’ bs is right there to re-activate. You could and probably should fire the sprayer back up but if you’re like me you need a break and probably won’t. We’ll get back on the offensive in the spring!

This spray program used along with the cultural suggestions are a highly effective preventative bs program! We hope this information helps you to be able to enjoy your rose garden to the fullest!