Powdery Mildew & Blackspot

A Quick Review

Monica Valentovic and Gary Rankin  

About this time each year we begin to think about preventing fungal infections on the roses. It is important that you already have started your spray program. If you have not yet begun to spray, then start today.  The main reason that we use fungicides is that fungal infections can be quite disfiguring to rose bushes and lead to serious loss of vigor and decreased flower production. For the home gardener, fungal infections on roses can occur because of a lack of preventative care (either too infrequent application of fungicides or no applications), the use of outdated products, or the emergence of resistant strains of fungus.

 

The main fungal infections that rosarians encounter in our area are powdery mildew and blackspot. Another fungal infection, anthracnose, is seen less often by rosarians in our area, but can appear and is controlled by the agents that control blackspot. However, conditions that favor fog formation also favor the appearance of anthracnose. May is usually the time of year when powdery mildew first appears in our area of West Virginia with blackspot appearing later in the spring or early summer.

 

Powdery Mildew

Powdery mildew is a fungal infection that appears mainly on new growth. It appears as a fuzzy white powder covering tender new shoots and leaves. Once you can see the presence of powdery mildew, it has already invaded the plant's tissues. If left untreated, powdery mildew can kill the cells of the plant leaves and cause the leaves to curl or ripple.

 

Powdery mildew begins to show up in the garden when days are warm (50-80 degrees F) and nights are cool with elevated humidity. Cool foggy or dew laden nights with warm days can be ideal for powdery mildew to take off. The fungal spores are everywhere and easily attach themselves to new moist rose leaves. However, the spores grow when the leaf surface dries out during the day.

 

The best control for powdery mildew is prevention. Make sure plants are spaced far enough apart to allow for adequate airflow to help prevent mildew formation. Spores can also be washed from the plants before they imbed in tissue by using a strong stream of water early in the day. Fungicides (e.g. Immunox, Banner Maxx) used regularly (every 7 to 14 days) can also be used to prevent mildew infestation and help prevent the spread of powdery mildew once an outbreak has occurred. Other treatments include baking soda (1-3 teaspoons/gallon) alone or in combination with insecticidal soap or Sunspray ultrafine oil (2 tablespoons/gallon). Baking soda makes the leaf surface more alkaline and discourages spore growth. A study by one of the extension agencies in the United States noted better success if an oil or spreader sticker was included with the baking soda solution, presumably due to better dispersal along the leaf.  These treatments produced some success when sprayed weekly and can offer an alternative to the use of traditional fungicidal chemicals.

 

Blackspot

Like powdery mildew, blackspot is a common fungal infection in roses. The blackspot spores are mainly found in the ground and get onto the lower leaves when the rose bush is watered or when rainwater splashes the spores onto the plant.  Thus, unlike powdery mildew that starts at or near the top of the rose bush, blackspot usually begins on the lower portions of the bush.

Blackspot infections look just like the name suggests - black fuzzy-edged spots on green leaves that eventually turn yellow (due to ethylene production by the fungus) and fall off. If left unchecked, roses may lose most of their leaves and be more susceptible to winter damage and killing. Conditions that promote blackspot disease are warm temperature (70-80 degrees F) and moist conditions (e.g. rain followed by high humidity). Blackspot spores need at least seven to nine hours of warm, moist conditions for the spores to germinate.

 

Prevention is again the best way to keep blackspot infections away from roses. Make sure that your bushes are planted far enough apart and away from structures to allow for adequate airflow. Don't splash water on the leaves when watering. Watering during the day so that leaves can dry before evening is also better than watering early in the evening. Remove all blackspot leaves from the ground each fall and start a preventative spray program early in the spring. If blackspot appears, remove the blackspot leaves from the rose bush and from the ground. Spraying with fungicides such as Daconil, Manzate, Banner Maxx or Compass is necessary for controlling an outbreak of blackspot. Rotation of two or more fungicides also helps to keep resistant strains of blackspot from getting established in the garden.