For The Union-Tribune
Roses do not have thorns; they have prickles. But there ARE “thorns” in the rose garden. Their names are botrytis, black spot, rust, downy mildew, anthracnose and cerospora leaf spot. They are all water-initiated fungal diseases, and you might be seeing them in your garden right now.
How do these fungal diseases develop in our garden? Think of it like a recipe for making bread, which requires yeast and flour. But the recipe also needs specific conditions for the yeast to activate. In the rose garden, the fungal spore is the yeast, and the susceptible rose is the flour. Add a thick marine layer, dense fog, heavy dew, overcast cloudy skies, cool temperatures, mist, rain and drizzle and you have the ideal triggers to activitate the spores of botrytis, downy mildew and anthracnose. Throw in a few warm days with wet or humid weather and this activates black spot, rust and cerospora spores.
Sunny skies and air circulation can help dissipate disease by reducing humidity. But true to its reputation, the month of May gave us a lot of gray and June will likely give us some gloom. As much as we love rain, it is much more perfect for roses when it arrives in January and February before our roses are leafed out and in bloom.
We can’t change the weather. Can we change any ingredients in the garden to minimize the ouch of these thorny diseases? Fortunately, the answer is “Yes!”
Fungal diseases on blooms
Botrytis blight is caused by Botrytis cinerea and can be a problem on susceptible roses in early spring and fall. It thrives in high humidity and cool, wet weather. Botrytis can infect blooms and canes but not leaves. The petals on blooms develop unattractive soft, gray or brown edges. On some roses the entire bloom can become a brown or gray, soggy mess. On multipetalled roses, the buds may ball up and fail to open.
Damaged blooms must be deadheaded and cannot be salvaged, as the fungal disease consumes all the petal material and can transform into an ugly grey ball. Master rosarian Rich Baer has a very descriptive name for this. He calls it a “botrytis bomb” — a very accurate description. There are millions of spores within the ball that, when released, will blow about in the air and spread disease to other uninfected buds and blooms.
When the fungus affects canes, you will see die-back and large diffuse splotches form on canes. Botrytis can live, nourish itself and multiply on live and even dead plant material. It can overwinter even in cold climates, and spores can remain dormant until the weather conditions are right for it to come back to life.
Ghost spotting is the term used to describe the small pink spots on the outer petals of blooms on susceptible roses during prolonged periods of rain and consecutive days of dense fog and high humidity. They are caused by several fungi including botrytis. These blooms should also be deadheaded and removed from the garden.
Water-initiated leaf fungal diseases
Downy mildew is caused by the fungus Peronospora sparsa and can be found in nearly every state across the U.S. The fungus is active usually in spring and in cool, damp conditions.
Downy mildew’s red, maroon, purple or black lesions appear angular in shape as they are limited within the leaf veins. A white, downy fungal growth may be apparent on leaf undersides. Downy lesions are usually more purple in color than black spot, but unlike black spot, downy mildew usually starts at the top of the bush. The spores can remain dormant on plants and serve as reservoirs of the pathogen for following years.
Downy mildew fungi germinate and infect leaves with six hours of constant leaf wetness. When the sun comes up, leaves start to dry, and the spores are released. A severe outbreak progresses very quickly, with air moving spores from plant to plant.
Downy mildew also affects rose canes and stems with purplish, brown or black spots. Within three days of infection, damage can be observed on the plants. Plants can defoliate all their leaves before any sign of the disease is obvious.
Spores can live on fallen leaves for up to one month. The good news is that this disease becomes inactive when humidity drops below 85 percent, and it is killed when temperatures stay at more than 80 degrees for several days.
Anthracnose is a fungal disease caused by Sphaceloma rosarum. Anthracnose has lesions with discrete edges. The centers of the lesions are gray and have “pepper specks” that are the fruiting bodies of the pathogen. Spores are spread by splashing rain. The disease can move out of the leaf into the petiole and into the cane. This disease is seen most often in cool humid weather in early and late spring.
‘Neil Diamond,’ ‘Ambridge Rose’ and ‘Graham Thomas’ are varieties in my garden that are susceptible to anthracnose in the spring, but this pathogen does not appear to affect these roses’ vigor. By summertime, the disease is not noticeable on their leaves.
Rust is caused by the fungus Phragmidium mucronatum and P. tuberculatum and is easily identifiable with yellow-orange leaf lesions on the top of leaves and orange pustules on leaf bottoms. Later in the growing season, rust pustules on the underside of the leaf can be orange and black.
Rust grows only on live tissue but makes overwintering structures that survive in leaf debris. In the spring, spores blow or splash up onto newly emerging rose foliage and can germinate with conditions of as little as two to four hours of moisture.
Black spot caused by the fungal pathogen Diplocarpon rosae is a water-induced fungus. Black spot lesions have diffused feathery borders, and this foliar disease usually starts at the bottom of the bush. It is prevalent in warm, wet weather or when there is high humidity or conditions such as overcrowded plants that prevent foliage from drying.
Part of the life cycle of this fungus is entirely within the leaf. A week to 10 days later, each of the small black fruiting domes releases thousands of spores. The spores require at least six hours of water to germinate and infect plants.
Black spot spores can overwinter on rose canes and leaves. The fungi can even survive and use fallen dead leaves as a reservoir to produce spores for new infections. The previous year’s spores splash up and infect leaves at the base of the plant.
Cercospora leaf spot is a fungal disease caused by Cercospora rosicola and resembles anthracnose, but unlike anthracnose it is most common when the weather is hot and humid. The purple-red lesions have gray centers as they age but they do not have anthracnose’s “pepper specks” in the center of the lesions.
Management
• Our best and easiest control is to plant disease-resistant cultivars.
• Prevention controls include considering ways to minimize the length of time there is moisture on our plants’ leaves and blooms. Drip irrigation is best to avoid wetting our plants. Blasting plants with water to wash off pests must be calculated to occur early in the day and when there is enough sun to ensure that our plants will dry promptly.
• Reducing humidity and increasing air circulation around plants helps plants dry. We can achieve this by properly spacing our plants, pruning them annually, and cutting back, staking or tying back companion plantings when necessary.
• Spotting disease as soon as it starts and cutting out and removing infected blooms, leaves and stems.
• Sanitation is of utmost importance to reduce the spread of disease to other susceptible roses, and the spread of disease in the following year.
• Botrytis-infected blooms and ghost-spotted blooms must be deadheaded, and all diseased, fallen petals must be picked up and removed from the garden. It is preferable, simpler and saves time to deadhead blooms before the petals drop to the ground.
• Leaves that are infected with rust, black spot or downy mildew must be cut out and removed from the garden.
• Picking up and removing fallen leaves is a must, and so too is a thick layer of mulch laid down after our winter pruning and clean up.
• Do not compost leaves that have fungal disease.
• If you decide to spray with a protectant fungicide, be sure to spray every 10 days, and be aware that fungicide does wash off with rain.
Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension. For more information, visit rose-harmony.com.
