In San Diego, it is taboo to grumble about getting too much rain. You certainly won’t hear gardeners complaining. We love the gift of free irrigation, and our roses love the water. But this year we had more than our usual share of rain without our usual fair share of sun. When it wasn’t pouring, drizzling and misting, it was cloudy, foggy, windy and cold. April was so lacking in sun and was so gray that it was nicknamed “Gray-pril” by the National Weather Service.
This year’s atypical weather patterns delayed our roses’ first blooms by several weeks. Here are some questions you may have in your rose garden this May. Selective Herbicide For Wild Onions
Why on earth are there leaf-like structures emerging from the center of my rose bloom?
Phyllody, or vegetative center, is an interesting bloom deformity we sometimes see in our rose garden even when the plant is otherwise healthy. The cause for the strange blooms is plant hormones that are out of balance. Reasons for the imbalance are due to environmental conditions such as fluctuating weather. There is no reason to be concerned, and the bush will resume producing normal flowers when the weather stabilizes.
Why am I seeing so many stems that terminate without a bloom?
Blind shoots are short stems with no flower at the end. The reason that blind shoots develop is not fully understood. It is believed that they are caused by extreme fluctuations in weather or temperatures that slow and stop the tip growth. They are also thought to occur because the rose is putting out more stems than it can support with corresponding blooms.
There is nothing we can do to prevent blind shoots. But don’t just ignore them. When they are growing in the center of the bush, remove the stem altogether to open the center of the bush to light and air. If the blind shoot is vigorous and growing toward the outside of the bush, you can prune back the blind tip, which will allow the rose to try to set a bud again.
Why am I seeing so many more aphids on my new rose growth this year?
Aphids are the first pest in the spring. This year’s rains prompted a lot of lush, green, succulent growth, and the aphids have responded by reproducing rapidly in even larger numbers on our roses’ new growth and buds. They distort the new spring growth by piercing and sucking on the tender new leaves, stems and buds. The colder weather may have delayed their natural enemies.
Management: We can manage aphids without pesticides by checking for them daily. Generally, all that is required is squishing them with our fingers or knocking them off with a strong spray of water. Most of the aphids will not be able to return to the plant. It is best to do this early in the day to allow plants to dry more rapidly and avoid conditions for fungal diseases. Soon, the natural enemies — which include the lacewings, lady beetles, adult wasps, insect larvae, earwigs and birds — will arrive to assist us in aphid management.
Why am I seeing black spots with diffuse, feathery margins on the upper surface of leaves?
Black spot: This is caused by the fungal pathogen Diplocarpon rosae. It is prevalent in wet weather. 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, which require at least six hours of water to germinate and infect plants.
Management: As with the other water-induced fungi, cultural best practices for black spot include planting resistant cultivars and lessening humidity around plants by proper pruning to ensure good air circulation. Remove infected leaves on the bush as soon as they appear. Dispose of fallen leaves around the plant to reduce spreading disease to other susceptible roses and to avoid overwintering. The source of an infection during this year’s rainy season is very likely the previous year’s spores splashing up and infecting leaves at the base of the plant. Since this pathogen is known to make genetic changes, it is important if you decide to spray, to rotate fungicides with different modes of action.
Why do I have round spots with defined red, maroon or purple borders on my rose’s leaves?
Anthracnose: This is a fungal disease caused by Sphaceloma rosarum. 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 weather in late spring to early summer. The only rose in my garden that gets anthracnose in the spring is Neil Diamond. The leaves are unsightly, but the bush tolerates the pathogen and continues to grow and bloom vigorously even during the outbreak. It doesn’t pass on the fungal disease to neighboring roses and grows out of the problem in the drier, less moist summer months. For all these reasons, I do not remove the anthracnose-diseased leaves from the bush.
Management: Since this is a water-induced disease, spacing plants helps the foliage to dry.
Why do I have orange pustules on the undersides of leaves?
Rust: This is caused by the fungus Phragmidium mucronatum and P. tuberculatum. It is most severe in cool, moist weather, which makes it most common in coastal areas of San Diego and in rainy years inland. 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.
Management: Some cultivars tolerate rust, but intolerant cultivars can defoliate completely, so our best management tool is to plant resistant cultivars. Try to spot rust as soon as it starts. Cut out leaves with rust pustules rather than pulling them off as the latter action disperses the spores onto other leaves, into the air and onto the soil. Removing fallen leaves is a must. Do not compost leaves that have fungal disease.
Why do I have gray, brown or tan-color mold or rotting edges on some of my petals?
Botrytis blight: This fungal disease is caused by Botrytis cinerea and is worst in cool, wet weather. You will notice gummy-soft, gray or brown edges on the petals of older blooms. Deadhead and remove infected blooms as they cannot be salvaged. On multi-petalled roses, the buds can become soggy and ball up. The next stage is a fuzzy, gray mold that can release millions of spores and spread the disease.
Management: The remedy for botrytis is sanitation. Because the spores reproduce quickly and can infect other uninfected blooms, the prompt removal of affected blooms and fallen petals is imperative. Space plants and reduce companion plantings to increase air circulation and lessen humidity.
Why do I have small pink spots on some of my roses’ petals?
Ghost spotting: This is caused by several fungi, including Botrytis, Bipolar, Cercospora and Cladosporium. The small pink spotting can be most severe during prolonged periods of rain and consecutive days of dense fog and high humidity.
Management: Affected blooms should be promptly deadheaded. Protective fungicides are of little use as they wash off with rain, and systemic fungicides are also ineffective as little fungicide accumulates in petals.
The two essential ingredients for beautiful, big blooms are water and sun. Fewer sunny days delayed our blooms a little this year and prompted a few ‘hiccups’ in the rose garden, but look around at your spectacular spring blooms and thank the rain!
Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension.
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