I am seeing a fine white powdery substance on the upper leaf surfaces of the new leaves, buds and sepals on some of my roses. What is it? What should I do?

You are describing powdery mildew. It is often most active when cool nights are followed by warm, humid days and limited rainfall. This fungal disease does not require free water on the plant surfaces to develop but it does need high humidity for spore germination. Spores are disseminated in air currents. The fungus overwinters in infected rose canes and in fruiting bodies found in leaf litter.

Management Plant resistant cultivars. If you catch it early you can wash it off the foliage and you can also cut out and remove diseased foliage and buds from garden. There are fungicides that can be applied preventatively.

I am seeing black spots with diffuse, feathery margins on the upper surface of some of my roses’ leaves. What is this and what should I do?

Blackspot is a water-induced fungus. It is prevalent in wet weather or when there is high humidity or conditions like over-crowded plants that prevent foliage from drying. The spores require at least six hours of water to germinate and infect plants. Blacspot can survive on fallen dead leaves so the source of an infection during this year’s rainy season is very likely the previous year’s spores splashing up and infecting leaves.

Management Plant resistant cultivars. Lessen humidity around plants by proper pruning, spacing and reducing wet foliage by using drip irrigation. Remove infected leaves on the bush and fallen leaves around the plant to reduce spreading disease to other susceptible roses. Fungicides, if used, are most effective if sprayed preventively.

A few of my roses had round spots with defined red, maroon or purple borders on the leaves in the spring but they went away in the summer.

Anthracnose is a less common fungal disease. The spores are spread by splashing rain. This disease is seen most often in cool weather in late spring to early summer. Neil Diamond is highly susceptible to anthracnose in the spring months but continues to grow and bloom vigorously even during the outbreak.

Management Since this is a water-induced disease, space plants and use drip irrigation. This helps foliage to stay dry which slows the spread of the disease.

What are the orange pustules on the undersides of my roses’ leaves?

Rust is most severe in cool, moist weather and in rainy years. The 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 Plant resistant cultivars. Some cultivars tolerate rust, but intolerant cultivars can defoliate completely. 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. If you decide to spray, use a protectant fungicide.

What are the bright yellow to white line patterns, ring spots and/or mosaics on some of my rose leaves?

This is rose virus. Viruses cannot be spread to other roses. Sometimes the vigor of the rose is affected, and a rose may produce fewer blooms and shorter stems. Most viruses are disseminated not by pruning or by vectors but by the propagation of infected plants. The remedy is to inspect plants before purchasing and not buy symptomatic plants.

Why do I have gray, brown or tan-color mold or rotting edges on some rose petals? And why do some of my rose buds ball up and fail to open?

Botrytis blight is a fungal disease and is worst in cool, wet weather. It is most problematic in coastal areas in early spring and fall. 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 grey mold which can release millions of spores and spread the disease to other as yet uninfected buds and blooms.

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. Also, check and modify irrigation and curtail overhead watering. Space plants and reduce companion plantings to increase air circulation and lessen humidity. Botrytis can live, nourish itself and multiply on live or dead plant material.

Why do I have small pink spots on the petals?

This is known as ghost spotting and is caused by several fungi including botrytis. The small pink spotting can be most severe during prolonged periods of rain and consecutive days of gloom 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. 

I am seeing black, yellow, gray, brown or purplish-red spots or blotches on some of my rose canes. On some canes the entire cane is discolored. What should I do?

 Stem and Cane Cankers can be caused by several fungi and diagnosis of the fungi or bacteria responsible is difficult. Cankers are more likely to occur on plants that have been weakened by pest, disease, sunburn, poor nutrition, or wounds, including pruning wounds, that open the inner cambium to disease.

Management Fungal spores can start on blooms and leaves and move on to invade the canes, making it very important to remove any fungal-infected flowers and leaves from the garden. Cut out cankers as soon as they are detected, pruning at least two- to three-inches below the infected portion. Avoid wounding the cane which allows penetration and infection by disease. Fungicides are generally not effective for management of cane disease.