Rose: Identify and Manage Problems

A location with a minimum of six hours of direct sunlight daily is ideal for most types of roses. Morning sun is preferable, as it helps to dry out foliage quickly. Minimizing leaf wetness helps in the management of diseases. 
Roses need rich, loamy well-drained soil. Before planting amend the planting site with organic matter. Roses do not grow well in wet, poorly drained soils.

Space to allow for good air movement around plants to reduce localized high humidity. This helps in the prevention of diseases. Avoid crowding plants closely together to reduce competition for water, nutrients, and sunlight. Plant roses away from fruit trees to minimize shared disease and pest problems.

Seek and select disease-resistant varieties of roses (very important!). There are hundreds of hybrid roses and cultivars available and they vary in their susceptibility to diseases. Weather conditions play a large role in disease severity.

Disease and Insect Resistant Ornamental Plants: Roses | Cornell University

All roses require some pruning each year to manage diseases and maintain vigor. Refer to our guide to pruning roses. 

If there is less than 1” of rainfall per week, water roses to maintain even soil moisture. Water early in the day and aim the water at the base of the plant rather than overhead to minimize leaf wetness as much as possible. 

Fertilize roses if a soil test indicates a nutrient deficiency and adjust soil pH if recommended by the soil testing lab. Soil pH affects the availability of nutrients to plants. Roses grow best in slightly acidic soil with a pH between 6.0 - 6.5. Fertilizer should not be applied to roses after early August.

Diseased foliage, flowers, and stems should be removed from the planting site and composted. Remove old mulch and replace it with a fresh layer of mulch in the spring to minimize problems with overwintering pests and pathogens.

Environmental (weather) stresses, shady conditions, excessive lush growth from too much fertilizer, improper pruning (either too much, too little, or at the wrong time), winter-kill, and pesticides are some possible reasons for poor flowering. Some roses bloom early in the season and only bloom once a year. Knock Out roses bloom early, may take a break in the heat of the summer, and rebloom into the fall. 

Few or no flowers

Leaves: pale yellow-green color, stunted growth

nitrogen deficiency

Causes of leaf browning (leaf scorch) 

heat and drought stress

excess fertilizer

Causes of leaves turning green between veins with yellow veins.  

overwatering

lack of oxygen due to poor drainage or too much water. 

Cause of unusually small leaves, leaf twisting/curling/cupping

herbicide injury

Proper care after planting will help roses get off to a healthy start from the beginning. 

Problems due to poor care after planting

Chlorosis symptoms, caused by a deficiency of iron, appear as yellow leaves with prominent green veins. These symptoms are usually caused by high soil pH. 

Iron

• Black spot symptoms appear as dark lesions that have feathery margins.  
• Early spring infections often begin on the older lower leaves and progress to the newer growth near the top of the plant. 
• Infected leaves often turn yellow and drop from the plant, even with only a few lesions present.
• This disease causes severe loss of foliage on susceptible rose cultivars which over several seasons can lead to dead plants.
• Purple-to-black lesions may form on the canes in the fall which should be pruned out before new growth begins in the spring. 

This disease used to be very common, however, most landscape roses grown in Maryland are the highly black spot-resistant Knock Out® rose cultivars. They are, however,  very susceptible to another fungal disease called Cercospora leaf spot.

There have been reports of black spot-resistant roses getting black spot, possibly from the mutation of this fungal pathogen to form new pathogenic strains. Resistance is dependent on the strains of black spots in your area. Learn more about black spot disease and how to manage it.

• Symptoms of Cercospora leaf spot, caused by the fungus Cercospora rosicola, first appear on lower leaves as small, maroon to dark purple lesions with smooth margins.
• These symptoms appear during warm weather, usually by mid-summer, and will progress from the base of the plant toward the growing tips.
• Older lesions tend to have a dead tan center with a purple border.
• Roses that are resistant to black spot are often susceptible to Cercospora leaf spot disease and may defoliate during warm rainy summers.
• This disease is becoming much more prevalent in Maryland on roses that are resistant to black spot and may be as severe as black spot in some cases.
• Management is the same as for black spot disease. 

• White, powdery fungal growth on the upper surfaces of leaves and sometimes flower buds is typical of powdery mildew.
• Leaf distortion, yellowing, or reddening can occur when leaves are infected with this disease, which is caused by the fungal pathogen Podosphaera (Sphaerotheca) pannosa.
• Powdery mildew advances in warm, humid weather with limited rainfall.
• The fungus survives the winter on infected cases and leaves. Learn about powdery mildew and how to manage it. 

• Spots or blotches on rose stems are a symptom of stem canker disease.
• The spots may be purplish, reddish, or pale yellow and gradually enlarge and change color to brown or grayish-white.
• Infected areas may fully encircle stems and lead to the dieback of canes.
• Several types of fungal pathogens cause stem canker on roses.
• All are managed in the same way: prune out infected canes.
• Learn more about canker diseases and how to manage them.

• Rose rosette disease is a virus transmitted by the feeding of microscopic rose leaf curl mites (Phyllocoptes fructiplilus).
• Symptoms include abnormally thin leaves, a proliferation or clustering of stems (“witch’s broom”), excessive thorniness, reddening of stems, distorted flower buds, and stunted growth.
• Flowering is reduced and flowers may be aborted from the plant.
• Normal new growth on roses may be reddish in color and mistaken for rose rosette disease.

• Symptoms of rose mosaic may appear as irregular yellow or white lines, bands, ring spots, or blotches on the foliage.
• Rose mosaic disease may be caused by different types of viruses including Apple Mosaic Virus (AMV) and Prunus Necrotic Ringspot Virus (PNRV). Infections may reduce flowering and can make plants more susceptible to winter injury.
• Virus diseases are systemic, meaning they are present throughout the entire plant.
• Removal of symptomatic leaves or stems will not rid the plant of the virus.
• There is no cure for viruses in roses and those with virus systems should be removed.
• Learn more about viruses of shrubs and how to manage them.

• Spotted, withering flower petals and rotting buds are symptoms of gray mold on roses.
• Infected flowers and buds may turn completely brown.
• Gray, fuzzy mold may be visible on infected flowers, leaves, and/or stems.
• This disease is caused by the fungal pathogen Botrytis cinerea.
• The disease thrives in cool, wet, humid conditions.
• Plants can recover when the weather becomes warm and dry.
• Learn more about gray mold and how to manage it.

• Leaf etching (scraped surfaces) or small holes in rose leaves are symptoms of feeding damage from sawflies. Heavy infestations can cause leaves to turn brown and curl.
• The roseslug sawfly is one of three common sawflies that attack roses (others are curled and bristly roseslugs).
• These insects are related to wasps and have four life stages: egg, larva, pupa, and adult.
• The larvae look like caterpillars, but they are not. They are about 1/16 - 1/2 inch long and are greenish-yellow in color.
• Check roses in May and June (in Maryland) for worm-like larvae. They are usually found on the undersides of the leaves and a hand lens may be needed to see them. Roseslugs can be active through the fall.

• These are common pests of landscape roses, including ‘Knock-out’ roses.
• If the infestation is light, pick off the larvae by hand and destroy them. They can be dropped into a container of soapy water to kill them. To control heavy infestations, use horticultural oil or spray with spinosad. Target the undersides of the leaves.
• Control is the same for all three species.

Aphids  

• Aphids are small insects, winged, or non-winged, about  1/16 inch to 1/4 inch in length, and appear in various colors including green, yellow, orange, red, black, or white.
• They have piercing-sucking mouthparts used to feed on plant sap.
• During feeding, they excrete digested excess sap, a sticky sugary waste product called honeydew.
• The honeydew serves as a substrate for the growth of several fungi that appear as a dark coating called sooty mold.
• Aphids prefer to feed on tender new plant growth and are typically found in groups on newly emerging buds and leaves beginning in the spring.
• High rates of nitrogen fertilizer can increase problems with aphids.
• Their feeding on roses may cause the weakening of buds, leaf distortion, and reduction in the aesthetic value of the flowers.
• Aphids have many natural enemies (predators) like ladybugs and lacewings in the environment that can help to control their populations naturally.
• It is important to choose control measures that minimize impacts on natural enemies, or else secondary (recurring) pest outbreaks can be problematic. Learn more about aphids and how to manage them.

Japanese beetles

• Japanese beetles (Popillia japonica) may cause severe chewing damage on rose flowers and foliage.
• On the leaves of roses, they produce a symptom called skeletonization -- irregular chewing between the leaf veins.
• Adult Japanese beetles are approximately ½ inch long, with brown wing covers and a metallic green body.
• Beetles may be present and actively feeding from June to early August.
• Handpick beetles, drop them into a container of soapy water, and discard them in the trash. Japanese beetle larvae (grubs) overwinter in the soil.

Spider mites

• The presence of very fine silk webbing on leaves and stems is a sign of spider mites.
• Very small yellow or white dots (stippling) is a symptom of spider mite feeding.
• Most adult spider mites are about ½ millimeter long and they are typically found on the undersides of leaves.
• A hand lens may be needed to see them. They use piercing mouthparts to feed on cellular contents including chlorophyll.
• Spider mites tend to be a problem under hot, droughty, dusty conditions, and also where nitrogen fertilizer and pesticides are overused (the latter resulting in the decline of natural predators of mites).

Thrips

• Irregular white or brown streaks on flowers and/or irregular streaking on the upper surfaces of leaves are symptoms of damage from thrips.
• These insects are very small, about 1 to 2 millimeters in length.
• There are several species that are pests of roses and other plants.
• Some species overwinter in the soil while some species migrate annually from the south.
• They may be white, yellow, brown, or black in color.
• Thrips generally do not require control in the home garden.

Rose midge

• Flower buds that do not form correctly, turn brown or black and wither, or fall off before they are fully developed may indicate the presence of rose midge (Dasineura rhodophaga).
• This is an uncommon pest but may be present in Maryland rose plantings.
• The rose midge is a very tiny fly. The females lay eggs in developing buds. When the eggs hatch, tiny white larvae (maggots) feed on plant tissue, resulting in damage to the buds. There are several generations per year.
• This insect overwinters in cocoons in the soil.
• Remove mulch under and around rose shrubs in late winter to remove pupating rose midges.
• Put fresh mulch on in early spring to bury any remaining midges even deeper.
• Monitor roses weekly in spring, summer, and fall, and prune off abnormal buds.
• Do not wait until the shoots look black (larvae will be gone by then)