Diseases of Roses John Mitchie Moe ARS

Diseases of Roses John & Mitchie Moe ARS Master Rosarians Pacific Northwest District Revised March 2015 May 25, 2012

This program is eligible for one CR Credit if pre-approved by the District and National CR Chairs An American Rose Society Presentation © 2015

Acknowledgements • Most photos used in this program are copyrighted. • Permission was received to use the copyrighted photos for this educational program! Please do not copy or reproduce for any other uses. • An excellent source – the ARS Consulting Rosarian Manual • Another excellent source - http: //www. sactorose. org Baldo’s Bugs & Roses, the website of Baldo Villegas, known to rosarians as “The Bugman” • Please – no commercial use of this program!

Introduction • There are many diseases of roses that have described in various books, but there are three that seem to afflict the rose garden at one time or another: • Mildew • Blackspot • Rust • If these three can be controlled, the rose garden will be generally disease free. They can often be managed by following these recommendations: • Buy certified stock free of diseases • Keep free of weeds, fallen leaves, and disease-infested plants or canes • Use regular spray programs, and practice good cultural methods Note: This program deals primarily with identification of diseases, and does not get into specific chemical controls, which is a subject by itself.

Powdery Mildew Sphaerotheca pannosa • A very common disease of roses – some roses more susceptible than others! • Starts on leaves, but spreads to buds and young shoots • Common in crowded plantings where air circulation is poor • Optimum conditions for spore germination are 71°F and 98 percent relative humidity Jack Kelly Clark Photo by Jack Kelly Clark Warm days and cool nights makes any climate just right for this!

Powdery Mildew Control • Over-winters on infected canes, spores move to new leaves in spring to begin disease cycle • Aggressive spring pruning will remove a large source of this fungus • Fungicides work best at onset of disease, not after massive infection has occurred • A regular fungicide spray program is generally necessary to prevent it David Hughes, Gardenseeker. com This is why the first fungicide spray is so important!

Black Spot Diplocarpon rosae Photo by Jack Kelly Clark • One of the most common diseases of roses • Disease cycle typically begins on canes and leaves infected the previous season • Leaves less than two weeks old are the most susceptible • Small 2 mm black spots enlarging to 12 mm with generally circular appearance, but having a feathery edge found on the upper leaf surfaces as well as immature canes

Blackspot (cont. ) • Lower areas are more likely to be infected as spores are splashed up on new foliage during rains or irrigation– will continue up the stem - as disease spreads, leaves go from green to yellow and then drop • Becomes very active in a wet environment with temps around 70°F – needs about 7 hours to produce new spores to infect additional leaves and canes within a few days Clemson University - USDA Cooperative Extension Slide Series, Bugwood. org

Blackspot Control • Good sanitation will help – remove diseased leaves • Spores can over winter on canes, and the entire cycle will repeat next year if a fungicide spray program is not started early and maintained! • Aggressive spring pruning will remove a large source of fungus • Prune to allow air circulation, and water without wetting the foliage • Buy disease resistant varieties of roses Photo by Jack Kelly Clark

Rust • A fungal disease common in many areas, easily identified • Phragmidium spp Appears on the underside of the foliage as red-orange spots • Thrives in cool, moist weather • The summer stage is cyclic, and may repeat every 10 to 14 days in favorable weather • Fungus overwinters on infected leaves and canes • Spores are wind borne • Can defoliate the plant Clair Martin

Rust Control • Good sanitation will help to prevent early season infections • Remove affected leaves and dispose in trash • Don’t water in the evening so that the leaf surface is wet over night • Fungicide you use for powdery mildew or black spot is usually effective Univ of GA

Downy Mildew Peronospora sparsa • A very serious disease that can affect all roses! • May appear when favorable conditions of humidity over 85% and temps of 65 - 75° F • Under cool and moist spring conditions, young leaves, stems and flowers may show purple to red or brown irregular blotches that tend to follow the leaf veins • Advanced infections will have yellowing of leaves Gail Trimble

Downy Mildew (cont. ) • White to grayish fungal threads containing spores coat the undersides of leaves • Spores may germinate in as little as 4 hours, and appear on leaf surfaces in 3 days • Can defoliate a plant in a day or so • Fungus can winter-over as spores in or on plant parts Photo by Jack Kelly Clark

Treating Downy Mildew • Once seen, it is generally too late to prevent severe leaf drop, which begins at the top! • Before treating, severely prune heavily infected and defoliated plants to encourage new growth • Destroy all cut material, spores can live for a month • Treating generally requires using some very potent and costly fungicides Gail Trimble

Anthracnose Sphaceloma rosarum • Initially spots are small and black easily confused with blackspot • Spores believed to be carried by water or rain to new leaves and stems • As disease progresses, spots become purple to brown and centers turn gray or white with dark margin • This light-colored center best defines the difference with blackspot • Can be severe under cool, moist spring conditions Baldo Villegas

Anthracnose (cont. ) • Tissue may drop out of the center of the spots, giving infected leaves its other name of “Shot Hole Fungus” • Fungus overwinters as spores mainly in old lesions on canes • Can cause leaf drop that weakens the plant • In extreme cases, it can result in complete defoliation Gail Trimble

Anthracnose Control • Sanitation is best means of control – remove diseased leaves, and dispose in trash • Plant spacing, use of drip lines or soaker hoses rather than sprinklers should help • Prune out canes that have infections in fall – dispose in trash, not in compost pile • Fungicide used for black spot is usually effective Paul Bachi

Botrytis Blight • When the weather is cool and wet, the Botrytis cinerea conditions are right for this fungus • Most common symptom - young flower buds droop, turn black at the base • Later produce the cottony grey-black mycelium of Gail Trimble the fungus Gail Trimble • Can affect any part of the plant except roots

Botrytis Blight (cont. ) • Spores are moved to other plants by wind or blowing rain • Note the grayish fuzzy growth on the cane that indicates botrytis • Good ventilation is also essential in reducing disease incidence • Fungus may overwinter in dead plant material • Remove blighted flowers, leaves, or entire plants if infected at the base – discard in trash! • Fungus becomes quickly resistant, so alternate fungicides Univ of GA

Rose Canker • A fungal disease that can affect any part of the plant, but most common on stems and canes Coniothyrium spp • After a cold wet winter, pruned stems provide wound sites for canker causing fungi • Canker can also enter thru leaf scars – do NOT tear off leaves during fall pruning! John Moe

Rose Canker (cont. ) • Tissue within the canker begins to dry out and shrink, giving it a shriveled look • Stems will yellow, often have red spots and later become a dark brown or black • Can be acute on old roses that have lost vigor, and on new bare root roses coming from cold storage John Moe

Rose Canker Control • Time fall prune so tips can callus before winter • Dormant spray can protect pruning cuts • Disinfect pruners before moving to another bush • In spring, remove infected canes and spray for fungal disease • Fungicides that control black spot will also help to control canker John Moe

Bacterial Crown Gall Agrobacterium tumefaciens • Bacterial disease usually seen on bud union, or on the trunk • Thrives in warm, sunny weather; not active in cold weather • Cork-like substance that will turn brown and harden as it ages • Plants will usually be stunted in their growth habits John Moe

Bacterial Crown Gall (cont. ) • Can persist in the soil in a dormant state for years • The growth can be removed, however, chances are it will eventually return • If you do cut out galls, use a sharp knife or pruning shears, brush on full strength bleach • Destroy the galls; disinfect cutting tools with Lysol® after each cut John Moe This cut was most likely made with infected pruners!

Bacterial Crown Gall Control • Avoid injury to canes and stems when planting or cultivating • Maintain good garden sanitation and cultural practices • If heavily infected, remove and discard the plant • If you can, replace soil where rose was growing with new soil, or saturate the soil well with a 10% bleach solution before replanting • Disease is un-treatable

Rose Mosaic Prunus Necrotic Ringspot Virus • Probably the most commonly found virus in roses • Mosaic is primarily transmitted by propagation • Usually appears in spring as a distortion of growing tips and leaves • Symptoms include coloration patterns (shown), distorted leaf growth, slow to develop, and fewer quality blooms Univ of Georgia

Rose Mosaic (cont. ) • No cure – buy quality plants showing no symptoms of the disease • Commercial rose growers purchase rootstock that has been “virus indexed”, which means it has been lab tested to confirm the absence of the virus in the plants tissue • Overall damage from the disease may be mostly cosmetic with some reduction of plant vigor Univ of GA Jack Kelly Clark

Rosette Disease (RRD) • Exact cause unknown • The distinctive red stems, distorted and wrinkled leaves are symptoms • The main host is R. multiflora • New growth appears unusually soft and pliable • RRD is suspected to be carried from plant to plant by the tiny eriophyid mite • Can be confused with plant damage from chemicals like Roundup® Photo by James W. Amrine Jr.

Rosette Disease • The stems grow slowly and produce excessive thorns • There may be so many thorns that the stem may be non-visible! • Thorns are often red -tinged Photo by John Hartman

Rosette Disease • If you are sure that it is this disease, you need to act fast! • Very serious disease – no cure, fatal to plant! Photo by James W. Amrine Jr. • Remove the diseased plant as soon as possible (including the entire root system) and destroy it! • Considered to be one of the most damaging problems of roses!

Questions? Thank You

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