Seeing Spots Before Your Eyes?

By: Mary Peterson, meg21@stny.rr.com

It may be that “Rose is a rose is a rose is a rose is a rose” according to Gertrude Stein in “Sacred Emily” 1913, but “a spot is a spot is a spot” is not necessarily Blackspot.

There are several fungal diseases that exhibit spots on rose foliage and it is important to be able to recognize and correctly treat the causative agent. Most rosarians are familiar with Blackspot but let’s review the signs and symptoms.

• BLACKSPOT ( Diplocarpon rosae, Marssonina rosae ) Spots are nearly circular with fringed or feathery margins. Spots vary in size. Surrounding leaf tissue turns yellow before dropping. Bottom leaves affected first. Conditions that favor the development are 75-85 degree F temperatures with relative humidity above 85%, overcrowding and reduced air flow. Control can be achieved by planting blackspot resistant varieties, use of an adequate fungicide on a regular basis applied to all leaf surfaces in the recommended amounts and frequency, usually in early evening. Good housekeeping is essential to removed all infected foliage and canes and disposing of properly. Water using a soaker hose rather than spraying water as splashing water disperses spores to re-infect plant. Roses that yellow up more before dropping their leaves are more “resistant” than those that tend to stay more green. The ones that yellow up are ‘smart’ enough to know that they are being attacked and try to salvage some resources from the leaflet before it drops and those that stay more green don’t have a clue what’s going on and allow the fungus to grow and reproduce more readily.
• DOWNEY MILDEW (Peronospora sparsa ) Since the fungicide used to treat Powdery Mildew is not the same as those for Downy Mildew, it is critical that the correct diagnosis is made. The fungus that attacks roses is host-specific, meaning that it only attacks plants in the rose family. Downey Mildew occurs during the active growing season during periods of wet and cold weather. It is spread by air currents and splashing water. It can defoliate an entire plant in as little as 2 days causing loss of the plant. Unlike Blackspot that starts at the bottom, Downey Mildew starts at the top 1/3 of the plant and moves down. The canes can have purple spots/patches. The spots on the leaves are purple-red with a poorly defined border up to •••” in diameter. The spots do not grow in size or number while the leaves continue to yellow until only islands of green remain on a yellow leaf at which point the leaf falls from the plant. Unlike the damage to the leaves and buds by Powdery Mildew, Downey Mildew invades and kills all plant tissue. Downy Mildew is most closely related to Pythium and Phytophthora (water molds which have major biochemical and morphological differences) so fungicides that are active against them will give you good coverage for Downy Mildew. As with any fungal disease, be sure to clean up any infected foliage and dispose of properly.
• CERCOSPORA ( Cercospora puderi, Cercospora rosicola ) Characterized by the appearance of numerous tiny maroon to purple oval spots scattered randomly across the leaf surface. Later the center of the spot turns tan to gray in color while the margins remain maroon to dark purple. Heavily spotted leaves will turn yellow and prematurely shed. This infection can be easily confused with Blackspot. Same weather conditions that produce Blackspot will provide optimal growing conditions for Cercospora. It has the same method of spreading as Blackspot. Using a fungicide labeled for control of Blackspot will also control this fungus infection. Many of the roses resistant to Cercospora leaf spot are susceptible to Blackspot. Although no fungicides have been screened for the control of Cercospora leaf spot on roses, a number have activity against other Cercospora diseases on other woody ornamentals. Fungicides, such as Daconil, Immunox, Zyban, and Cleary’s 3336, which are recommended as weekly treatments for blackspot control, should also provide good protection from Cercospora leaf spot when applied on the same schedule.
• BOTRYTIS BLIGHT ( Botrytis cinerea ) Small light colored spots sometimes surrounded by a maroon halo. Under favorable conditions the spots quickly expand into large, brown, irregular blotches covering much of the petal. Infected buds fail to open and often droop. Discoloration, slightly sunken lesions often extend down the stem from blighted buds. Damaged petals and buds usually covered with gray-brown growth of fungus. Cool humid weather encourages this infection and spores are dispersed by air currants. Practice good garden sanitation and proper disposal.
• ANTHRACNOSE or SPOT ANTHRACNOSE (Elsinoe cornii, Sphaceloma rosarum) Usually seen in late spring. Spores spread by air currents and splashing water. Usually does not defoliate plant but weakens the plant somewhat leaving foliage and canes affected. Spots are circular 1/16” to 1/8” in diameter. They are dark brown and enlarge in size and turn white in the center while most of the leaf remains green. A ‘shot-hole’ effect may occur (the spot itself drops out of the leaf leaving a circular hole). Defoliation may occur if the disease is severe. Remove any canes with purple blotches as these also carry the spores for this fungus infection.

APPROVED TREATMENTS FOR FUNGUS INFECTIONS

(Used by permission: Larry L. Strand , UC Statewide IPM Program )

• Azoxystrobin (Heritage) is a locally systemic fungicide that is effective against rusts and as an eradicant and protectant against some powdery mildews.
• Chlorothalonil (Daconil) is effective for the control of Botrytis spp., Alternaria spp., Rhizoctonia spp., as well as other leaf-spotting fungi on many ornamentals.
• Fenarimol (Rubigan) is a systemic fungicide used for prevention or eradication of powdery mildew on roses and field and container-grown ornamentals.
• Fenhexamide (Decree) applied as a foliar or floral spray is very effective against Botrytis.
• Fixed copper (Kocide, BlueShield, Champ) is a general purpose fungicide and bactericide most often used as protectant against various leaf-spot pathogens, Botrytis spp., and anthracnose. Overall growth of some plants may be reduced by this material; follow label directions carefully to reduce the risk of phytotoxicity. Fosetyl-Al (Aliette) is active against Phytophthora species and some Pythium species. It is most effective when applied as a foliar spray because of its reverse systemic action: when sprayed on foliage it moves into the plant and is transported to the roots, providing better control than a soil application.
• Lime sulfur is a powdery mildew eradicant. Apply this material with caution when temperatures exceed 85° F. Because it is toxic to green plants, it is best used as a dormant spray.
• Mancozeb (Dithane) is a dithiocarbamate fungicide used to protect against leaf spots, Botrytis , rusts, and blight. It is not systemic so thorough coverage is important for control.
• Myclobutanil (Systhane) is a systemic fungicide applied as a foliar spray that is both a protectant and eradicant of rusts or powdery mildew on carnations, crepe myrtle, gerbera, roses, and snapdragons. It also controls Cercospora leaf spot. It is an important tool in the eradication program for chrysanthemum white rust.
• Neem oil (Triact) is a broad-spectrum botanical pesticide derived from the neem tree that is effective against various fungal diseases including black spot on roses, powdery mildew, downy mildew, anthracnose, and leaf spot.
• Piperalin (Pipron) is a foliar spray that eradicates powdery mildew on rose, lilac, dahlia, phlox, zinnia, chrysanthemum, and catalpa.
• Potassium bicarbonate (Kaligreen) can be used to protect against powdery mildew infections on roses. Because it is a contact-type fungicide, thorough coverage is essential for good protection.
• Propiconazole (Banner Maxx) is a preventative fungicide against powdery mildews, rusts, leaf spots, and blights.
• Stylet oil (JMS Stylet Oil) is available to control black spot and powdery mildew on roses, poinsettia, chrysanthemum, diffenbachia, and philodendron. There have been some phytotoxicity problems with this material, especially on greenhouse roses.
• Thiophanate methyl (FungoFlo, Cleary’s 3336F, etc.) is a systemic fungicide that is used to control many leaf-infecting fungi and some soil-borne fungi.
• Triadimefon (Strike) is a long-lasting systemic fungicide used for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries.
• Wettable sulfur can be used as a spray to protect ornamentals against powdery mildew. It has no eradication action and it leaves a residue on plants that could cause plant injury. Apply this material with caution when temperatures exceed 85° F.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental NurseriesR. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley

M. E. Grebus, Plant Pathology, UC Riverside

M. E. Grebus, Plant Pathology, UC Riverside

A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley

UC DANR Publication 3392

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries

To download the pdf version of this article click here.