3/30/2023 - By Dr. Aaron Palmateer

Leaf spots are among the most common plant diseases in the greenhouse and are mostly caused by fungi and bacteria. Some of the major fungal plant pathogens that cause leaf spots include Alternaria, Cercospora, Colletotrichum (anthracnose), and Myrothecium. The two most common bacterial plant pathogens that cause leaf spots include Pseudomonas and Xanthomonas.

When Does Leaf Spot Occur?

Alternaria alternata is the primary species associated with a wide host range of ornamental plants. Other Alternaria species exist with reports on Dianthus, marigold, impatiens, annual vinca, and zinnia. Depending on the host, spots can occur on the leaves, stems, and flowers. Seed transmission is a concern and especially for diseases of bedding plants. Favorable temperatures for Alternaria leaf spot range from 77 – 86 ℉.

Anthracnose

The fungus Colletotrichum gloeosporioides is one of the most common pathogens that cause anthracnose, but other Colletotrichum species and Gloeosporium species can cause anthracnose. Anthracnose is the term used to describe a symptom appearing as a black sunken lesion on the leaf, stem, or fruit of a plant. Anthracnose can be common on begonia, daisy, dahlia, dusty miller, geranium, gerber daisy, poinsettia, primrose, verbena, and vinca. Cyclamen is the most affected where the disease can severely disfigure leaves and petals. Depending on the host plant spots can occur on leaves, stems, and flower petals with disease development favoring the spring and summer months, when greenhouses are warm, and plants are frequently irrigated.

Myrothecium Leaf Spot

Myrothecium leaf spot is most common on pansies, salvia, and new guinea impatiens. The disease is not as common on poinsettia, but outbreaks do occur especially in the southeast US. Disease development favors temperatures in the 70s with high relative humidity. The fungus is an excellent opportunist and often the pathogen attacks wounded plugs during transplanting.

Leaf Spot Disease From Bacteria

Among the bacteria, Pseudomonas cichorii, P. syringae, and several species of Xanthomonas are the most common leaf spot pathogens of ornamental plants with, extensive host ranges including woody species, grasses, and vegetables. Leaf spots caused by Pseudomonas and Xanthomonas (varies based on the host plant) first appear water-soaked and as they expand in size the tissue turns black. Under dry conditions, the spots often turn brownish tan and do not expand in size. Leaf spots caused by bacteria are favored by wet conditions, so the disease is most common when plants are watered using overhead irrigation and during periods of heavy rainfall.

Diseases caused by Pseudomonas cichorii are most prevalent when temperatures range from the low 70s to mid-’80s whereas P. syringae favors cooler temperatures from the upper 50s to the upper 60s. Temperature range can vary based on the species of Xanthomonas, but generally favorable temperatures are the upper 70s to low 80s.

Spot Streaks and Bacteria

Bacteria are single-celled organisms and the cells of plant pathogenic bacteria are not capable of directly penetrating plant tissue and require wounds or natural openings. Thus, leaf spots often originate within the stomata or along the leaf margins where hydathodes are present. Spots are often delineated by the veins in the leaves and therefore can appear angular or sometimes like straight lines that run parallel with the leaf veins and are often referred to as a streak. These differ from leaf spots caused by fungi as they can actively penetrate plant host tissue where the spots often cross leaf vein tissue.

A Few Tips and Warnings

Whether it’s fungal spores or bacterial cells both are easily spread in splashing water and windblown rain where they come into contact with the leaves of plants. Growers should avoid overhead irrigation and minimize periods of leaf wetness when possible. Frequently scout highly susceptible plants and especially those that are closely spaced for symptoms. Minimize wounding of plants and remove symptomatic leaves and severely diseased plants to reduce inoculum. Pesticide applications should be preventative and applied before favorable conditions occur.

Azoxystrobin: has been a standard in ornamental fungicide disease control and is highly effective against leaf spot pathogens of bedding plants. Azoxystrobin is highly effective against leaf spot pathogens of bedding plants. Azoxystrobin is a strobilurin fungicide that enters the plant systemically to offer both preventative and curative control. Several fungicides for greenhouse production contain azoxystrobin, including Harrell's ProtectMAX® Azoxy, Heritage®, and Mural.

Propiconazole: is a systemic DMI fungicide that has activity against leaf spot pathogens and should be considered in disease control programs. Propiconazole can provide both curative and preventative control of a broad spectrum of ornamental diseases. Additionally, Banner Maxx® contains Propiconazole.

Thiophanate-methyl: is a member of the benzimidazole class of chemistry and can be a good rotational product for ornamental disease control programs. It has protective and curative properties and can be effective against leaf spot pathogens. T-methyl and Cleary's 3336 are two fungicides containing thiophanate-methyl.

Fluoxastrobin SC: One of the strongest solo active ingredients on anthracnose and a great option for a broad spectrum of other diseases on ornamentals. With systemic movement in the plant and excellent knockdown and curative activity, Fluoxastrbin is a versatile tool in any program. Check out the all new ProtectMAX® Fluoxastrobin as a great option.

Reach Out to Your Harrell’s Rep for More Information

If you have any further questions or want to know more about leaf spot/how to protect your plants from it’s appearance, reach out to your local Harrell’s rep.