Spotlight on leaf spot diseases

Ornamentals Advisory Blog

Fungal leaf spots are the most common and obvious diseases present during ornamental crop production. In most cases, they are easily noticed, and the temptation to guess specific causes is great. While certain characteristics may often be present with each type of leaf spot disease, there are no hard and fast rules about diagnosing by the "seat of your pants." Most serious diagnosticians recognize the need to culture the pathogen before making a recommendation for the best treatment.

Pathogens such as Alternaria and Colletotrichum (anthracnose) affect most ornamentals, including bedding plants, cut flowers, and cut foliage, tropical foliage plants and woody crops. Other diseases such as Fusarium leaf spot on dracaenas and fairy ring leaf spot on dianthus affect a narrow range of ornamentals but remain serious concerns for producers of those crops. Fungal leaf spots rarely kill a crop, but on rooted cuttings, such as pittosporum, Alternaria can result in massive losses. It is also possible to incur huge losses by planting plugs contaminated with foliar diseases. Unless environmental conditions are bad for the disease, it will continue while the product finishes.

Plant pathologists continue to stress use of cultural control strategies that minimize exposure to overhead irrigation and rainfall when possible; employ pathogen-free seeds, cuttings and plugs; and use of resistant cultivars. Many of these methods are impractical when the production area is the great outdoors. In addition, despite nearly constant warnings regarding use of pathogen-free propagative materials, seeds are still commonly contaminated with Alternaria and other pathogens and plugs or rooted cuttings infected with a variety of leaf spot diseases are easily obtained.

Fungicides will remain, for the foreseeable future, the most common and often the only way to manage some fungal leaf spots for many ornamentals. The difficulty faced in diagnosing even common leaf spots usually means that growers will choose a broad-spectrum fungicide to cover all the bases. There are many available fungicides with relatively safe, broad-spectrum characteristics that allow for undiagnosed control of many leaf spot diseases. A further difficulty is that, due to the increasingly large number of ornamentals grown, many specific diseases cannot be found on a fungicide label, even when a diagnosis is made. Researchers recognize these problems and struggle each year to fill the information gap for both fungicide users and manufacturers.

Symptoms and Diagnosis

The chief symptom of a leaf spot disease is spots on foliage. The spots will vary in size and colour depending on the plant affected, the specific organism involved, and the stage of development. Spots are most often brownish but may be tan or black. Concentric rings or dark margins are often present. Fungal bodies may appear as black dots in the spots, either in rings or in a central cluster. Over time, the spots may combine or enlarge to form blotches. Spots or blotches that are angular are generally referred to as anthracnose. Leaves may yellow and drop prematurely.

a. Black spot in roses    b. Leaf spot disease on hydrangea  c. Septoria leaf spot on chrysanthemums

Life Cycle

The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the crops. During wet weather, spores may splash or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering can also provide prolonged wet periods that are ideal for spreading leaf spot diseases.

Integrated Pest Management Strategies

Remove infected leaves and dead twigs. Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can reinfect the new leaves. This is not a cure but may help limit infection by reducing the total amount of inoculum.

Keep foliage dry. Avoid overhead watering. Use soaker hoses or water early in the day so the foliage can dry off before night. Watering can also spread the disease by splashing. Pruning plants to allow for good air circulation and reducing crowding will also help keep the foliage dry.

Keep plants healthy. Since most plants can tolerate some defoliation, keep them in good health so they can rebound quickly. Avoid over fertilizing by testing the soil first. Abundant, young, tender growth is very susceptible to attack by disease and insects. Overuse of nitrogen can cause an abundance of succulent growth.

Use fungicides if needed. In rare cases of severe infection and where the size and value of plants make it practicable, applications of fungicides may be helpful. Sprays will not cure infected leaves. Therefore, once the damage is noticed, spraying may have limited value. Spraying generally needs to be started as buds break in the spring and repeated at 10–14 day intervals. Recommendations will vary by disease and fungicide used. Have the disease identified before purchasing a control product.

Chemical control

The most used fungicides have been available to our industry. These include chlorothalonil, copper, mancozeb, iprodione, thiophanate methyl and various combinations of these active ingredients. More recent fungicide introductions often include active ingredients such as fludioxonil, propiconazole, myclobutanil, azoxystrobin and trifloxystrobin. The newer fungicides represent many classes of chemistry, most quite different from previously available products. This gives the grower a better tool for resistance management, since rotations between chemical classes are believed to be our best tool in reducing the potential for fungicide resistance. Certainly, we have a very large number of different chemical classes to choose from for fungal leaf spot control. The responsible producer must practice rotation between classes and/or the use of products with more than a single active ingredient.