4 minute read
MANAGING LEAF SPOT DISEASES IN THE LANDSCAPE
Black spot of elm. Photo: Richard Buckley, Rutgers PDL
by Bill Errickson, Agriculture Agent - Rutgers Cooperative Extension of Monmouth County
Leaf spot diseases are one of the most common problems that affect landscape plantings in New Jersey. There are many different types of pathogens that can cause leaf spot symptoms and there is a broad range of ornamental plants that can be affected. Most leaf spots are regarded as cosmetic issues, rather than a disease that is life-threatening to the plant. However, high levels of leaf spot can lead to premature leaf drop and can reduce the amount of green, photosynthetically active area on the leaf surface. This ultimately causes a reduction in the amount of carbohydrates that the plant is able to produce and that means the plant is not growing to its full potential. Nonetheless, clients and customers would also prefer not to have unsightly leaf spots on their plants and in their landscapes. Managing these diseases as part of a successful IPM program starts with an understanding of the pathogens, followed by proper fall sanitation and cultural practices to prevent outbreaks in the following season.
Most leaf spots are caused by several different genera of fungi, including but not limited to Alternaria, Cercospora, Diplocarpon, Phoma, Phyllosticta, and Septoria. There are also bacteria that can cause leaf spot (Psuedomonas and Xanthamonas) and even foliar nematodes (Aphelenchoides). Some common ornamental species that are affected by leaf spot pathogens include maple, hydrangea, cherry, crape myrtle, redbud, viburnum, oak, and rose, though there are many other plants that can also be infected.
The symptoms of leaf spot diseases can vary depending on the host plant and which pathogen is causing the infection. Leaf spots may be black, brown, tan, yellow, or reddish in color and can vary in size from several millimeters to several centimeters in diameter. Some leaf spots may be completely circular, while others may be irregular in shape, and some lesions may even drop out of the leaf, resulting in a shot-hole appearance. As the disease progresses, the lesions may coalesce. The leaves may turn yellow, then brown, and drop from the tree, resulting in premature defoliation.
Septoria leaf spot on red twig dogwood. Photo: Richard Buckley, Rutgers PDL
Many of the fungal pathogens that cause leaf spot diseases have a similar life cycle, in which the pathogen overwinters in the infected fallen leaves. In the spring, spores are released and dispersed by wind or rain onto new leaf surfaces. The pathogen will then infect the new leaf and start growing, though symptoms will usually not occur until later in the summer. This is why applying fungicides is not particularly effective in curing leaf spot diseases in the middle of the summer, as the infection had already taken place months earlier. A cool wet spring will be most conducive to pathogen development, so keeping foliage as dry as possible by avoiding overhead irrigation and promoting airflow during this period through proper pruning and plant spacing is important for managing the disease. Encouraging optimum site conditions and plant vigor through proper fertilization and water management is also important for helping the plants to resist leaf spot, as infection is more likely to occur on plants that are experiencing other forms of stress.
Because the majority of leaf spot pathogens overwinter in infected plant material, the key to reducing their infection potential is proper fall clean-up and landscape sanitation practices. Removing any diseased leaves or other plant debris from the landscape is critical to limit the population of pathogenic organisms that will infect next year’s newly emerging leaves. After the leaves are removed, properly applying a thin layer of mulch around the base of trees and shrubs or in planting beds can also help to reduce spore dispersal and encourage decomposition of any remaining plant debris that may have been infected.
In landscapes that have a history of heavy leaf spot infection, a preventative spring fungicide application may be warranted at bud break, when new leaves are emerging and infection potential is high. However, it is important to have an accurate diagnosis of which organism is being targeted, as certain materials may or may not be effective in controlling the many different species of pathogens that cause leaf spot, and the product must be labelled for the intended use. Additionally, there are several biological products that can be used as protectants, though again it is essential to understand which causal agent is being controlled before applying any materials.
While the majority of leaf spot diseases in the landscape are primarily aesthetic problems, they can be effectively managed with proper cultural practices and landscape sanitation. Attention to detail during fall clean-up, proper pruning, increasing airflow, and optimizing site conditions for the plant species being grown will reduce leaf spot issues in the following season. An ounce of prevention is worth a pound of cure when managing leaf spot diseases and customer satisfaction will be sure to follow.
Bill Errickson is the Agriculture and Natural Resources Agent for Rutgers Cooperative Extension of Monmouth County with a focus on sustainable nursery production, turfgrass, and agricultural innovation. He has worked in various aspects of the horticultural sector for the past 16 years, including operating his own diversified farm in coastal Maine and growing bare-root nursery stock for Fedco Trees. He has led courses focused on sustainable horticulture and soil fertility for commercial growers, homeowners, and college students throughout New Jersey and New England.
“Football” shaped leaf lesions of leaf spot and melting out of Kentucky bluegrass. Photo: Sabrina Tirpak, Rutgers PDL
