This is the ninth part of a series on fall tree diseases and disorders. This article examines maple tar spot and oak wilt.
During fall, the environment undergoes a drastic shift, with cool temperatures prevailing, and inclement weather often abundant. In this climate, many plant diseases flourish. The following describes some of the most common plant diseases to occur in fall, and how they affect their hosts.
Maple Tar Spot (Rhytisma acerinum, Rhytisma americanum, and Rhytisma punctatum)
Maple tar spot is a fungal disease that affects many species of maple. The disease occurs most frequently in North America and Europe. It is caused by three related fungi: Rhytisma acerinum, Rhytisma americanum, and Rhytisma punctatum. Symptoms vary between the three pathogens, but they all produce fungal spots on host plants that resemble tar. While maple tar spot occurs on many species of maple, it is most common on Norway, red, and silver maple.
Maple tar spot afflicts many species of maple, including big leaf, mountain, Norway, red, rocky mountain, silver, sugar, and sycamore maple. It may also occur on boxelder, willow, and tulip-tree.
Symptoms of Infection
Initial symptoms develop on infected leaves in early summer. At this stage, the leaf spots are light green to yellow green. As the growing season progresses, the spots enlarge, and the yellow coloring intensifies. On red and silver maple, a black lesion usually develops in the center of each yellow spot. These black lesions appear from mid-July to early August. The black lesion grows in diameter and thickness until late summer, when it resembles a spot of tar. The surface of the lesion may feature a pattern of wavy indentations, or ripples. Late in the growing season, infected leaves may drop early. Not all infected leaves are shed. Defoliations are often minor.
Symptoms differ on striped and Norway maple. On these trees, twenty to fifty small spots appear on infected leaves in late July or early August. On striped maple, the spots enlarge slightly. This type of tar spot is often referred to as speckled tar spot. On Norway maple, the spots grow significantly. They eventually coalesce, yielding a black mass that can measure up to 1 ½ inches in diameter. On Norway maple, the surface of the black mass may be slightly roughened or smooth.
- Maple tar spot does not cause significant damage to trees. The most effective management practice is raking and destroying infected leaves in fall. This will reduce the number of spores available the following spring.
- Mulching infected leaves will destroy many of the spots before they mature. As a precaution, the mulch pile should be covered, or shifted before new leaves emerge in spring.
- Fungicides are available for controlling maple tar spot. Applications should be performed in spring. To ensure success, complete coverage of the leaf tissue is required.
Oak Wilt (Ceratocytis fagacearum)
Oak wilt is an aggressive tree disease that affects many species of oak. The causal agent of oak wilt is the fungus Ceratocytis fagacearum. Oak wilt was first identified in North America in 1944, and has since become one of the most widespread tree diseases in the eastern United States. Symptoms vary by tree species, but generally consist of leaf discoloration, a wilting of foliage, defoliation, and tree mortality. The oak wilt fungus is spread by insects, or through root grafts. Oak wilt is responsible for killing thousands of oaks in forests, woodlots, and landscapes each year.
Oak wilt affects all oak species, but impacts the two main groups differently. Oaks in North and Central America are sorted into two groups based on their characteristics. Those in the white oak group have rounded leaf edges, and pores that are clogged with tyloses. Those in the red oak group have pointed leaf edges, and enlarged, open pores. Due to their open pores, oaks in the red oak group are more susceptible to oak wilt. When infected, they generally succumb to the disease within a year. Oaks in the white oak group are less susceptible to the disease, and can persist for years following infection. Some oaks in the white oak group, such as bur oak, are more easily infected than others, but still exhibit a greater resistance to the disease than oaks in the red oak group.
Symptoms in Red Oak
Infections on red oaks are most common in spring, when new vessel wood is being formed. Symptoms may occur as early as May. On red oaks, wilting progresses from the top of the tree downwards. Leaves turn dull green or bronze, and appear water-soaked. As the disease progresses, leaves turn yellow or brown, and curl around the midrib before dropping from the tree. Leaf damage begins from the tip and outer edges, and advances toward the midrib and base. Leaves fall in all stages of discoloration. Seemingly healthy leaves may be shed from infected branches. Discoloration of the vascular tissue also occurs, with brown streaks or spots appearing under the bark in the sapwood. Oak wilt progresses rapidly, with some red oaks succumbing to the infection within one to two months. Most diseased red oaks die within a year of infection.
Symptoms in White Oak
Symptoms are more variable in white oaks, although foliar symptoms tend to be similar. In white oaks that exhibit an increased susceptibility to the disease, symptoms resemble those of red oaks. The disease progresses rapidly, with many trees succumbing to infection within one growing season. This is particularly common in bur oaks. Most other white oaks decline more gradually. If the fungus persists, a few branches may be killed during the first year. Over subsequent years, the tree will decline, and may eventually die. Some white oaks can recover from the disease, but may continue to harbor the pathogen.
Symptoms in Southern Live Oak
In pockets of Texas, oak wilt can become prevalent. Southern live oaks, also called Texas live oaks, are especially vulnerable to infection. When infected, southern live oaks wilt and die. Disease progression varies depending on the timing of the infection, and the weather conditions. If conditions are favorable, infected southern live oaks may succumb to the disease within one to six months.
On infected Texas live oaks, leaves develop clorotic veins, a condition referred to veinal necrosis. Necrosis often spreads to the leaf tips, margins, and interveinal portions of the leaves. As the disease advances, the leaves turn brown, and eventually fall, with the tree crown becoming increasingly sparse. Fallen leaves may feature darker brown veins that persist for months.
- There is no permanent treatment for oak wilt. However, there are a number of preventative methods one can use to reduce the probability of infection.
- Susceptible oaks should be inspected for symptoms of oak wilt. If fungal mats, or beetles are present, remove the tree to prevent additional infections.
- Infected trees should be promptly removed, with the diseased wood safely disposed of. Logs from infected trees should not be moved to unaffected areas.
- To prevent beetle transmission, avoid pruning oaks during spring in the east and Midwest, and from February to June in Texas. Pruning wounds can attract beetles carrying the oak wilt fungus.
- Prevent mechanical and environmental injuries to susceptible oaks, especially when conditions are favorable for infection.
- Creating mechanical and chemical barriers between diseased and healthy trees can prevent the oak wilt fungus from spreading through root grafts. Trenchers or vibrating plows can be used to disrupt root grafts down to a depth of two to four feet.
- Soil fumigants may also be applied to sever root connections. Soil fumigants are restricted-use pesticides. They are primarily used in locations where vibratory plowing or trenching is not possible. Soil fumigants are often volatile, and can be difficult to manage. As such, they should only be handled by a licensed pesticide applicator. In locations that necessitate the use of a soil fumigant, holes are drilled into the soil at prescribed intervals, and the pesticide is applied therein. Once administered, the pesticide is absorbed into the soil where it kills any roots in a localized area.
- Fungicides have been developed that may be effective in preventing oak wilt when injected into healthy oak trees. A single treatment can protect red oaks from infection for up to two years. In southern live oaks, fungicidal treatments have prevented mortality, though foliar thinning generally still occurs.