Hardwood Plantation Diseases

Hardwood tree species have been grown in plantations throughout the Southeastern United States for more than 50 years, although the total acreage in hardwood plantations is much less than that in softwood species. Since the early 1990s, market conditions and new approaches to environmental issues have led to changes in cultural methods for growing hardwoods and the planting of many more acres of hardwoods. Fiber-farming technology has allowed industrial growers to plant bottomland hardwood species on upland sites where rapid growth is fostered by irrigation and liquid fertilization, a method referred to as fertigation. This cultural method allows year-round harvesting, whereas wintertime harvesting in natural bottomland stands is limited by wet soil conditions and associated environmental concerns. In the Lower Mississippi River Alluvial Valley (LMRAV) and other areas in the Southeast, agricultural land is being afforested in response to changing agricultural markets and increasing interest in ecosystem restoration (Stanturf and others 2000). Fiber farming and large-scale afforestation present unique challenges and opportunities to growers and pest management professionals.

Bacterial Leaf Scorch on American Sycamore

American sycamore (Platanus occidentalis L.) has good commercial value because of its rapid growth and excellent pulping qualities for the production of paper products. Sycamore is commonly used in afforestation efforts. During the early and mid-1970s, sycamore decline was the main problem of concern to sycamore producers in the Southeastern United States. Surveys conducted in the 1970s, focusing on leaf scorch, dieback, and cankers, found a complex of diseases associated with sycamore decline. These included canker stain, caused by Ceratocystis fimbriata (Ellis & Halst.) F. platani J. M. Walter; Botryosphaeria canker, caused by Botryosphaeria rhodina (Cooke) Arx; and anthracnose, attributed to two conidial stages of Apiognomonia veneta (Sacc. & Speg.) Höhn (Filer and others 1975). Leaf scorching occurred in all locations surveyed. Leaves were described as scorched, eventually turning completely brown, but not shedding prematurely. These symptoms are common for bacterial leaf scorch, a disease caused by Xylella fastidiosa Wells and others (Leininger and others 1999, Sherald and Kostka 1992), but which was attributed in the 1970s to late-summer symptoms of anthracnose caused by fungi, particularly species of Colletotrichum (McGarity 1976). Tree diseases caused by X. fastidiosa were considered hard to diagnose in the past because diagnosticians were unfamiliar with the pathogen and no diagnostic tools were available to detect it. Symptoms were easily confused with those of other biotic and abiotic factors such as moisture stress and herbicide damage. The presence of the bacterium in trees previously was difficult to confirm using routine laboratory techniques because of its fastidious nature (Sherald and Kostka 1992). The advent of enzyme-linked immunosorbent assays (ELISA) has made diagnosis of X. fastidiosa infections in plants routine, and has facilitated the detection of bacterial leaf scorch in sycamore throughout the Southeast. Polymerase chain reaction is also being used to detect this bacterium in plants. Recent visual surveys and ELISA testing of sycamore plantations across the Southeast showed that bacterial leaf scorch caused severe dieback, decline, and mortality to sycamore saplings growing on sites with or without irrigation (Britton and others 1998). Initially, necrotic zones appear along the midrib and main veins of leaves by late July of the second growing season. Severe marginal leaf scorching in foliage throughout individual crowns and the entire stand is common by the third year. Branch and top dieback occurs in 50 percent or more of a stand and some mortality may occur by the fifth year. In severe cases, premature salvage harvests are justified because of concerns that stands will not contain sufficient volume at the normal pulpwood rotation age to pay for the additional carrying cost. Research is currently underway to identify sycamore genotypes that are tolerant to bacterial leaf scorch disease (Chang and others 2002).

Eastern Cottonwood

Many of the same hardwood species used in fiber farming also are used for afforesting former agricultural fields. These include several oak species, eastern cottonwood (Populus deltoides Bartr. ex Marsh.), American sycamore, and green ash (Fraxinus pennsylvanica Marsh.). Forest restoration through afforestation is just beginning on a large scale in the LMRAV, and many successful planting and cultural methods are in use (Stanturf and others 2000). Development of new management methods for controlling insect and disease pests in these monocultural plantation settings is badly needed. Cherrybark oak (Q. falcata var. pagodifolia Ell.) seedlings growing in nursery beds are susceptible to leaf injury and stunting from Cylindrocladium scoparium Morg. (Smyly and Filer 1977). Newly emerged hardwood seedlings of many species are susceptible to damage from soil-borne fungi such as species of Fusarium, Rhizoctonia, and Pythium that cause damping-off (Filer and Cordell 1983). Insect and disease management guides for oaks (Solomon and others 1997), sycamore (Leininger and others 1999), cottonwood (Morris and others 1975), and ash (Solomon and others 1993) will aid in diagnosing many problems, especially in older stands. However, disease problems in nurseries and on stored and newly planted seedlings will require research and development of new control methods, especially since traditional controls such as methyl bromide have been eliminated. Concern for the surrounding environment is likely to lead to the development of biological and chemical controls that minimize long-term effects on ecosystems adjacent to plantations.