The importance of fungal plant pathogens

Fungal plant diseases have profound effects on food production and society in general. Modern industrial society has evolved a number of methods for the control of plant disease comprising hygenic agricultural practice, breeding plants for disease resistance and the use of anti-fungal chemicals. Such measures are needed to produce crops of consistent quality and yield but contribute greatly to the cost of food production. Despite these advances, some 5–10 per cent of crop plants in Europe and North America are still lost to disease. Fungal races which can overcome resistance and which are insensitive to fungicides are constantly arising. Most farmers in underdeveloped countries are unable to afford either fungicides or the latest resistance-bred seeds and losses of 50 per cent are not unusual in these areas. Increasing human populations and concern over the excessive use of many fungicides has emphasized the need to understand fully the biochemical and genetic interactions between plant and pathogen in order to find new strategies to combat disease.

Fungal plant disease occurs because some fungi have evolved the ability to penetrate and colonize plant tissue. Fungal pathogens have evolved along with their host plants for many millions of years and have had a major impact on crop production since the first attempts at organized agriculture. In addition to pathogens, there are many fungal species that interact intimately with plants symbiotically.

Introduction

Fungi are the main decomposers of dead plant material and many fungal species are known as plant pathogens. Saprophytic fungi are able to get energy and nutrients from a large variety of organic compounds and therefore they are found in practically all terrestrial habitats in which there is life. Fungi can colonize new locations efficiently by spreading through their spores. A saprophytic fungus is always competing for nutrients against bacteria and other fungal species in its habitat. In many cases fungi benefit from their ability to degrade complex plant polymers more completely than bacteria and they also compete by antagonism against other saprophytic microbes. Yeasts are not generally able to decompose plant cell walls with polymeric components and thus saprophytic fungi are mainly filamentous. Saprophytic fungal species can be found in all fungal classes.

The saprophytic fungi are divided into three groups according to their mode of action on plant material. Soft-rot fungi cause softening of wood, degrading preferably plant polysaccharides but are often capable of slow lignin degradation as well. These species mostly belong to Ascomycetes. Brown-rot fungi degrade plant polysaccharides and leave behind a brown, modified lignin residue. White-rot fungi invade the lumens of wood cells and cause progressive thinning of the cell wall. They can degrade both plant polysaccharides and lignin efficiently. The members of the two latter groups mostly belong to Basidiomycetes.

Introduction

An essential feature of living cells is their potential to replicate and produce exact copies of themselves. In single-celled organisms replication is achieved by cell division. In multicellular organisms programmed cell division is essential for growth and normal development. In this case, cell proliferation can be seen as a strategy for increasing overall size as well as permitting cells or groups of cells to specialize. Multicellular animals, plants and fungi, therefore, can develop different tissues each containing one or more cell types adapted to a particular function.

The coordination of the various processes required to bring about successful cell division is not trivial. Simply cutting a cell in two is unlikely to result in the production of two viable daughter cells (the products of a cell division). Mistakes at any stage during the entire division cycle can lead to mutant or non-viable daughter cells. To produce two viable cells from one, partitioning of all the necessary cellular components into the two daughters is essential. Each daughter must receive a complete copy of the genome of its mother cell (the cell undergoing division) if genetic integrity is to be maintained through subsequent generations. The genome contains the complete set of genetic information that encodes the proteins required for viability. This information is packaged into a set of chromosomes. The fidelity of replication of these chromosomes must be high in order to maintain genetic stability. Incomplete replication may lead to the loss of essential genes and, ultimately, death of daughter cells.

Introduction

Surveyor records and other historical data indicate that at the time of European settlement, there were approximately 920,000 ha of pine barrens landscape in Wisconsin. Approximately 3,500 ha remain (Chequamegon National Forest 1993). Pre-European jack pine (Finns banksiana) barrens are estimated to have covered 20,000 km2 in northern Minnesota, Wisconsin, and Michigan (Vora 1993; Figure 21.1). Barrens in the western Upper Peninsula of Michigan, such as the Baraga and Yellow Dog plains, were also dominated by jack pine. The Baraga Plains support serviceberry (Amelanchier) and cherry (Prunus) species, as well as scattered red oak (Quercus rubra), red maple (Acerrubrum), big-tooth aspen (Populus grandidentata), and black spruce (Picea mariana). The Yellow Dog Plains are less xeric than the Baraga Plains and thus support a larger component of white spruce (P. glauca), and some red (P. resinosa) and white pine (P. strobus) (Bourdo 1954).

Barrens communities similar to those found in Michigan, Wisconsin, and Minnesota (the Great Lakes states) also are found in New York, Manitoba, and Saskatchewan, and, historically, in eastern Ontario. The New Jersey pine barrens are similar in structure to barrens in the Great Lakes region.

Introduction

The term woodland is used for a variety of shrub- and tree-dominated communities on sites that are not favorable for the development of a forest, whereas the term savanna implies a grass/forb-dominated community with trees at a lower density than expected in the forests of the area. Tree-dominated woodlands are rare across the western Great Plains of North America, occurring mostly along rivers and occasionally on rocky escarpments. Various broad-leaved trees dominate the riparian zone, especially plains cottonwood (Populus deltoides) (plant nomenclature follows Kartesz 1994), boxelder (Acer negundo), green ash (Fraxinus pennsylvanica), and peachleaf willow (Salix amygdaloides). On escarpments, ponderosa pine (Pinus ponderosa var. scopulorum) is common (Figure 15.1). Associated species include Rocky Mountain juniper (Juniperus scopulorum), skunkbush sumac (Rhus trilobata), big sagebrush (Artemisia tridentata), mountain mahogany (Cercocarpus montanus), little bluestem (Schyzachyrium scoparium), and sideoats grama (Bouteloua curtipendula) (Hansen and Hoffman 1988). Bur oak (Quercus macrocarpa) is a common associate in some areas, such as in the Black Hills of South Dakota (Hoffman and Alexander 1987). Near the Rocky Mountains, ponderosa pine often occurs beyond the rocky escarpments, forming parklike savannas on the deeper, finer-textured soils of the adjacent grassland. Limber pine (Pinus flexilis) sometimes occurs with ponderosa pine, but usually it appears at higher elevations where the climate is drier and colder (Peet 1988).

Similarly, desert shrublands of the Southwest grade into woodlands near the mountains or on plateaus. Woodlands dominated by various species of oak (Quercus spp.), especially Gambel oak (Q. gambelii), are common in some areas (Dick-Peddie 1993).

Introduction

When Ponce de Leon first landed in Florida, overstory pines and a ground cover containing grasses, forbs, and shrubs were widespread over southeastern North America. The earliest descriptions were strikingly consistent. Journals of early explorers (e.g., Alvar Nunez Cabeza de Vaca and Hernando de Soto in the 1500s), travelers (e.g., John Latrobe, Mark Catesby, John Williams), and naturalists (e.g., John and William Bartram, Samuel Lockett, Andre Michaux, Thomas Nuttall in the 1700s and early 1800s) all depicted landscapes as open, with a low herbaceous ground cover and visibility for more than a kilometer through pines that most often did not form a complete overstory (Williams 1827, 1837; Small 1921a, b; Harper 1948; Tebo 1985; Frost 1993; Harcombe et al. 1993; Means 1996). More recent descriptions by field biologists reinforced the concept of open landscapes dominated by pines and herbaceous groundcover plants as the prominent upland landscape in the southeastern United States (e.g., Nash 1895; Schwarz 1907; Harper 1911, 1914, 1927, 1943; Harshberger 1914; Wells 1928; Wells and Shunk 1931). These descriptions still apply to the few old-growth stands present today (e.g., Platt, Evans and Rathbun 1988; Doren, Platt and Whiteaker 1993; Gilliam, Yurish and Goodwin 1993; Noel, Platt and Moser 1998).

These fragments of natural history all suggest that southeastern landscapes were predominately savannas.

Introduction

The aspen parkland is a mosaic of grassland and woodland plant communities forming a transitional zone between boreal forests and prairies of the interior plains of central Canada. The main tract of aspen parkland forms a crescent stretching from northern Montana northeastward along the foothills of the Rocky Mountains in southern Alberta, then curving through central Saskatchewan into southwestern Manitoba and adjacent areas of northwestern Minnesota (Figure 25.1). The largest area of parkland is in Saskatchewan, where it occupies about 72,000 km2 in a belt varying in width from 40 to 160 km. Trembling aspen (Populus tremuloides) is the dominant tree species throughout the parkland and typically grows in pure stands or groves (Figure 25.2). The southern boundary is fragmented, and the widely scattered groves of aspen are replaced by mixed-grass and shortgrass prairie. Grove size increases northwards, and eventually the distinctive, isolated character of the groves is lost as the trees merge with the mixedwood (conifer–hardwood) stands of the southern boreal forest. Extirpation of the bison and suppression of fire following settlement in the 1890s resulted in the spread of aspen throughout the parkland region (Archibold and Wilson 1980). Some authors (Sauer 1950; Stewart 1956) have suggested that aspen forest is the climax vegetation of the northern Great Plains; however, Looman (1979) considered the forest to be a subclimax formed by the southward extension of this pioneer boreal species.

Introduction

Scattered within the oak-hickory forest of southern Illinois and at Land Between The Lakes (LBL) in western Kentucky and northwest middle Tennessee are a variety of woodlands that have evolved from a barrens or savanna community of pre-European settlement time (before ca. 1820). In the present context, a barren is a community of widely scattered, short trees growing on relatively thin but continuous cover of extremely rocky soil; a savanna is dominated by scattered trees interspersed with prairie vegetation and located on somewhat deeper soil that may not have a large rock component. Trees commonly found in these communities are drought-tolerant scrub oak species (post oak, Quercus stellata; chestnut oak, Q. prinus; scarlet oak, Q. coccinea; southern red oak, Q. falcata; blackjack oak, Q. marilandica).

Woodland represents a late (successional) stage in the development of barrens and savanna communities and results from an extended period of protection (i.e., absence of disturbance, specifically fire). Like a barrens or savanna, it is a community dominated by extremely slow-growing, usually short, trees. The community has a higher tree density and a more closed (although still relatively open) canopy structure than a barrens or savanna. The generally gnarled, large, twisted limbs and crowns result from a combination of the open character of the overstory canopy, extreme temperatures (40 °C; Fralish, unpublished data) and low soil water-holding capacity, which produce high stress.

Introduction

Shale barrens have been a source of fascination to naturalists for the past one hundred years. In the late 19th and early 20th century, botanists noticed sites in the Middle Appalachians supporting an unusual herbaceous flora distinct from the surrounding eastern deciduous forest. Steele (1911) formally introduced shale barren communities to botanists with the following passage:

Thus, the first description delineated shale barren communities on the basis of substrate and presence of a unique flora. In addition, Steele (1911) speculated that in at least some cases soil chemistry must be a factor creating barrens. According to Platt (1951), a surface layer of rock fragments and its rapid erosion are important in maintaining shale barrens. Therefore, it is the structural characteristics of the ground surface and root zone that set shale barren communities apart from the surrounding sandstone ridges and limestone valleys.

Nevertheless, soil features alone are not adequate to delineate a shale barren community.

Introduction

“Alvars are naturally open areas of thin soil over essentially flat limestone or marble rock with trees absent or at least not forming a continuous canopy” (Catling et al. 1975; Catling and Brownell 1995). Workers familiar with similar habitats in Sweden, Denmark, and Estonia first applied the term alvar in North America to areas near Kingston, Ontario. The term is still widely used in Europe (e.g., Krahulec, Rosen and van der Maarel 1986). Although alvars of the Great Lakes region may be structurally similar to sites in northern Europe and subject to similar ecological processes, they are not necessarily more closely related to the European alvars than to habitats in adjacent regions of North America that are classified by other names. To the west, alvars grade into dry prairies over limestone or calcareous gravel (Curtis 1959; Erickson, Breener and Wraight 1942). Northward, similar habitats exist within the boreal forest region, where they are referred to as “limestone barren”. The “cedar glades” of Kentucky and Tennessee (e.g., Freeman 1933) are similar, but differ in having more endemic species (Baskin and Baskin 1986, 1988; Catling and Brownell 1995) and a different floristic composition (Catling and Brownell 1995). Nevertheless, the term alvar has been widely adopted, and over the past several years alvars have become a major focus of conservation efforts.

A mosaic of plant associations is a characteristic feature of alvars.

Introduction

The New Jersey Pine Barrens, or Pinelands, comprise a 550,000-ha mosaic of upland and wetland vegetation on the outer Coastal Plain of southern New Jersey (Little 1979; McCormick and Forman 1979). As noted by Little (1979), the physiognomy of much of the area does not resemble the common perception of areas termed barrens; that is, areas relatively “bare” of tree growth or with only stunted trees (Heikens and Robertson 1994; Homoya 1994; Tyndall 1994). In the lowlands, Atlantic white cedar (Chamaecyparis thyoides) swamp forests approach 26,200 trees ha–1 with a basal area of 56–57 m ha–1 (McCormick 1979). In the upland oak–pine forest, black oak (Quercus velutina), white oak (Q. alba), scarlet oak (Q. coccinea), chestnut oak (Q. prinus), pitch pine (Pinus rigida), and shortleaf pine (P. echinata) tree density can average 824 trees ha–1 with a basal area of 22 m2 ha−1 (Gibson, Collins and Good 1988). Nevertheless, within the Pinelands there exist a number of distinct regions of dwarfed pitch pine forest known as the Pine Plains or pygmy forest (McCormick and Buell 1968; Good, Good and Andresen 1979; Windisch 1986).