Plants as Tree Pathogens

Various Higher Plants and Lichens

Various Trees

Plants growing in or on trees are often distinctive and highly visible. To some, the presence of plants attached to or hanging from the trunks and branches of yard trees is often disconcerting. In certain cases, concern may be justified, while in others there is little, if any, reason for alarm.

Some higher plants actually attach themselves to and draw all or part of their sustenance from their host trees (PARASITES). Others simply hang from or grow on their hosts while manufacturing (producing carbohydrates via photosynthesis) or acquiring their nutritional requirements independently (EPIPHYTES). A third group of plants which often represents problems for trees consist of vines which root in the soil, manufacture their own food via photosynthesis, but climb on available trees (CLIMBERS).

Parasitic plants are often harmful to their hosts, especially if present in abundance. Epiphytes, on the other hand are rarely harmful. Climbing vines are usually harmless when small, but can, over time, develop into real problems as they grow around the trunks and throughout the crowns of trees. Any plant that develops to the point of significantly shading the foliage of its host tree should be considered harmful.

Plants growing in or on trees in Florida come in varying sizes, shapes, and colors.

The common MISTLETOE is readily recognized in the winter on hardwoods which have lost their leaves by the globose (spherical) growth habit of the evergreen, leafy plants. Close inspection of this parasitic plant reveals typical attachments to host branches at the base of repeatedly branched, central stems.

SPANISH MOSS, perhaps Florida's best known epiphyte, is recognized easily by the hanging or draped appearance of its gray-green foliage.

BALL MOSS, a bromeliad similar to Spanish moss, occurs in ball-like clusters, as opposed to the hanging or shroud-like habit of the latter. OTHER BROMELIADS, are vase-like in appearance and resemble their well known cousin, the pineapple.

Epiphytic LICHENS typically appear as masses or patches of small gray-green, blue-green, or reddish-green flakes or bushes along the branches of their hardwood hosts.

ROOT PARASITES vary from chlorophyllous (green, having chlorophyll) plants with brightly colored and distinctive, seasonal flowers (for example, senna seymeria on pines) to drab brown, achlorophyllous (not green, lacking chlorophyll) plants with inconspicuous foliage and flowers (for example, beechdrops on beech and squawroot on oaks).

CLIMBERS, of course, are simply vines of various descriptions.

Most plants growing in or on trees in Florida reproduce by seed. LICHENS AND RESURRECTION FERNS are exceptions, however, in that these plants reproduce by means of minute, wind-disseminated spores. Some lichens also spread by means of small vegetative fragments (flakes) which break off from parent plants and are disseminated by the splattering action of rain. MISTLETOE is typically spread from tree to tree by birds that feed on its seed only to deposit them later through their excrement or by wiping the sticky seeds from their beaks onto host branches. The seeds of BROMELIADS (Spanish moss, etc.) are spread through the air by the wind, and to some extent the splashing of rain.

Seeds produced by CLIMBERS (vines) and ROOT PARASITES are spread by a variety of agents including wind, water, birds, and rodents. EPIPHYTES (bromeliads, lichens, resurrection ferns, etc.) simply use their hosts for support. They do not draw sustenance by way of parasitism from their hosts. These plants obtain their nutritional requirements from the air or rainwater and manufacture their own carbohydrates via photosynthesis . CLIMBERS likewise manufacture their own carbohydrates through photosynthesis, but these plants draw their mineral supplies, from the soil where they are rooted. In contrast, PARASITIC PLANTS draw all or part of their nutritional requirements directly from their hosts. Mistletoes attach themselves to their hosts and produce a root system (haustorium; pl. haustauria) within the woody tissues of infected branches. Through these haustoria the mistletoes draw water and mineral nutrients, at the expense of their hosts. In a similar vein, root parasites attach themselves to host roots by means of haustoria and extract water and nutrients from the roots of their hosts.

Table X: Some Common Types of High Plants Growing In or On Trees in Florida