Tropical epiphytes and collateral control with copper fungicide
Weeds are plants growing in the wrong place at the wrong time. Grasses and broadleaved weeds grow under and around trees to compete with their root systems for water and nutrients, and in the case of tall vigorously growing weeds around small stature trees for light and space as well.
Branches on cocoa trees weighed down by large bromeliad epiphytes (Picture: Dr Terry Mabbett)
‘Lower’ plants like lichens, mosses and ferns will grow on trees. In natural woodland situations they are invariably regarded as an important addition to biodiversity, but in many agricultural tree crop situations in the wet and humid tropics, including cocoa, coffee and citrus, they are sufficiently frequent and damaging to be classed as weeds. The scientific term for such plants is epiphytes derived from the Greek words ‘Epi’ meaning ‘on’ and ‘phyton’ meaning plant. Epiphytes growing on trees are also called ‘air’ plants because they have no tangible contact with the earth.
Trees support a wide range of epiphytes
Epiphytic plants include lichens, algae, bryophytes (mosses and liverworts), pteridophytes (ferns), bromeliads (pineapple-like plants), cacti and orchids. These green plants use the trees for anchorage and support but do not compete with the trees for water and nutrients. Epiphytes obtain these essential requirements from rain water and reservoirs of free water remaining on the trees.
However, they do harm the trees and will affect the yield and quality of fruit (e.g. cocoa pods, coffee berries and citrus fruit), both passively and actively, in a variety of ways. For instance, thick layers of lichen growing on the surfaces of coffee and citrus leaves can block and intercept a high proportion of light from entering the mesophyll leaf tissues, and therefore absorption of light by chlorophyll pigments (contained in chloroplasts).
Light blocking by epiphytes is especially damaging for cocoa and coffee traditionally grown under shade because the amount of light reaching the leaves is inherently low even without a layer of lichen over the leaf surface. Lichen isn’t a single plant but a symbiotic relationship between an alga and a fungus. The alga provides the light interception and energy production function and the fungus the attachment and anchorage, as well as absorption of water and nutrients from the surface layer of water on the leaf or bark, wherever the lichen happens to be growing.
There is an additional risk of leaf stomata being blocked, with corresponding inhibition of gaseous exchange (carbon dioxide diffusing in and oxygen and water out). However, in coffee and citrus most lichen is found on the adaxial (upper) surface of the leaf, whereas stomata are confined to the abaxial (lower) surface of the leaf.
Lichen growth is found mostly on the adaxial (upper) surface of hard stiff leaves such as coffee, citrus and avocado, with their natural thick and waxy cuticles, while lichen growth is minimal or absent on soft leaves such as those of cocoa which lack a thick waxy cuticle even when mature. It would appear that a thick wax-rich cuticle and the stiffness that imparts provides the most suitable anchorage for the growth of lichen.
This is a preview of a full article published in International Pest Control – July/August 2013 issue.
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Author: Dr Terry Mabbett
Director, Dr Terry Mabbett Consultants.
Category: Forestry-Plantation
