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Ways of controlling weed on trees

Weeds are simply green plants in the wrong place at the wrong time growing under and around trees competing with root systems for water and nutrients

Plants also grow on trees and in many agricultural tree crop situations in the wet and humid tropics, including cocoa, coffee and citrus, are sufficiently numerous 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 cover, among others, 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 them [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 several other 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 pallisade mesophyll and spongy mesophyll leaf tissues, and therefore absorption of light by chlorophyll pigments (contained in chloroplasts).

Light blocking by epiphytes can be especially damaging for cocoa and coffee which are traditionally grown under shade, because the amount of light that reaches 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 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 the corresponding inhibition of gaseous exchange (oxygen diffusing in and carbon dioxide 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. This leaf surface is devoid of the thick waxy cuticle which overlays the adaxial (upper) surface of the leaf.

Epiphytes on leaves and bark

Lichen growth is most prevalent on the adaxial (upper) surface of leaves such as coffee, citrus and avocado, which possess a natural thick waxy cuticle, while lichen growth is minimal or absent on soft leaves lacking a thick waxy cuticle (even when mature). It would appear the thick wax-rich cuticle provides the most suitable anchorage for the growth of lichen.

Lichen growth is prevalent on the trunks and branches virtually all trees in wet and humid tropical climates. Trees affected include cocoa, coffee, citrus, avocado, mango, durian, mangosteen, rambhutan, breadfruit and many others.

Epiphytic plant growth is particularly prolific on cocoa with its inherently closed canopy and accompanying shade trees which creates a very high humidity within the cocoa stand. This is required for maximum growth, production and longevity of Theobroma cacao trees which exist naturally as a wild under-storey in the rain forests of Central America and the Amazonian jungle of South America.

But high humidity and free water also provides ideal conditions for spread and development of cocoa diseases and especially Phytophthora pod rot (black pod of cocoa) caused by a number of fungus-like pathogens belonging to the genus Phytophthora. Phytophthora palmivora is universally found wherever cocoa is grown – West and Central Africa, South and South East Asia, Oceania and Latin America including the Caribbean. Phytophthora megakarya, a particularly aggressive pathogen, is prevalent in West Africa including Nigeria and Ghana