Green Horizon Newsletter - Issue No. 4 March 1996

A Newsletter of the Kenyan Forest Health Centre, Forest Department, Ministry of Environment and Natural Resources, Nairobi, Kenya

Dying Casuarinas Pose Another Environmental Concern

The devastating outbreak of the cypress aphid in 1990 has kept environmentalists in Kenya on their toes. But, the environmental scene has turned out to be a Hydra, with three other infestations reported as the battle to bring the Cypress aphid one rages.  Attention is now turned to the leucaena psyllid infestation and the African Pencil Cedar's reported die-back but, just as the ink dries on the proposals for funding projects to help deal with these two, another infestation was reported in August 1995.  The tree is Casuarina equisetifolia which grows on the Kenya Coast.

Casuarina is native to New South Wales, Queensland and Northern Australia but is widely planted throughout the tropics.  It grows fast on low nutrient, saline, sandy soils and is often used as a sand dune stabiliser and as an ornamental shade tree.  Casuarina wood makes good building poles.  The tree is also ideal for rehabilitating areas such as quarries and other degraded and denuded surfaces.

The tree has been relatively free of pest problems in Kenya but, in the recent past, there have been a number of reports on its mortality and the plantation failure associated with its specimens following which, the Forest Health Management Centre made an evaluation to identify causal factors associated with the accelerated mortality.

It was discovered that death of affected trees is rapid and is characterised by an initial yellowing of foliage followed by browning and drying.  There are no stem cankers or any evidence of foliar diseases on the affected trees.

Examination of a tree with fading or yellowing of foliage indicated that the cambium layer in the stem and branches was still green but an area of discoloured, necrotic tissue was found at the root collar.

Trees with brown and dried foliage have a dead cambium and contain extensive discoloration of woody tissue, presumably by saprophytic fungi.  These trees also contain infestations of termites which were also believed to be secondary in nature.

When the root system was excavated, a heavy infestation of a sucking insect was detected.  The insects occurred in large numbers on the main roots as well as on the feeder roots.  The insects were covered by masses of  a protective, powdery coating, yellow-brown in colour typical of some species of the family Phylloxeridae.  The protective coating bore a superficial resemblance to fungal spore masses.  The interior of the masses was white and contained large numbers of sessile, wingless adults, approximately 0.25 cm in length and light rose pink in colour.  Motile nymphs with a similar but somewhat paler body colour were also observed.

Specimens of typical damage, associated insects and fungi were collected and sent to the International Institute of Entomology (IIE) in the UK and the Mycological Institute (IMI) also in the UK.  The insect has since been identified as a root infesting, sucking insect of the order Homoptera.  The challenge now lies in finding a way of arresting the invasion with methods that are environment friendly.

Some Alternatives To Mexican Cypress Hedges

The cypress aphid continues to take its toll on Kenyan cypress trees in the natural forests, the forest plantations and where the previously popular tree was planted as an ornamental or used for hedges.  Though work on environmental-friendly control measures is being undertaken, chemical control remains the only way to curb spread.  Chemicals are used sparingly as they are known to be environmentally unfriendly.  The breeding of cypress species resistant to aphid attack to replace susceptible trees is another way of curbing the spread of the pest.  This is known as genetic control, a field the Forest Health Centre is currently engaged in.  As the country continues to count its losses before a remedy is found, the public is being asked to replace cypress with other species not susceptible to aphid attack.  Kenya's wealth of trees offers many choices.

Fact-File
Some Of The Trees Recommended As Alternatives To Cypress

Kei-apple (Dovyalis caffra) is a spiny shrub found in open bush and acacia woodlands.  It does well above 1,200mm, prefers deep well drained soils, tolerates loamy-clay and is drought resistant once established.  It makes a thorny evergreen hedge, usually 3-5m high and has grey bark and strong thorns some as long as 6cm.  Leaves are thin, shiny and dark green with rounded or notched tips.  Kei-apple also acts as a wind and fire break.  It forms a very thick, strong fence and has no serious pathological problems.

Acacia mellifora.  This is an indigenous shrub acacia widespread in all the arid areas of Kenya.  It has distinctive small, grey hooked pairs of pickles with black tips.  It is used as a live fence.  The bark is medicinal and he hedge can conserve soil.  The flowers help produce excellent quality honey.  It can be propagated trough coppicing which it does rather well.

Rosa multiflora is an exotic species from South Africa.  It is a thorny flowery shrub which makes fences that are practically impenetrable.  The fences also serve as fire and wind breaks and make good ornamentals.  The shrub is popular in the studied areas and is managed by trimming and weeding.   No serious pathological problems have been experienced where it has been in use.  It is propagated through cuttings.

Hakea saligna is an exotic species from Australia.  It is common in highland tea growing areas.  It has no thorns.  Its hedge is ornamental, a fire and wind bark, and helps conserve soil.  It grows fast in high potential areas and establishes quickly.  The hedge sheds leaves but does not form a thick fence.  The seeds have a germination rate of 70% and can store well.

Dichrostachys cinerea is an indigenous species found in a variety of habitats: open grassland, river banks, rocky hillsides and coastal plains.  It has curved thorns which grow as long as eight centimetres.  It can be reared to form an impenetrable hedge.  The hedge must be managed well by trimming and weeding.  Failure to do so could make it an aggressive weed because its root suckers form a dense thicket.  Propagation is through seedling, direct sowing and root suckers.

Prosopis juliflora is an exotic thorny shrub though it grows widely within the tropics.  It also grows well in arid regions, producing deep roots which reach far into the water enabling it to tolerate sandy, rocky or poor and saline soils.  The bark contains paired thorns which grow as long as five centimetres.  Trimming produces a good hedge that also acts as a wind bark, dune fixer and helps with soil conservation.  The species grows fast.  The seeds store well in pods even after being extracted as they are not attacked by insects.

Coffee-beam strychnos is an indigenous shrub of the drier areas which is also found along river banks in scrub as well as coastal areas.  The species can do well as a live fence if well trimmed.  Its stem and leaves can be used to make soup and its roots, fruits and barks are medicinal.  It can be propagated through seedlings, root suckers and coppicing.