Page 6 - Vol.39-No.2

CROP PROTECTION

PEST, DISEASE AND WEED

INTERACTIONS IN COCOA

(THEOBROMA CACAO)

PART II PESTS AND PATHOGENS

Formation of ‘capsid pockets’ allows an in-
creasing spread and intensity of light into
the canopy, antagonising mirid infestation
and encouraging development of sun loving
DR. TERRY MABBETT weeds which would not normally be found
under a closed, self-shading cocoa canopy.
Light penetration Relationships between insect pest
and mirid bugs damage and pathogen infection occurs
throughout the tropics and that involving
Cocoa mirids (capsids) are sucking in- mirid-bug damage and infection by Nectria
sects that penetrate young pod and green rigidiuscula in West Africa is no exception.
shoot tissue injecting toxic saliva as they There is also speculation about interac-
feed. Tissue surrounding the feeding point tion between mirids, the most important
darkens in colour and collapses due to the group of insect pests of cocoa worldwide,
toxic effects of insect saliva to provide an and Phytophthora pod rot (Phytophthora
easy entry point for secondary disease such species) and globally the most damaging
as green point gall caused by the fungus disease of cocoa.
Nectria rigidiuscula, a common wound
parasite. Mirid damage predisposing young pods
to infection by Phytophthora fungi is an
Mirids rarely develop into economic pests ‘attractive’ proposition but there is no Mirid bugs target the new cocoa shoots
if the cocoa is well maintained and has ad- ‘hard and fast’ evidence to support such and leaves (Picture Dr Terry Mabbett)
equate shade whether from tall legumi- speculation. Mirids are a wide group of Phytophthora pod rot,
nous trees such as Erythrina (immortelle) insects comprising several different fam-
or shorter legume trees including Gliricidia ilies with the proportion of pod-feeding ants and rodents
and Leucaenea. Losses are usually less than damage and shoot-feeding damage varying A range of fungal, bacterial and viral
25% even in the high infestation areas of widely. For instance, Sahlbergella singularis pathogens infect cocoa but only Phy-
West Africa but in out-break years, and and Disantiella theobroma the main mirid- tophthora fungus-like pathogens are en-
especially on neglected cocoa estates, bug pests of cocoa in West Africa cause countered in quantity wherever cocoa is
losses can rise up to 75%. grown. These ubiquitous microbes belong
relatively little pod damage, while much
Ghanaian cocoa farmers suffer some of the damage caused in the Americas by to the Oomycetes and infect a wide range
of the heaviest and most frequent mirid Monalonion annulipes is to the pods. of shrub and tree hosts, but an inherent
attacks, and comparable with anything, an- liking for and dependence on free water and
ywhere in the cocoa growing world. And high humidity means they were virtually
Ghanaian cocoa farmers say it all by call- ‘custom made’ for cocoa. Early mycologists
ing mirid attack and damage ‘Sankonuabe’, called the Phytophthoras the ‘water fungi’.
which means ‘go back to planting oil palm’! Three unassailable facts about Phytoph-
In the absence of proper, sufficient shade, thora species infecting cocoa are their
and with full sunlight striking the canopy, widespread distribution, high frequency
mirid bug populations and consequent and accompanying high levels of pod loss.
damage levels surge to leave cocoa trees Phytophthora species are found in all co-
with a scorched look commonly called cap- coa growing regions of the world and are
sid blast. In patchy shade mirids thrive only responsible for some 45% of crop loss.
on cocoa trees located outside the shade Main species are Phytophthora megakarya
influence of tree canopies above and where and Phytophthora palmivora but a number
they create groups of mirid- damaged trees of others including Phytophthora capsici
called ‘capsid pockets’. and Phytophthora citrophthora have been
If left unchecked mirid damage becomes isolated from cocoa.
debilitating, with trees suffering repeated Mirid bugs become a problem once Phytophthora megakarya is the most
and continual shoot damage and defolia- shade over the cocoa canopy is broken aggressive and fittingly confined to West
tion leading to die-back and even death. and light gets into the cocoa stand Africa where a large proportion of the
(Picture Dr Terry Mabbett) world’s cocoa is grown. Severe infection
4 Vol. 39 No. 2