The cacao disease trilogy: black pod, Monilia pod rot, and witches'-broom

Tags: cocoa bean, equipment industries, Monilia Pod Rot, Latin America, Westpark Drive, McLean, VA, chemical management, climatic conditions, cultural practices, pod, spray coverage, excessive amounts, disease management, world population, chocolate manufacturers, sensitivity ranges, Environmental Protection Agency, protectant fungicides, Brasier & Griffin, copper concentrations, soil surface, copper fungicide, Research Institute, bean production, Plano da Lavoura Cacaueira, Theobroma cacao, ROBERT H. FULTON, American Cocoa Research Institute, copper fungicides
Content: The Cacao Disease Trilogy: Black Pod, Monilia Pod Rot, and Witches'-Broom
ROBERT H. FULTON, American Cc)coa Research Institute, 7900 Westpark Drive, McLean, VA 22102
For decades, cacao plantations in the Caribbean basin and Latin America have flourished, declined, then flourished again. These drastic production swings have been triggered by world bean prices, political turmoils, disastrous disease outbreaks, and the sociological nature of growing what was once considered a "peasant crop." As the world population has increased, so has the demand for the cocoa bean. Currently, one-third of the world's supply of cocoa is produced in the Americas, and monetary support for basic and field research on breeding, epidemiologic aspects, pollination, etc., is steadily increasing. The world's chocolate manufacturers are sponsoring this research ,via the American Cocoa Research Institute and the International Office of Cocoa and Chocolate. Cocoa organizations such as Centro Agron o m i c ~Tropical de lnvestigacion y Ensenanza (CATIE) in Turrialba, Costa Rica, and Comissao Executiva do Plano da Lavoura Cacaueira (CEPLAC) in Itabuna, Brazil, are endowed by the manufacturers as well as by their governments. The Crop Cacao (Theobroma cacao L.) originated and is grown commercially in the Americas in the moist, tropical "cocoa bean belt," 20° above and below the equator. In this habitat, the trees form a closed, integrated foliar canopy within 2 yr, depending on planting density. The canopy area, initiated from the spreading scaffold limbs, is 2-3 m above the ground. Taller trees (Erythrina sp.) are planted at intervals to provide a light overhead shade to the dense cacao canopy, which supplies the energy necessary for vegetative growth and bean production (17). The environment is confining, shaded, and cooler, with soft air currents, than ambient field temperatures. During rains, the shade trees and the cacao foliar canopy act like giant sponges, producing rivulets of water over the scaffold branches and down the trunks, while releasing congregated large (2-4 mm in diameter) rain droplets from the undersurface of the canopy. These drops fall vertically Presented 12 August 1986 for the symposium "International problems in Tropical Plant Pathology" at the annual meeting of The American Phytopathological Society, Kissimmee, Florida. Accepted for publication 15 February 1989 1989TheAmerican Phytopathological Society
for roughly 2 m onto the soil and leaflittered surfaces and produce an impressive number of ballistic droplets that rebound at least 0.5 m up the tree trunk. Other, more aerosol droplets move high into the cacao understory of branches and foliage (8). The flowering phenomenon of cacao is unusual. Tiny flowers are produced on dense, cushionlike structures growing directly from the bark of the trunk, scaffold limbs, and older branches. The flower primordia arise from leaf axils, and their overall orientation on the bark surface follows the original leaf placement patterns. The cushions arise linearly on branches and spirally on "chupons" (vertical shoots). The number of flowers is immense, commonly 3,000-5,000 per tree. Usually, pod set is relatively low, the number of maturing pods per tree varying from 10 to 120. This can be offset, however, by scattering banana or plantain pseudostem pieces over the planting floor. These serve as nesting sites for the midge pollinator (18). A pod takes 6 mo to mature from pollination t o harvest. Because of rhythmic flower flushes during the rainy season, pods in all stages of maturation are present on the tree. Inside these pods are some 30-40 cocoa beans, the economic units for chocolate manufacture. The Challenge Cacao is a suitable crop for small farms as well as large plantations, especially since the advent of hybrids with their increased productivity, but it is not without problems. Cacao is subjected at repeated intervals t o a trilogy of crippling fungal diseases: black pod (Phytophthora palmivora (Butler) Butler), Monilia pod rot (Moniliophthora roreri (Ciferri) Evans et al), and witches'-broom (Crinipellis perniciosa (Stahel) Singer). The management of these diseases under tropical conditions challenges the aspiring and experienced pathologist alike. On the one hand are the extremes in rainfall, humidity, heterogenicity in soil types, and genetic stock with continued rhythmic flushes of new, unprotected growth while the maturing pod is at risk for months. On the other hand is the tortoise-slow pace of quantified chemical control schemes. The decades of spraying fungicides have proved three major shortcomings: 1) the practice is costly, 2) the results are poor, and 3) the available products are not fitted or formulated for the actual targets (4,7,9,15). A review of all facets of each disease is not within the scope of this article.
Rather, I will focus on a series of disease management hypotheses to generate changes, ideas, and hopefully advances by the chocolate, chemical, and equipment industries acting as a team.
Black Pod
The incidence of black pod peaks at
harvest. In drought years, when infection
levels are low, growers and spray masters
become lax with their programs. Then
when climatic conditions are conducive
and disease levels skyrocket, they apply
excessive amounts of fungicides at short
intervals. The solution to such manage-
ment is to complement the spray pro-
gram with a "program package," in
which cultural practices such as pruning,
drainage, weed control, frequent removal
of infected pods, and general tree
sanitation are followed at intervals
throughout the year and continued on
an annual basis. These steps change the
microclimate, improve spray coverage,
and help to lower inoculum potential,
making chemical management more
feasible.
Some of the factors influencing disease
development could be better handled.
The infection gradient concept for
"originating" sources of pod infection
delimited the soil and infected flower
cushions as the prime candidates (8).
After the rainy season starts, sporangia
of Phytophthora are
on the
surfaces of infected rootlets. These
liberate motile zoospores that swim
upward and accumulate at the soil
surface (20). These in turn are splash-
dispersed upward onto the host. Early
studies with soil chemicals and/or
protectant fungicides applied directly to
this target area around the tree had
mixed results (20). Also, copper
fungicides have been applied for years
and have accumulated in the soil surface.
Further, P. capsici Leonian, rather than
P. palmivora, is now the species to be
dealt with in the Americas, and P.
megakarya (Brasier & Griffin) is waiting
in the wings (9,15,17). Their individual
ED,, levels based on copper fungicide
classes need to be identified. Also, in view
of the potential copper concentrations on
and in some of the cacao plantation
floors, sensitivity ranges among the
Phytophthora spp. could differ.
On the basis of today's soil strategy
and in tune with the philosophy of the
Environmental Protection Agency, there
are two possibilities for a chemical
mulch: the octylphenoxy ethanols
(OPES) and the isothiazolone corn-
pounds (1 1). Treatment of zoospores of
Plant Disease/July 1989 601

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