Thèse Ndo - Montpellier SupAgro

Annexe : Effet de l’application de l’urée
Abstract
Phaeoramularia leaf and fruit spot disease of citrus (PLFSD) caused by Pseudocercospora
angolensis is one of the most serious diseases which attacks citrus in tropical Africa. It also
constitutes a threat for the other producer countries located in tropical zone. Chemical
treatment is the only method used for effective control of this disease. However, in addition to
causing ecological problems, this method lies beyond the reach local smallholders. The
development of ecologically friendly strategies to ensure effective protection of citrus against
P. angolensis, is thus necessary. To this end, a knowledge of PLFSD risk factors is prerequired.
The purpose of this study was to specify, by means of a survey and experiments
carried out in natural conditions, the effects of certain biotic and abiotic factors on the
development of the disease. The survey was carried out in 39 sites belonging to 13 citrus
production basins of the humid zones of Cameroon and targetted a collection of
environmental and biological tree characteristics as well as a focus on the most
epidermiologically important ones. Experiments on 8 sites enabled a confirmation and
evaluation of suspected factors. A stepwise logistic regression analysis of survey data made it
possible to target altitude, soil type, citrus species and vegetation type as main factors
influencing disease development. Relations between these factors and disease incidence are
illustrated in segmentation trees which define the levels of disease according to the evolution
of these factors. Experiments confirmed the great sensitivity of grape fruit and orange trees to
PLFSD while tangerine and Tahiti lime trees were less sensitive. Disease incidence increased
with a rise in altitude and a reduction in temperature and rainfall. Soil texture (clay, sand and
silt contents) proved to be more determinant in disease development than its chemical
composition. The role of nitrogen fertilization was not demonstrated in disease epidemiology
while the effect of shade was significant. An experimentation showed that trees planted under
shade are less affected than those located in sunshine and increase in shade intensity conferred
a reduction of disease incidence. A spatial structure analysis of the disease made it possible to
highlight an aggregate structure indicating small scale disease dispersion. An analysis of the
spatial structure of an agro_forestry farm showed an aggregate structure of the sub
populations of constituent tree families. This structure suggested that the dispersion of PLFSD
within such a farm would be limited between the various citrus aggregates. PLS regression
analysis suggested that several of these parameters play a determinant role in contributing to
the development of disease on grape fruit, orange trees and Satsuma mandarin trees. All
results indicate the need for developing parameter specific strategies for controlling the
disease. The development of integrated agro forest systems which enable a decrease of the
pressure of pest and diseases is envisaged.
Key-words: Pseudocercospora angolensis, leaf and fruit spot disease, citrus, epidemiology,
risk factors
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