Abstract
Black pod is an important fungal disease of Theobroma cacao L. which causes significant yield losses in the field and consequently, inhibits the growth and development of the plant. For this purpose, the objective of this work was to evaluate the impact of phosphates solubilizing bacteria on growth of cocoa-tree and the biocontrol of Phytophthora megakarya, which is the most aggressive agent of black pod disease. Five strains of phosphate solubilizing bacteria (PSB) were characterized on petri dishes and in liquid media in order to determine their aptitude to solubilize different types of rock phosphate. The aptitude of the strains to inhibit P. megakarya was been evaluated on discs of leaves under in vitro and in vivo culture. The results shown that, Pantoea sp. (63B) with index of solubilization (IS) =5.5 and Enterobacter sp. (196B) with IS = 434.364 are the strains able to solubilize the five different types of rock phosphates used. Moreover, the antagonism test shown that Pantoea sp. (111B) with 90.35%, presented the greatest percentage of inhibition on plate. Bacillus sp. (104B) with 0.5 for the tolerant hybrid genotype and 1.75 for the sensitive hybrid genotype and Pantoea sp. (111B) with 1 for the tolerant hybrid genotype and 1.75 for the sensitive hybrid genotype are the highest foliar sensitivity reduction. In addition, the growth of cocoa tree has been very improved by the strain Enterobacter sp. (196B) for the tolerant hybrid genotype which had increased the plant high up to 50%, 35% for the number of leaves, 38.88% for the stem base diameter and 67.37% for the total dry mass compared to the negative control which did not receive microorganisms. With regard to the hybrids of sensitive cocoa-tree, Pantoea sp. 63B involves an increase of 43.28% of plant high and of 61.11% of the total dry mass, Enterobacter sp. (196B) involves an increase of 22.05% of number of leaves and 31.76% for the stem base diameter compared to the negative control. These strains can therefore be recommended to farmers in order to improve cocoa cultivation in the field. Keys words: Rock phosphates, Phytophthora megakarya, biocontrol, black pod disease, Thebroma cacao.
Highlights
Cocoa tree (Theobroma cacao L.) represents one of the major crops originating from tropical rainforests of Central and South America (Pokou et al, 2019)
The results obtained (Figure 1) indicated that Pantoea sp. 63B was the one which had more the strong potential of solubilization with an index of solubilization (IS) equal to 5.501 followed by Klebsiella sp.113B which had an IS equal to 5.029; between these two strains there was no significant difference while the strains Enterobacter sp. 79B and Enterobacter sp. 196B presented a weak solubilizer aptitude with IS equal to 3.603 and 3.065 respectively
All the rock phosphates do not have the same capacity of solubilization, the phosphate of Cameroon and Mali were solubilized by the various strains with a IS of 6.911 and 6.543, respectively, followed by the phosphate of Algeria with 4.803 for IS and of phosphates of Morocco and Mexico with IS 1.935 and 1.385, respectively
Summary
Cocoa tree (Theobroma cacao L.) represents one of the major crops originating from tropical rainforests of Central and South America (Pokou et al, 2019). In Cameroon, whatever this plant is economically important, its culture is subjected to the low productivity and several causes explain poor yield of the cocoa production: weak fertility of the soils, related to an unavailability of the assimilable phosphorus due to his precipitation by the mineral elements like iron and aluminum for acidic soils or calcium for alkaline soils (Fankem et al, 2008). Another fact is the black pod disease of cocoa tree (Ndoumbe-Nkeng, 2002). In Cameroon, these losses can reach 90 to 100% according to the area, the cultivar and the environmental conditions, in absence of treatments of plant (Ndoumbe-Nkeng et al, 2004; Nyadanu et al, 2012)
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