The plant material used in this study was constituted by organs (stems, leaves and pods) of cowpea plants with symptoms of cowpea brown blotch disease. They were collected in forty-four villages in seven divisions in the northern and far northern regions of the Sudano-Sahelian zone of Cameroon. The collection was done in the cowpea production basins according to the arrangement of the cardinal points [10]. These samples of diseased organs were transported to the plant pathology laboratory of the regional agricultural research center of Maroua.

The infected organs collected were cut with scapel and then superficially desinfected with sodium hypochlorite (NaOCl) 1% for 5 minutes and immersed in 70% alcohol (v/v) for one minute followed by three successive rinses with sterile distilled water [11] [12]. After rinsing, the disinfected fragments were dried for at least two hours on sterile absorbent paper and then placed in the petri dishes previously dressed with three layers of blotting paper moistened with sterile distilled water. The boxes were incubated for 5 to 7 days at room temperature in the laboratory under alternating light and dark (12h/12h) [13]. After the incubation period, the cultured samples were taken under a stereo microscope for identification of Colletotrichumcapsici. Identification was made on the basis of the morphological characteristics of Colletotrichumcapsici using the identification key defined by Mathur and Kongsdal (2003) [14]. After identification, isolates were transplanted onto the Potato Dextrose Agar (PDA) culture medium supplanted by streptomycin and ampicilin and incubated for seven days for purification [15].

The single spores production was made from previously isolated and purified population isolates. A diameter of 5 mm conidian mass of each population isolate was transferred to 10 ml of sterile water. The mixture was stirred at the vortex and 1ml of the suspension was removed and added to 9 ml of sterile distilled water contained in a tube. The action was redone twice to obtain a less concentrated suspension (10⁻³ of the initial concentration) [16]. One to two drops of the suspension was sprayed on PDA medium. After sixteen to 24 hours of incubation, 3 to 4 sprouted conidia were individually transplanted onto PDA.

Morphological characterization was done based on qualitative and quantitative characteristics from the work of other scientists on different species of Colletotrichumspp. [17] [18] [19].

After purification of the single spore isolates, a 5 mm diameter fragment of each of the isolates was transferred and deposited in the centre of the petri dishes containing the PDA medium. The incubation of the cultures was done at room temperature of the laboratory (23˚C - 25˚C). The morphological characteristics selected for the study focused on the colony colour, the mycelial shape, the abundance of acervulis and the presence or absence of saltations [20]. Observations and notations were made on the seventh day after incubation of single spores isolates. The biometrics used for the study included mycelial diameter, daily radial growth rate, conidia length and width [21]. The reverses of petri dishes containing the single spores isolates were previously crossed by two diagonals (x, y) perpendicular to each other. The measurement of the evolution of the colony was taken every other day until the sixth day.

The mycelial diameter was calculated by the formula: D = d 1 + d 2 2 − d 0 [18].

The radial growth rate was calculated by the formula:

R g r = ( D M J 1 − D M J 0 ) / n + ( D M J 2 − D M J 1 ) / n + … + ( D M J n − D M J n − 1 ) / n [22]

where DMJ_n is the average growth diameter on the n^th day of measurement.

The daily radial growth rate (DMJ_n) is the average daily increase in crop diameter; it’s calculated by the formula:

DMJ_n = (d₁ + d₂)/2 where d₁ is the first diameter and d₂ is the second diameter measured on the n^th day.

DMJ₀ = 5 mm = diameter of the transplanted explant.

On the tenth day after transplanting, 10 ml of sterile water was poured into the culture and after stirring, the suspension was returned to the test tubes. A drop was mounted on a micrometer and the reading was made using the Motic microscope at magnification 400. Thus, measurements (Length and width) of conidia were taken on 30 conidia per strain [23].

The data was entered into the Excel 2010 spreadsheet. SPSS software was used for qualitative data analysis and STATGRAPHICS Centurion XVI.II software for quantitative data. These data were subjected to analysis of variance (ANOVA) and separate means using Fisher’s smallest significant difference test (PPDS) at the 5% probability threshold [24] for quantitative data and the khi two test for qualitative data. Using the averages of the measured and scored parameters, the principal component analysis was performed using XLSTAT 2007 software.

The results obtained from this study are presented under labels such as identification of Colletotrichumcapsici isolates in Cameroon, their morphological characters, biometric characters, spatial arrangement and similarity.

After collection, 151 samples of plant organs were obtained. Of the 151 samples, 51 single spores isolates were isolated. These results show that isolates could not be identified and isolated at each collection site. However, Thio’s work (2016) [1] shows that at least one isolate has been identified in each of the collection sites.

From the same work, 44.44% of isolates were identified from the stem organ, 8.33% from the leaf organ and 47.22% from the pod organ compared to 50.92%, 14.3% and 34.55% respectively from the stem, leaf and pod organs in the present study. However, it should be noted that both in this study and in the other the high percentage of isolate comes from the stem organ, followed by the pod organ.

This could be explained by the fact that Colletotrichumcapsici isolates come more from stems [25]. Also, the infectious process is highly developed during flowering and fruit formation [27].

The results of this study indicate that there are various colony colors of Colletotrichumcapsici: black, brown-black, brown-orange, brown, black, gray colors. These colors are characteristic of C. capsici and C. gloeosporiodes [28] [12].

This colour variation could be related either to the host plant or to the nature of the isolate or the environmental conditions of the isolate. The identification of Colletotrichum species is based on the morphocultural characteristics coupled with the origins of the isolates. However, several isolates of Colletotrichum show great variations in culture. [29].

Of the identified C. capsici isolates, 56.36% have abundant acervulis, which is much lower than the proportion (90%) observed by Thio (2018) [30]. In addition, Sereme [2] indicated that the abundance of acervulis is a discriminating character for the characterization of C. capsici.

Various forms of the mycelial have been noted such as compact, cottony, airy... These results corroborate those obtained by Ehui et al. [12].

The differences established between isolates by the different parameters confirm their use by several scientists including Ehui et al. [12]; Appiah et al. [26]; Sereme [2] characterized Colletotrichumspp isolates using these parameters. The revelation of several homogeneous groups of isolates indicates that there is a morpho-metric diversity of Colletotrichumcapsici in the Sudano-Sahelian zone of Cameroon.

The average growth diameter on the sixth day is 66.23 mm and smaller than that (68 mm) obtained by Sereme [2] and the average daily growth rate is 11.03 mm/day. These values are higher than those obtained by Ehui et al. [12] during the characterization of C. gloeosporiodes Penz manihotis causative agent of cassava anthracnose.

During his study, the average diameter was 65.40 mm and the radial growth rate was 9.20 mm/day. The average value of the growth diameter is higher than that obtained (57.87 mm) by Kuete et al. [17] when the radial growth rate is higher than that obtained by the same author (12.39 mm/day).

The conidia dimensions (length and width) observed in C. capsici isolates are not similar to those reported in other Colletrotrichum [18] [19], Appiah et al. [26] and similar to those reported in other Colletotrichum and fungal species Sereme [2] [12] [22].

Indeed, conidia lengths and widths ranged from 48.10 to 72.79 μm and 4.41 μm and 7.71 μm respectively. These values do not fall within the range described by Yun et al. [31]; who demonstrated that C. capsici conidia had dimensions ranging from 13.21 μm to 16.21 μm for length and from 1.79 to 3.28 μm for width.