Diagnosis of fungal diseases in quinoa (Chenopodium quinoa Willd) in Burkina Faso

Sample size

A total of 138 samples presenting various symptoms were collected and analyzed during the two collection seasons. These were distributed as follows: 24 samples in the Sahelian zone, 48 in the North Sudanese zone, and 66 in the South Sudanese zone (Table 1).

Description of symptoms

Six primary symptom categories were identified across the three agro-ecological zones. These included damping-off, brown crown rot, brown stalk rot, plant wilting, grey mold, and chlorosis with necrotic brown leaf spots (Fig. 1). Damping-off is manifested by seed rot, followed by severe browning of the root and hypocotyl, and death of the seedlings. This damage is observed approximately 10 days after sowing (Fig. 1A). Collar rot is characterised first by wet patches on the upper part of the collar, then by the foliage becoming chlorotic, and finally by the stalk bending downward and tending to break easily (Fig. 1B). This symptom manifests when the plant reaches the 12-true-leaf stage until flowering. Brown stalk rot begins with the appearance of necrotic spots on the stalk. Over time, the stalk assumes a strangulated shape, and the plant collapses (Fig. 1C). This occurs at the same stages on the plant as crown rot. Leaf chlorosis accompanied by necrotic spots appears when the plants were full vegetation. The disease persists on the plant until physiological maturity (Fig. 1D). The disease manifested as gray mold on the quinoa plant as soon as the panicle inflection occurred, or as the panicle entered flowering. At this stage, the disease was most visible when the panicle took on a flaccid, drooping shape. The plant began to dry, and within one week of attack, the main and secondary panicles had completely dried (Fig. 1E). The disease spread rapidly, with all plants in the field being affected within 10 days. Wilt was observed on several plants in various locations, manifesting as a loss of plant rigidity, with curling of the upper apical bud (Fig. 1F).

Disease distribution in relation to the agroecological zone

The average frequencies of the various symptoms observed in each agro-ecological zone are presented in Table 2. Analysis of the table shows that all the symptoms observed were present in all agro-ecological zones. The frequencies of plant wilt varied from 8.33% in the southern Sudan and Sahel zones to 62.5% for crown rot in the northern Sudan zone (Table 2). In all zones, analysis of variance revealed a significant difference between the mean frequencies of all symptoms observed (Table 2). In the southern Sudanian and Sahelian zones, gray mold was significantly more frequent, with 55.55% and 30.55% of the samples in these zones, respectively, than plant wilt, which was the least frequent, with 08.33% of the samples in each zone. However, brown rot of the crown was the most frequent in the North-Sudanese zone, with 62.50% of samples from the zone, against chlorosis and necrotic spots of the leaves, with 08% of samples from the zone (Table 2). On average, gray mold and brown rot of the crown were significantly more frequent in all the samples collected, with 45.36% and 41.20%, respectively.

Disease distribution in agro-ecological zones

The geographical distribution of diseases across the country

The symptom map was drawn on the basis of the field diagnosis (Fig. 2). The geographical distribution of the six (6) symptoms observed on the crop. In this way, we can see a strong presence of disease in the study localities in the southern Sudanian zone, and as we progress toward the northern Sudanian and Sahelian zones, we see a decrease in the number of symptoms per site. Gray molds were present at all localities except one (1) in both campaigns. Leaf chlorosis with necrotic spots and damping off was observed at fifteen (15) localities. Throughout the country, no symptoms were observed at any locality in any agroecological zone.

Distribution of diseases in Burkina Faso.

Morphological characterization of the fungi

The number of parasite species found on each symptom varied. Rhizoctonia solani, Fusarium oxysporum, Sclerotinia sclerotiorum, and Ascochyta rabiei were the pathogens identified in samples of damping-off castings (Fig. 3a, a’; b, b’; c, c’ and j, j’). In the case of brown stalk rot, the fungi responsible are Phoma exigua, Botrytis cinerea, Colletotrichum dematium, Macrophomina phaseolina, Curvularia lunata and Curvularia pallescence (Fig. 3h, h’; g, g’; i summer i’ and p, p’).The pathogens associated with brown crown rot include Rhizoctonia solani, Fusarium solani, Fusarium oxysporum, Fusarium moniliforme, and Sclerotium sclerotiorum (Fig. 3b, b’; c, c’; j, j’; r, r’ and s, s’). On leaves exhibiting chlorosis and necrotic lesions, the following pathogens were isolated M. phaseolina, Melanospora zamiae, Botryodiplodia theobromae, Cercospora sesami, Phoma sorghina, P. lingam, and Nigrospora oryzae (Fig. 3a, a’; d, d’; e, e’; i, i’; k, k’ and q, q’). In samples exhibiting gray mold, Fusarium oxysporum and Botrytis cinerea were predominantly identified, with the occasional presence of Phoma sorghina (Fig. 3b, b’; d, d’ and p, p’). Wilting of the plants was attributed to species of the Fusarium genus, with the involvement of other terrestrial species such as R. solani, S. sclerotiorum, and Ascochyta rabiei (Fig. 3a, a’; c, c’ and j, j’).

Morphological structure of some fungi characterized.

Frequency of fungal pathogens in the samples

A total of 29 fungal species were identified in the 138 samples examined, with varying frequencies (Table 3). The number and infection rates of the various species identified varied significantly between agro-ecological zones. In the Sahelian zone, 11 species were identified, with the most common being Rhizoctonia solani and Fusarium solani, occurring with the same frequency (37.5%). In the northern Sudan zone, 18 fungal species were identified, with Fusarium oxysporum being the most common in the sample (52.40%), followed by Phoma lingam (38%), Phoma sorghina and Botrytis cinerea (22.22%). In the southern Sudan zone, 22 fungal species were detected on organs, with Fusarium oxysporum being the most prevalent (80%), followed by Fusarium solani (35%), Phoma sorghina (30%), and Melanospra zamiae (30%). From the Sahelian zone, the least-watered area of the country, to the moderately-watered North Sudanian zone, seven additional fungi were observed. In addition to the shift from the northern Sudanese zone to the southern Sudanese zone, four additional fungal species have been identified as diseases of quinoa. These include Fusarium oxysporum, Fusarium solani, Curvularia lunata, Macrophomina phaseolina, Phoma lingam, Phoma sorghina, and Rhizoctonia solani. These species were present in the different agro-ecological zones of Burkina Faso.

Koch’s postulates

Koch’s postulate was employed to demonstrate a causal relationship between the observed symptoms and the identified fungi. This confirmed the significant involvement of the various fungal species isolated from infected organs, exhibiting a wide range of virulence levels. Figure 4 illustrates the diverse representative symptoms induced by the fungi inoculated on healthy plants. The most common fungal genera that produced necrotic spots with marked chlorosis on the leaves were Phoma, Botrytis, Curvularia, Colletotrichum and Macrophomina (Fig. 4b–d). For telluric fungi, the genera Fusarium, Rhizoctonia, Sclerotinia, and Ascochyta were identified as the primary causes of post-alkergen damping-off (Fig. 4e, e’). Additionally, the plants exhibited dwarfing compared to the control (Fig. 4f, f’).

Symptoms observed with pathogenicity tests. (a) control; (b) necrosis limited to the infected area; (c) chlorosis beyond the infection area; (d) generalized chlorosis on the leaf; (e) damping-off; (f) control; (g) dwarf plant; (h) control.

Pathogenicity of fungal species isolated from infected organs of quinoa

The infection of the aerial organs of quinoa revealed a diversified pathogenicity of different fungal species tested (Table 4). At 7 days after inoculation (7 DAI), fungi such as B. theobromae and E. rostratum exhibited symptoms of eight additional species (B. micropus, C. eragrostidis, C. glaesporioides, C. sesami, C. pallescens, M. zamiae, N. oryzae, and Pyricularia sp.). The species B. theobromae, E. rostratum, C. sesami, C. pallescens, M. zamiae, and N. oryzae exhibited the most pronounced symptoms and symptom traits on the plants, while the species B. microspora, C. demantium, C. glaesporioides, E. rostratum, N. oryzae, and Pyricularia sp. demonstrated the least pronounced symptoms. Fourteen days after inoculation (14DAI), six species exhibited very symptoms (B. micropus, B. theobromae, C. demantium, C. glaesporioides, E. rostratum, N. oryzae, Pyricularia sp.), while seven species exhibited very few symptoms (C. eragrostidis, C. pallescens, C. sesami, M. zamiae, Pyricularia sp., S. sclerotiorum) in comparison to the former. The species (B. micropus, B. theobromae, C. demantium, C. glaesporioides, E. rostratum, N. oryzae, Pyricularia sp.) exhibited very pronounced symptomatic, while seven other species (C. eragrostidis, C. pallescens, C. sesami, M. zamiae, Pyricularia sp, and S. sclerotiorum) demonstrated very pronounced resistance to the non-symptomatic fungus. Twenty one days later (21DAI), nineteen (19) phytopathogenic fungi exhibited at least some symptoms. The remaining four (4) species (F. equiseti, M. phaseolina and P. exigua) had moderate symptoms on their leaves.

Impact of telluric fungi on plant vigor

The impact of fungi on quinoa cultivation was evaluated through the measurement of plant vigor (height). The results of the analysis of variance showed that the different species induce a significant reduction in plant growth at the various observation periods (Table 5). F. oxysporum was identified as the most detrimental plant-parasitic agent, exhibiting the greatest decrease in growth compared ther other species and the control group across all observation periods. At 20 days after sowing (20 DAS), the following pathogens were observed: A. rabiei, F. solani, F. moliniforme, and R. solani.