Plant Pathology Journal

Year: 2021 | Volume: 20 | Issue: 1 | Page No.: 1-10
DOI: 10.3923/ppj.2021.1.10

The Occurrence and Distribution of Sorghum Diseases in Major Production Regions of Senegal, West Africa

L.K. Prom , M.P. Sarr, C. Diatta, A. Ngom, O. Aïdara, N. Cisse and C. Magill

Abstract: Background and Objective: Sorghum ranks third behind millet and maize among dry land cereal production and plays a critical part in subsistence farming in Senegal. This study aimed to establish the occurrence, distribution and identify ‘hotspots’ of sorghum foliar and panicle diseases in major production regions of Senegal, West Africa. Materials and Methods: In 2019, a survey was conducted on 206 farmers’ fields along paved and unpaved roads and around rural villages in major sorghum production regions of Thies, Kolda, Kaolack, Tambacounda, Kaffrine, Diourbel and Fatick. At each field, 40 plants were assessed arbitrarily using a W-shaped pattern to cover the whole field. Stops were made at intervals of 30 km and at each stop, 2-5 fields were surveyed for disease prevalence and incidence. Results: Fifteen different sorghum diseases were documented. Leaf blight was the most prevalent disease, followed by anthracnose, zonate leaf spot, rough leaf spot, sooty stripe and target leaf spot across the regions. Prevalence of covered kernel smut, gray leaf spot, oval leaf spot, long smut, grain mold and maize mosaic virus were observed in low frequencies. The highest incidence of leaf blight and anthracnose was recorded in the region of Kolda. The incidence of sooty stripe was highest in the regions of Thies and Kaffrine, while zonate leaf spot incidence was highest in Tambacounda. Conclusion: The work is significant because for the first time this information was documented and can be utilized as a guide by researchers such as Plant Pathologists, students, government and funding agencies and producers on the occurrence, distribution and relative importance of each sorghum disease in major sorghum-growing regions in Senegal, West Africa. The “hot spots” for evaluating sorghum germplasm for resistance to leaf blight, anthracnose, zonate leaf spot, sooty stripe and rough leaf spot also were identified.

Keywords: prevalence, Senegal, sorghum fungal diseases, hot spots, sorghum, disease survey and incidence

INTRODUCTION

Sorghum [Sorghum bicolor (L.) Moench] is one of the most important crops, especially in the arid and semi-arid tropics of Africa, Asia, Australia and Latin America^1-3. Globally, sorghum provides the daily calorie needs for hundreds of millions people^1-4. The crop is used primarily for human consumption in the form of baked foodstuff, boiled and traditional beverages, pasta, syrup and semolina accompanied sauce and in industries, sorghum is utilized in the production of fiber, starch, paper, recently, it used as a source of biofuel^1,2,4-6. Sorghum is also used in animal feed, especially their haulms^1,3,7. In Senegal, sorghum ranks third behind millet and maize among dry land cereal production and plays a critical part in subsistence farming^4,8. The crop is planted in over 221,329 ha of land and producing about 225,865 tons with an average yield reaching 1,020 kg ha^{–1 8}. When compared to North America, Mexico and India sorghum yield in some African Countries is still low and this can be attributed to the unpredictability of rainfall, low soil fertility, low farm input and biotic stresses^3,8. In Senegal, landraces yields are low; however, some of the improved sorghum varieties developed and released by Institut Sénégalais de Recherches Agricoles/ Centre National de Recherches Agronomiques, yields can range from 2-5 t ha^–1 in Experiment Stations with high farm inputs^4,9. However, these improved varieties often yield about half when planted in farmers’ fields⁹. Sorghum will continue to play an important part in food security, especially in the drier tropics as the world’s population increases which is predicted to reach around 9.1 billion by 2050^3,10. New challenges due to climate change and increases in sorghum production will likely increase diseases incited fungi, bacteria and other micro-organisms. Yield will be severely impacted and in addition, some of these pathogens are mycotoxigenic, thereby limiting the crop uses in human consumption and animal feed¹¹. Therefore, an effective management strategy will require knowledge of the occurrence, distribution and relative economic importance of each disease across major sorghum growing regions. Due to the inadequate information regarding the biotic stresses that impact sorghum in major production regions of Senegal, West Africa, this study was undertaken to investigate the occurrence, distribution and ‘hotspots’ of sorghum foliar and panicle diseases in farmers’ fields.

MATERIALS AND METHODS

Study area: Major sorghum production regions of Thies, Kolda, Kaolack, Tambacounda, Kaffrine, Diourbel and Fatick in Senegal, West Africa were surveyed for foliar and panicle diseases during the 2019 growing season (Fig. 1)¹². Senegal is located between latitudes 12°30’ and 16°30’ N and longitudes 11°30’ and 17°30’ W, with the Sahelian zone lying in the northern part of Country, while the southern part which receives more rains belongs to the Sudanian zone¹³. There are two distinct seasons, a long dry period and 3-4 months rainy season^14,15. Annual rainfall for the surveyed regions ranges from 380 mm (Thies) in the north to 1072 mm in Kolda which lies in the south and the soil types consisted of cambisols, arenosols, gleysols, regosols, acrisols, lixisols and solonchacks either individually as in Fatick or in combination as in the other regions as shown in Table 1. Sorghum cultivation in the Country is mainly by rainfed.

Data collection: The survey was conducted on 206 farmers’ fields located along paved and unpaved roads and around rural villages. To identify farmers’ fields, department agents from each region were contacted and team drove to the locations to conduct the survey. Due to the late planting in most regions, plants at soft to early hard dough stages of development were assessed for disease prevalence and incidence. Stops were made at intervals of 30 km. At each stop, 2-5 fields (40 plants/field) were surveyed using a W-shaped pattern to cover the whole field.

For sampling plants in a diseased field, the use of a W-shaped pattern is more effective than a simple random sample because diverse locations within the field will be sampled¹⁶. Equations for prevalence and incidence as per Prom et al.¹¹ and Njoroge et al.¹⁷ used.

Statistical analysis: Pearson correlation coefficients between weather conditions and sorghum disease prevalence and incidence were calculated:

RESULTS

A total of 206 farmers’ fields in 7 major sorghum producing regions of Thies, Tambacounda, Kolda, Kaolack, Kaffrine, Diourbel, Fatick, Senegal and West Africa were surveyed for the prevalence and incidence of diseases during the 2019 growing season (Fig. 1). The vast majority of fields evaluated were planted with different landraces with few exceptions that were cultivated with improved varieties developed and released by Institut Sénégalais de RecherchesAgricoles/Centre National de Recherches Agronomiques (ISRA/CNRA). The fields surveyed consisted of different production practices, including pure sorghum stand, intercropping with either peanut or cowpea and field sizes ranged from 0.25-2.5 ha. The fields were relatively well maintained. Weather parameters and the soil types for the 7 regions which consisted of cambisols, arenosols, gleysols, regosols, acrisols, lixisols and solonchacks either individually as in Fatick or in combination as in the other regions are noted in Table 1.

Disease prevalence and incidence: In each field, 40 plants were assessed for disease prevalence and incidence using a W-shaped pattern to cover the whole site. Fifteen different sorghum diseases, including leaf blight, anthracnose, long smut, target leaf spot zonate leaf spot, rough leaf spots, oval leaf spot, sooty stripe, covered kernel spot and gray leaf spot were observed across the surveyed regions.

Figure 2 shows 2a Leaf blight; 2b Anthracnose infected leaf with acervuli; 2c Anthracnose infected midrib with acervuli; 2d Zonate leaf spot; 2e Rough leaf spot; 2f long smut; 2g Covered kernel smut and 2h Oval leaf spot. Overall, leaf blight was the most prevalent disease found in 96% of the 206 fields surveyed, followed by anthracnose (68%), zonate leaf spot (61%), rough leaf spot (42%), sooty stripe (41%) and target leaf spot (35%) across the regions (Table 2). Prevalence of covered kernel smut was 19%, gray leaf spot (13%), oval leaf spot (12%), long smut (10%) and bacterial stripe (10%). Out of the 206 fields surveyed, bacterial leaf streak was observed in 8 fields, bacterial leaf blight (9 fields), grain mold (5 fields) and maize mosaic virus (2 fields) accounting for less than 10% prevalence for each of these diseases (Table 2). The mean prevalence of leaf blight ranged from 88% in Thies region to a 100% in the regions of Kolda, Kaolack and Fatick (Table 3). The highest prevalence of anthracnose (88%) was recorded in the region of Thies, followed by 82 and 80% in the regions of Tambacounda and Kaffrine, respectively.

Zonate leaf spot was most prevalent in the region of Tambacounda (97%), while rough leaf spot (83%) and sooty stripe (83%) were most prevalent in the regions of Kaffrine and Fatick, respectively. The highest mean incidences of leaf blight (81%) and anthracnose (49%) were observed in the region of Kolda, while the highest incidences of long smut (20%) and target leaf spot (17%) were recorded in the region of Fatick (Table 4). Among the regions, the highest mean incidence of sooty stripe (30%) was noted in Thies region followed by 28% in Fatick. Whereas, the mean incidences of rough leaf spot (30%) and covered kernel smut (19%) were highest in the region of Kaffrine. Grain mold was recorded in 5 fields out of the 206 fields surveyed. In two separate fields, one each in Tambacounda and Diourbel where grain mold was observed, the incidences were 33 and 53% (Table 4).

Out of the 51 fields surveyed in the region of Kolda, 39% (i.e., 20 fields) and 14% (i.e., 7 fields) recorded 100% incidence of leaf blight and anthracnose in each field, respectively. The locations and their coordinates also are listed in Table 5. The highest incidence of sooty stripe was recorded in three different fields and regions, field # 5 (98%) located in Pointe Sarene, Thies region; field # 112 (98%), Fass Bamba, Kaffrine and field # 134 (90%) located in Keur Kory, Kaolack (Fig. 3a). Figure 3b also shows the highest incidence 98% of zonate leaf spot was recorded in field # 78 located in Djinkore, Tambacounda. The other three fields with next highest zonate leaf spot incidence also were located in the region of Tambacounda. The highest incidence of rough leaf spot was observed in field # 160 (95%) located in Keur Ismaila, in the region of Kaffrine (Fig. 4). These aforementioned fields can be considered ‘hot spots’ for leaf blight, anthracnose, sooty stripe and zonate leaf spot evaluation sites to identify resistant sources.

Pearson correlation coefficients were calculated among rainfall, maximum temperat ure, minimum temperature and various sorghum diseases (Table 6 and 7). A significant positive correlation (p = 0.80710, r = 0.0282) between rainfall and leaf blight was noted, indicating that higher rainfall results in higher prevalence of leaf blight (Table 6).

A significant negative correlation (p = -0.75306, r = 0.0507) between maximum temperature and oval leaf spot also was noted during the 2019 growing season. Similarly, there was a highly positive significant correlation (p = 0.87651, r = 0.0096) between rainfall and leaf blight incidence, indicating that higher rainfall results in a higher incidence of leaf blight (Table 7). Also, a positive significant correlation (p = 0.77783, r = 0.0395) between rainfall and the incidence of target leaf spot was noted.

A non-significant positive correlation between rainfall and anthracnose and a non-significant negative association between rainfall and long smut were observed (Table 6 and 7).

DISCUSSION

The ability to increase yield and garner profit for farmers in rainfed cultivation of sorghum in Senegal and other arid and semi-arid sorghum growing regions are hampered by many challenges, including lower farm inputs, low soil fertility, weather pattern and biotic stresses^3,8,18. For food security and the expected global increase in population to around 9.1 billion by 2050, increase in annual cereal production, including sorghum will be required¹⁰. Increases in sorghum production coupled with climate change will likely increase diseases caused by fungal, bacterial and viral micro-organisms¹¹. In Senegal, there is little or no information on the occurrence and distribution of sorghum diseases in farmers’ fields. In addition, sound management strategy will require knowledge of the occurrence, distribution and relative economic importance of each disease across major sorghum growing regions in Senegal. Thus, this is the first time that an extensive survey of sorghum foliar and panicle diseases in production fields was conducted.

In this survey, leaf blight was the most prevalent disease in all the 7 regions surveyed and in three of the regions Kolda, Kaolack and Fatick, the prevalence was 100%. Out of the 51 fields surveyed in the region of Kolda, 20 fields exhibited 100% of leaf blight. Beshir et al.¹⁹, also noted the presence of sorghum leaf blight in all 45 fields surveyed in Central Sudan with incidence ranging from 65-100%. In Niger, the prevalence of leaf blight was 89% across farmers’ fields surveyed in four major sorghum-growing regions¹¹. Survey conducted by Njoroge et al.¹⁷ on sorghum grown across different agroecological zones in Tanzania and Uganda, revealed that leaf blight was the most prevalent disease in Tanzania. The prevalence of leaf blight was 84.8% on sorghum fields surveyed in South Tigray, Ethiopia in 2015²⁰. However, survey conducted in three climatic zones (Sudan, northern and southern Guinea Savanna) in major sorghum-growing regions in Ghana, revealed low frequency of leaf blight²¹. This is not surprising due to the different weather patterns, farming systems and different landraces that are planted in different regions and Countries in West Africa. In Senegal, anthracnose was the second most frequently documented disease with 68% prevalence. Tsedaley et al.²² observed the presence of anthracnose in all fields surveyed in Southwestern and Western Ethiopia. Similarly, Eshte et al.²³, recorded 100% incidence of anthracnose in three districts of South Omo and Segen People zones in Ethiopia. while Prom et al.¹¹, noted the presence of anthracnose in 120 out of 121 fields surveyed in four major sorghum growing regions of Niger. In the current survey, 7 fields out of 51 surveyed in the region of Kolda had 100% incidence of anthracnose. In some Countries of West Africa, anthracnose infected foliage can result in yield loss of up to 46%²⁴. During the survey in Senegal, other diseases such as zonate leaf spot, rough leaf spot, sooty stripe, target leaf spot covered kernel smut, gray leaf spot, oval leaf spot and long smut were also documented, while bacterial leaf streak, bacterial leaf blight, grain mold and maize mosaic virus were observed in low frequency. In Senegal, one of the primary priorities involves the development of high yielding hybrids with resistance or tolerance to grain mold⁹. However, Denis and Girard²⁵ reported that location was a significant factor in grain mold severity and the concentration of grain molding fungi such as Fusarium and Curvularia in the kernel. In this study, the low frequency of grain mold observed can also be attributed to the fact that some of the surveyed field did not reach physiological maturity. Nevertheless, where it occurred in the regions of Tambacounda and Diourbel in this survey, grain mold was more severe in improved varieties released by ISRA/CNRA with compact shaped panicles. The landraces (loose panicles) planted in farmers’ fields during the survey were mainly from the Guinea race. In Puerto Rico, Cuevas et al.²⁶ evaluated 158 accessions from the Senegalese germplasm collection maintained by the USDA National Plant Germplasm, Georgia and reported that Guinea race population favors resistance to anthracnose and grain mold. Anthracnose, oval leaf spot, sooty stripe and gray leaf spot were the most prevalence diseases documented in farmers’ fields surveyed across four major sorghum-growing climatic zones in Nigeria²⁷. In Western Kenya, anthracnose, leaf blight, gray leaf spot and zonate leaf spot were found to be the most frequently observed diseases in farmers’ fields²⁸. In Niger, West Africa anthracnose, leaf blight, oval leaf spot, rough leaf spot and long smut were the most prevalence diseases found in farmers’ field across four regions¹¹. In this study, similar diseases with different frequencies were also documented in farmers’ fields in Senegal. Rainfall is vital for plant growth, pathogen spore germination, infection, disease development, severity and spread^8,29-35. The annual rainfall was highest in the region of Kolda which is located in the southern part of the Country (Table 1). In Kolda, 20 fields and 7 fields exhibited 100% incidence of leaf blight and anthracnose, respectively. These two diseases also were the most prevalent during the survey. In this survey, significant positive correlation between rainfall and leaf blight prevalence and highly significant association between rainfall and leaf blight incidence were noted, indicating that rainfall has an influence in the development of leaf blight. However, Hennessy et al.³⁶ observed that high leaf blight severity was associated with minimum temperature between 14 and 16°C and average temperature of 20.8-22.2°C, but that rain was not as critical in determining the severity of leaf blight as temperature. In the sorghum/ergot pathosystem, Montes-Garcia et al.²⁹ reported that ergot infection was enhanced by high humidity provided by rain or drizzles during the morning or afternoon. In the current survey, non-significant positive correlation was noted between rainfall and the prevalence and incidence of anthracnose. However, Chala et al.³⁰ and Prom et al.³¹ noted significant positive association between total rainfall and anthracnose development. A positive significant correlation between rainfall and the incidence of target leaf spot was also noted in this study. Long smut of sorghum is most severe during drought and low soil moisture conditions³⁷ and in this survey, non-significant negative correlation between rainfall and long smut was noted.

CONCLUSION

This survey showed that leaf blight was the most prevalence sorghum disease, followed by anthracnose, zonate leaf spot, rough leaf spot and sooty stripe. The work is significant because for the first time this information was documented and can be utilized as a guide by researchers such as Plant Pathologists, students, government and funding agencies and producers, on occurrence, distribution and relative importance of each sorghum disease in major sorghum-growing regions in Senegal, West Africa. During this survey “hot spots” for leaf blight, anthracnose, zonate leaf spot, sooty stripe and rough leaf spot were identified. These locations can be used to screen sorghum germplasm for resistance against the pathogens inciting the aforementioned diseases. With the global population increases coupled with climate change, the proliferation of crop diseases constitutes a direct threat to food security and an indirect threat to the economy, to human and animal health and to the environment. Integrated disease management, especially providing farmers with genetically resistant sources to diseases that of economic importance in the arid and semi-arid sorghum producing Countries or regions, including Senegal will be critical in ensuring food security, current interactions with farmers on disease identification, control methods and importance of planting resistant sources during the survey will facilitate the acceptance of new technology in improving farming in these regions.

SIGNIFICANCE STATEMENT

Sorghum leaf blight, anthracnose, zonate leaf spot, rough leaf spot, sooty stripe and target leaf spot were the most widely distributed diseases in major production regions of Senegal.

“Hot spots” for resistance evaluation of diseases such as leaf blight, anthracnose, sooty stripe, zonate leaf spot and rough leaf spot were identified. This is the first extensive survey of sorghum diseases in major production regions of Senegal. This document will serve as a guide for present and future sorghum workers, especially sorghum pathologists.

ACKNOWLEDGMENTS

This research was supported in part by the US Department of Agriculture, Agricultural Research Service. This research was made possible with the support of the American people provided to the Feed the Future Innovation Lab for Collaborative Research on Sorghum and Millet through the United States Agency for International Development (USAID). The contents are the sole responsibility of the authors and do not necessarily reflect the views of USAID or the United States Government. Program activities are funded by the United States Agency for International Development (USAID) under Cooperative Agreement No. AID-OAA-A-13-00047.

REFERENCES